virtio-blk: add multiread support

Signed-off-by Peter Lieven <[email protected]>

---
 block.c                    |    2 ++
 block/accounting.c         |    7 +++++++
 block/qapi.c               |    2 ++
 hmp.c                      |    6 +++++-
 include/block/accounting.h |    3 +++
 qapi/block-core.json       |   10 +++++++++-
 6 files changed, 28 insertions(+), 2 deletions(-)

+# @wr_merged: Number of write requests that have been merged into another request

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

          flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
          flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

          flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
          flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

          flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
          flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

          flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
          flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Signed-off-by Peter Lieven <[email protected]>

---
  hw/block/dataplane/virtio-blk.c |   10 +-
  hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
  include/hw/virtio/virtio-blk.h  |   23 ++--
  3 files changed, 156 insertions(+), 99 deletions(-)

hw/block/dataplane/virtio-blk.c +5 -5

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
      event_notifier_test_and_clear(&s->host_notifier);
      blk_io_plug(s->conf->conf.blk);
      for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
          int ret;
 
          /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
                                                          req->elem.in_num,
                                                          req->elem.index);
 
-            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
          }
 
-        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
 
          if (likely(ret == -EAGAIN)) { /* vring emptied */
              /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +33 -9

index 490f961..f522bfc 100644
@@ -22,12 +22,15 @@
  #include "dataplane/virtio-blk.h"
  #include "migration/migration.h"
  #include "block/scsi.h"
+#include "block/block_int.h"
  #ifdef __linux__
  # include <scsi/sg.h>
  #endif
  #include "hw/virtio/virtio-bus.h"
  #include "hw/virtio/virtio-access.h"
 
+/* #define DEBUG_MULTIREQ */
+
  VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
  {
      VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

@@ -88,6 +91,11 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
 
      trace_virtio_blk_rw_complete(req, ret);
 
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_blk_rw_complete p %p ret %d\n",
+           req, ret);
+#endif
+
      if (ret) {
          int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
          bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
      virtio_blk_free_request(req);
  }
 
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
  {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
 
-    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
          return;
      }
 
-    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
-                virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c +20 -59

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                      virtio_multireq_cb, mrb);
      }
 
-    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
  }
 
  static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

@@ -283,9 +330,9 @@ static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)
                       BLOCK_ACCT_FLUSH);
 
      /*
-     * Make sure all outstanding writes are posted to the backing device.
+     * Make sure all outstanding requests are posted to the backing device.
       */
-    virtio_submit_multiwrite(req->dev->blk, mrb);
+    virtio_submit_multireq(req->dev->blk, mrb);
      blk_aio_flush(req->dev->blk, virtio_blk_flush_complete, req);
  }
 

@@ -308,61 +355,8 @@ static bool virtio_blk_sect_range_ok(VirtIOBlock *dev,
      return true;
  }
 
-static void virtio_blk_handle_write(VirtIOBlockReq *req, MultiReqBuffer *mrb)
-{
-    BlockRequest *blkreq;
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_write(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_WRITE);
-
-    if (mrb->num_writes == VIRTIO_BLK_MAX_MERGE_REQS) {
-        virtio_submit_multiwrite(req->dev->blk, mrb);
-    }
-
-    blkreq = &mrb->blkreq[mrb->num_writes];
-    blkreq->sector = sector;
-    blkreq->nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
-    blkreq->qiov = &req->qiov;
-    blkreq->cb = virtio_blk_rw_complete;
-    blkreq->opaque = req;
-    blkreq->error = 0;
-
-    mrb->num_writes++;
-}
-
-static void virtio_blk_handle_read(VirtIOBlockReq *req)
-{
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_read(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_READ);
-    blk_aio_readv(req->dev->blk, sector, &req->qiov,
-                  req->qiov.size / BDRV_SECTOR_SIZE,
-                  virtio_blk_rw_complete, req);
-}
-
-void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
+void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb_wr,
+                               MultiReqBuffer *mrb_rd)
  {
      uint32_t type;
      struct iovec *in_iov = req->elem.in_sg;

@@ -397,7 +391,7 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
      type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
 
      if (type & VIRTIO_BLK_T_FLUSH) {
-        virtio_blk_handle_flush(req, mrb);
+        virtio_blk_handle_flush(req, mrb_wr);
      } else if (type & VIRTIO_BLK_T_SCSI_CMD) {
          virtio_blk_handle_scsi(req);
      } else if (type & VIRTIO_BLK_T_GET_ID) {

@@ -414,13 +408,71 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
          iov_from_buf(in_iov, in_num, 0, serial, size);

Quoted 58 lines · Peter Lieven

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

          flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
          flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

          flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
          flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

          flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
          flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

          flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
          flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Quoted 6 lines

Signed-off-by Peter Lieven <[email protected]>

---
  hw/block/dataplane/virtio-blk.c |   10 +-
  hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
  include/hw/virtio/virtio-blk.h  |   23 ++--
  3 files changed, 156 insertions(+), 99 deletions(-)

So, since you CC-ed me, I guess you're expecting a reply from me. I feel like I'm saying this more often recently (about anything, not just about virtio), but I don't know anything about virtio, so you should take anything from me with a not just a grain but more of a pack of salt.

Quoted 96 lines

hw/block/dataplane/virtio-blk.c +5 -3

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
      event_notifier_test_and_clear(&s->host_notifier);
      blk_io_plug(s->conf->conf.blk);
      for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
          int ret;
            /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
 req->elem.in_num,
 req->elem.index);
  -            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
          }
  -        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
            if (likely(ret == -EAGAIN)) { /* vring emptied */
              /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +31 -6

index 490f961..f522bfc 100644
@@ -22,12 +22,15 @@
  #include "dataplane/virtio-blk.h"
  #include "migration/migration.h"
  #include "block/scsi.h"
+#include "block/block_int.h"
  #ifdef __linux__
  # include <scsi/sg.h>
  #endif
  #include "hw/virtio/virtio-bus.h"
  #include "hw/virtio/virtio-access.h"
  +/* #define DEBUG_MULTIREQ */
+
  VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
  {
      VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

@@ -88,6 +91,11 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
        trace_virtio_blk_rw_complete(req, ret);
  +#ifdef DEBUG_MULTIREQ
+    printf("virtio_blk_rw_complete p %p ret %d\n",
+           req, ret);
+#endif
+
      if (ret) {
          int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
          bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
      virtio_blk_free_request(req);
  }
  -void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
  {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
  -    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
          return;
      }
  -    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
- virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 307

Quoted 10 lines

hw/block/virtio-blk.c +9 -0

+                          virtio_blk_rw_complete, mrb0->reqs[0]);
          }
+        qemu_iovec_destroy(&mrb0->qiov);
+        mrb0->num_reqs = 0;
+        return;
+    }
+
+    mrb = g_malloc(sizeof(MultiReqBuffer));
+    memcpy(mrb, mrb0, sizeof(MultiReqBuffer));
+    mrb0->num_reqs = 0;

I guess you're making mrb0->num_reqs == 1 a special case so you don't have to duplicate mrb0 in that case. But why are you duplicating it at all? And you're just moving over the IO vector without duplicating mrb->qiov.iov, which seems risky to me considering the qemu_iovec_destroy() in virtio_multireq_cb().

I could imagine you're duplicating it so the request can run in the background while you're again filling up the MultiReqBuffer, but that doesn't fit together with not duplicating it in the case of mrb0->num_reqs == 1 which can run in the background just the same.

Quoted 107 lines

hw/block/virtio-blk.c +17 -2

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                      virtio_multireq_cb, mrb);
      }
  -    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
  }
    static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

@@ -283,9 +330,9 @@ static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)
                       BLOCK_ACCT_FLUSH);
        /*
-     * Make sure all outstanding writes are posted to the backing device.
+     * Make sure all outstanding requests are posted to the backing device.
       */
-    virtio_submit_multiwrite(req->dev->blk, mrb);
+    virtio_submit_multireq(req->dev->blk, mrb);
      blk_aio_flush(req->dev->blk, virtio_blk_flush_complete, req);
  }
  @@ -308,61 +355,8 @@ static bool virtio_blk_sect_range_ok(VirtIOBlock *dev,
      return true;

  }
  -static void virtio_blk_handle_write(VirtIOBlockReq *req, MultiReqBuffer *mrb)

hw/block/virtio-blk.c +3 -57

-{
-    BlockRequest *blkreq;
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_write(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_WRITE);
-
-    if (mrb->num_writes == VIRTIO_BLK_MAX_MERGE_REQS) {
-        virtio_submit_multiwrite(req->dev->blk, mrb);
-    }
-
-    blkreq = &mrb->blkreq[mrb->num_writes];
-    blkreq->sector = sector;
-    blkreq->nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
-    blkreq->qiov = &req->qiov;
-    blkreq->cb = virtio_blk_rw_complete;
-    blkreq->opaque = req;
-    blkreq->error = 0;
-
-    mrb->num_writes++;
-}
-
-static void virtio_blk_handle_read(VirtIOBlockReq *req)
-{
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_read(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_READ);
-    blk_aio_readv(req->dev->blk, sector, &req->qiov,
-                  req->qiov.size / BDRV_SECTOR_SIZE,
-                  virtio_blk_rw_complete, req);
-}
-
-void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
+void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb_wr,
+                               MultiReqBuffer *mrb_rd)
  {
      uint32_t type;
      struct iovec *in_iov = req->elem.in_sg;

@@ -397,7 +391,7 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
      type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
        if (type & VIRTIO_BLK_T_FLUSH) {
-        virtio_blk_handle_flush(req, mrb);
+        virtio_blk_handle_flush(req, mrb_wr);
      } else if (type & VIRTIO_BLK_T_SCSI_CMD) {
          virtio_blk_handle_scsi(req);
      } else if (type & VIRTIO_BLK_T_GET_ID) {

@@ -414,13 +408,71 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
          iov_from_buf(in_iov, in_num, 0, serial, size);
          virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
          virtio_blk_free_request(req);
-    } else if (type & VIRTIO_BLK_T_OUT) {
-        qemu_iovec_init_external(&req->qiov, iov, out_num);

hw/block/virtio-blk.c void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb) · line 411

Quoted 6 lines

hw/block/virtio-blk.c +1 -5

-        virtio_blk_handle_write(req, mrb);
-    } else if (type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {
-        /* VIRTIO_BLK_T_IN is 0, so we can't just & it. */
-        qemu_iovec_init_external(&req->qiov, in_iov, in_num);
-        virtio_blk_handle_read(req);
+    } else if (type & VIRTIO_BLK_T_OUT || type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {

Please don't omit the comment about VIRTIO_BLK_T_IN being 0, I spend quite a while thinking about why VIRTIO_BLK_T_BARRIER is seemingly handled just the same as VIRTIO_BLK_T_IN. I still don't know what VIRTIO_BLK_T_BARRIER is supposed to do (and I don't really want to grab the virtio spec right now, although I should do it some time...), but now it at least seems to me like it doesn't really matter.

hw/block/virtio-blk.c void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb) · line 417

Quoted 7 lines

hw/block/virtio-blk.c +6 -0

+        bool is_write = type & VIRTIO_BLK_T_OUT;
+        MultiReqBuffer *mrb = is_write ? mrb_wr : mrb_rd;
+        int64_t sector_num = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
+        BlockDriverState *bs = blk_bs(req->dev->blk);
+        int nb_sectors = 0;
+        int merge = 1;

I shouldn't be criticizing such small issues in an RFC, but it's the most I can do for a virtio-blk series: I'd like this to be a bool instead of an int, please.

Judging from the not-so-quick-in-quantity-but-pretty-brief-in-quality scan through this RFC, it looks OK to me. There are some minor things (e.g. "too" instead of "to" in the commit message) which should be fixed eventually, but logic-wise, I didn't spot anything major.

Perhaps we could put the merge logic into an own function, though, just like bdrv_aio_multiwrite() did with multiwrite_merge().

I trust you that overlapping requests are not an issue, at least when not sorting the requests. Other than that, nothing bad from me. The patch looks good, but I don't want to be too optimistic, as I don't want to definitely judge anything related to virtio-blk. :-)

(Apart from the first two patches of this series, they were completely fine, but I seem to remember having them reviewed once already...)

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

         flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
         flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

         flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
         flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

         flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
         flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

         flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
         flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Signed-off-by Peter Lieven <[email protected]>

---
 hw/block/dataplane/virtio-blk.c |   10 +-
 hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
 include/hw/virtio/virtio-blk.h  |   23 ++--
 3 files changed, 156 insertions(+), 99 deletions(-)

hw/block/dataplane/virtio-blk.c +5 -5

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
     event_notifier_test_and_clear(&s->host_notifier);
     blk_io_plug(s->conf->conf.blk);
     for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
         int ret;
 
         /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
                                                         req->elem.in_num,
                                                         req->elem.index);
 
-            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
         }
 
-        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
 
         if (likely(ret == -EAGAIN)) { /* vring emptied */
             /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +0 -0

index 490f961..f522bfc 100644

hw/block/virtio-blk.c +2 -0

 #ifdef __linux__
 # include <scsi/sg.h>
 #endif
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
 
+/* #define DEBUG_MULTIREQ */
+
 VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
 {
     VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

hw/block/virtio-blk.c +25 -9

     if (ret) {
         int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
         bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
     virtio_blk_free_request(req);
 }
 
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
 {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
 
-    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
         return;
     }
 
-    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
-                virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c +17 -46

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                      virtio_multireq_cb, mrb);
     }
 
-    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
 }
 
 static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

@@ -283,9 +330,9 @@ static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)
                      BLOCK_ACCT_FLUSH);
 
     /*
-     * Make sure all outstanding writes are posted to the backing device.
+     * Make sure all outstanding requests are posted to the backing device.
      */
-    virtio_submit_multiwrite(req->dev->blk, mrb);
+    virtio_submit_multireq(req->dev->blk, mrb);
     blk_aio_flush(req->dev->blk, virtio_blk_flush_complete, req);
 }
 

@@ -308,61 +355,8 @@ static bool virtio_blk_sect_range_ok(VirtIOBlock *dev,
     return true;
 }
 
-static void virtio_blk_handle_write(VirtIOBlockReq *req, MultiReqBuffer *mrb)
-{
-    BlockRequest *blkreq;
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_write(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_WRITE);
-
-    if (mrb->num_writes == VIRTIO_BLK_MAX_MERGE_REQS) {
-        virtio_submit_multiwrite(req->dev->blk, mrb);
-    }
-
-    blkreq = &mrb->blkreq[mrb->num_writes];
-    blkreq->sector = sector;
-    blkreq->nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
-    blkreq->qiov = &req->qiov;
-    blkreq->cb = virtio_blk_rw_complete;
-    blkreq->opaque = req;
-    blkreq->error = 0;
-
-    mrb->num_writes++;
-}
-
-static void virtio_blk_handle_read(VirtIOBlockReq *req)
-{
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_read(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);

hw/block/virtio-blk.c +29 -9

-void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
+void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb_wr,
+                               MultiReqBuffer *mrb_rd)
 {
     uint32_t type;
     struct iovec *in_iov = req->elem.in_sg;

@@ -397,7 +391,7 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
     type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
 
     if (type & VIRTIO_BLK_T_FLUSH) {
-        virtio_blk_handle_flush(req, mrb);
+        virtio_blk_handle_flush(req, mrb_wr);
     } else if (type & VIRTIO_BLK_T_SCSI_CMD) {
         virtio_blk_handle_scsi(req);
     } else if (type & VIRTIO_BLK_T_GET_ID) {

@@ -414,13 +408,71 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
         iov_from_buf(in_iov, in_num, 0, serial, size);
         virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
         virtio_blk_free_request(req);
-    } else if (type & VIRTIO_BLK_T_OUT) {
-        qemu_iovec_init_external(&req->qiov, iov, out_num);
-        virtio_blk_handle_write(req, mrb);
-    } else if (type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {
-        /* VIRTIO_BLK_T_IN is 0, so we can't just & it. */
-        qemu_iovec_init_external(&req->qiov, in_iov, in_num);
-        virtio_blk_handle_read(req);
+    } else if (type & VIRTIO_BLK_T_OUT || type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {
+        bool is_write = type & VIRTIO_BLK_T_OUT;
+        MultiReqBuffer *mrb = is_write ? mrb_wr : mrb_rd;
+        int64_t sector_num = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
+        BlockDriverState *bs = blk_bs(req->dev->blk);
+        int nb_sectors = 0;
+        int merge = 1;
+
+        if (!virtio_blk_sect_range_ok(req->dev, sector_num, req->qiov.size)) {
+            virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
+            virtio_blk_free_request(req);
+            return;
+        }
+
+        if (is_write) {
+            qemu_iovec_init_external(&req->qiov, iov, out_num);
+            trace_virtio_blk_handle_write(req, sector_num, req->qiov.size / BDRV_SECTOR_SIZE);
+        } else {
+            qemu_iovec_init_external(&req->qiov, in_iov, in_num);
+            trace_virtio_blk_handle_read(req, sector_num, req->qiov.size / BDRV_SECTOR_SIZE);
+        }
+
+        nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
+#ifdef DEBUG_MULTIREQ
+        printf("virtio_blk_handle_request p %p sector_num %ld nb_sectors %d is_write %d\n",
+               req, sector_num, nb_sectors, is_write);

hw/block/virtio-blk.c +22 -0

+
+        /* merge would exceed maximum transfer length of backend device */
+        if (bs->bl.max_transfer_length &&
+            mrb->nb_sectors + nb_sectors > bs->bl.max_transfer_length) {
+            merge = 0;
+        }
+
+        /* requests are not sequential */
+        if (mrb->num_reqs && mrb->sector_num + mrb->nb_sectors != sector_num) {
+            merge = 0;
+        }
+
+        if (!merge) {
+            virtio_submit_multireq(req->dev->blk, mrb);
+        }
+
+        if (mrb->num_reqs == 0) {
+            qemu_iovec_init(&mrb->qiov, MAX_MERGE_REQS);
+            mrb->sector_num = sector_num;
+            mrb->nb_sectors = 0;
+        }
+

Am 02.12.2014 um 15:33 hat Peter Lieven geschrieben:

Quoted 100 lines

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

          flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
          flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

          flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
          flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

          flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
          flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

          flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
          flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Signed-off-by Peter Lieven <[email protected]>

---
  hw/block/dataplane/virtio-blk.c |   10 +-
  hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
  include/hw/virtio/virtio-blk.h  |   23 ++--
  3 files changed, 156 insertions(+), 99 deletions(-)

hw/block/dataplane/virtio-blk.c +5 -5

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
      event_notifier_test_and_clear(&s->host_notifier);
      blk_io_plug(s->conf->conf.blk);
      for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
          int ret;
 
          /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
                                                          req->elem.in_num,
                                                          req->elem.index);
 
-            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
          }
 
-        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
 
          if (likely(ret == -EAGAIN)) { /* vring emptied */
              /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +0 -0

index 490f961..f522bfc 100644

hw/block/virtio-blk.c line 25

Quoted 4 lines

hw/block/virtio-blk.c +1 -0

@@ -22,12 +22,15 @@
  #include "dataplane/virtio-blk.h"
  #include "migration/migration.h"
  #include "block/scsi.h"
+#include "block/block_int.h"

No. :-)

We could either expose the information that you need through BlockBackend, or wait until your automatic request splitting logic is implemented in block.c and the information isn't needed here any more.

Quoted 14 lines

hw/block/virtio-blk.c +2 -0

  #ifdef __linux__
  # include <scsi/sg.h>
  #endif
  #include "hw/virtio/virtio-bus.h"
  #include "hw/virtio/virtio-access.h"
 
+/* #define DEBUG_MULTIREQ */
+
  VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
  {
      VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

@@ -88,6 +91,11 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
 

hw/block/virtio-blk.c static void virtio_blk_rw_complete(void *opaque, int ret) · line 97

Quoted 6 lines

hw/block/virtio-blk.c +4 -0

      trace_virtio_blk_rw_complete(req, ret);
 
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_blk_rw_complete p %p ret %d\n",
+           req, ret);
+#endif

In the final version, please use tracepoints instead of printfs.

Quoted 47 lines

hw/block/virtio-blk.c +25 -9

      if (ret) {
          int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
          bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
      virtio_blk_free_request(req);
  }
 
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
  {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
 
-    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
          return;
      }
 
-    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
-                virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 307

Quoted 10 lines

hw/block/virtio-blk.c +9 -0

+                          virtio_blk_rw_complete, mrb0->reqs[0]);
          }
+        qemu_iovec_destroy(&mrb0->qiov);
+        mrb0->num_reqs = 0;
+        return;
+    }
+
+    mrb = g_malloc(sizeof(MultiReqBuffer));
+    memcpy(mrb, mrb0, sizeof(MultiReqBuffer));
+    mrb0->num_reqs = 0;

We probably want to avoid allocations as much as we can. If at all possible, we don't want to memcpy() either.

Most parts of MultiReqBuffer don't seem to be actually needed during the request and in the callback any more. Essentially, all that is needed is the qiov and a way to find all the requests that belong to the same MultiReqBuffer, so that they can all be completed.

We could create a linked list by having a VirtIOBlockReq *next in the request struct. For the qiov we can probably just use the field in the first request (i.e. we extend the qiov of the first request).

With these requirements removed, MultiReqBuffer can live on the stack and doesn't need to be copied to the heap.

With the additional patch that you send me, the next few lines read:

hw/block/virtio-blk.c +6 -0

+    qemu_iovec_init(&mrb->qiov, mrb->num_reqs);
+
+    for (i = 0; i < mrb->num_reqs; i++) {
+        qemu_iovec_concat(&mrb->qiov, &mrb->reqs[i]->qiov, 0, mrb->reqs[i]->qiov.size);
+    }
+    assert(mrb->nb_sectors == mrb->qiov.size / BDRV_SECTOR_SIZE);

The long line is more than 80 characters, but probably more importantly, mrb->num_reqs is a really bad estimation for niov. If the guest uses any vectored I/O, we're sure to get reallocations. What you really want to use here is the old mrb->qiov.niov (which is an abuse to store in the qiov, see below).

(I also wonder whether it would make sense to keep qiovs allocated when putting VirtIOBlockReqs into the pool, but that's not for this series.)

Quoted 85 lines

hw/block/virtio-blk.c +17 -51

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                      virtio_multireq_cb, mrb);
      }
 
-    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
  }
 
  static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

@@ -283,9 +330,9 @@ static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)
                       BLOCK_ACCT_FLUSH);
 
      /*
-     * Make sure all outstanding writes are posted to the backing device.
+     * Make sure all outstanding requests are posted to the backing device.
       */
-    virtio_submit_multiwrite(req->dev->blk, mrb);
+    virtio_submit_multireq(req->dev->blk, mrb);
      blk_aio_flush(req->dev->blk, virtio_blk_flush_complete, req);
  }
 

@@ -308,61 +355,8 @@ static bool virtio_blk_sect_range_ok(VirtIOBlock *dev,
      return true;
  }
 
-static void virtio_blk_handle_write(VirtIOBlockReq *req, MultiReqBuffer *mrb)
-{
-    BlockRequest *blkreq;
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_write(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
-                     BLOCK_ACCT_WRITE);
-
-    if (mrb->num_writes == VIRTIO_BLK_MAX_MERGE_REQS) {
-        virtio_submit_multiwrite(req->dev->blk, mrb);
-    }
-
-    blkreq = &mrb->blkreq[mrb->num_writes];
-    blkreq->sector = sector;
-    blkreq->nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
-    blkreq->qiov = &req->qiov;
-    blkreq->cb = virtio_blk_rw_complete;
-    blkreq->opaque = req;
-    blkreq->error = 0;
-
-    mrb->num_writes++;
-}
-
-static void virtio_blk_handle_read(VirtIOBlockReq *req)
-{
-    uint64_t sector;
-
-    sector = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
-
-    trace_virtio_blk_handle_read(req, sector, req->qiov.size / 512);
-
-    if (!virtio_blk_sect_range_ok(req->dev, sector, req->qiov.size)) {
-        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
-        virtio_blk_free_request(req);
-        return;
-    }
-
-    block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,

hw/block/virtio-blk.c static bool virtio_blk_sect_range_ok(VirtIOBlock *dev, · line 363

Quoted 5 lines

hw/block/virtio-blk.c +0 -5

-                     BLOCK_ACCT_READ);
-    blk_aio_readv(req->dev->blk, sector, &req->qiov,
-                  req->qiov.size / BDRV_SECTOR_SIZE,
-                  virtio_blk_rw_complete, req);
-}

Handling reads and writes in a common place makes sense, but the code is long enough that I would still keep it in a separate function, like virtio_blk_handle_rw().

Quoted 53 lines

hw/block/virtio-blk.c +29 -9

-void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
+void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb_wr,
+                               MultiReqBuffer *mrb_rd)
  {
      uint32_t type;
      struct iovec *in_iov = req->elem.in_sg;

@@ -397,7 +391,7 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
      type = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
 
      if (type & VIRTIO_BLK_T_FLUSH) {
-        virtio_blk_handle_flush(req, mrb);
+        virtio_blk_handle_flush(req, mrb_wr);
      } else if (type & VIRTIO_BLK_T_SCSI_CMD) {
          virtio_blk_handle_scsi(req);
      } else if (type & VIRTIO_BLK_T_GET_ID) {

@@ -414,13 +408,71 @@ void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb)
          iov_from_buf(in_iov, in_num, 0, serial, size);
          virtio_blk_req_complete(req, VIRTIO_BLK_S_OK);
          virtio_blk_free_request(req);
-    } else if (type & VIRTIO_BLK_T_OUT) {
-        qemu_iovec_init_external(&req->qiov, iov, out_num);
-        virtio_blk_handle_write(req, mrb);
-    } else if (type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {
-        /* VIRTIO_BLK_T_IN is 0, so we can't just & it. */
-        qemu_iovec_init_external(&req->qiov, in_iov, in_num);
-        virtio_blk_handle_read(req);
+    } else if (type & VIRTIO_BLK_T_OUT || type == VIRTIO_BLK_T_IN || type == VIRTIO_BLK_T_BARRIER) {
+        bool is_write = type & VIRTIO_BLK_T_OUT;
+        MultiReqBuffer *mrb = is_write ? mrb_wr : mrb_rd;
+        int64_t sector_num = virtio_ldq_p(VIRTIO_DEVICE(req->dev), &req->out.sector);
+        BlockDriverState *bs = blk_bs(req->dev->blk);
+        int nb_sectors = 0;
+        int merge = 1;
+
+        if (!virtio_blk_sect_range_ok(req->dev, sector_num, req->qiov.size)) {
+            virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
+            virtio_blk_free_request(req);
+            return;
+        }
+
+        if (is_write) {
+            qemu_iovec_init_external(&req->qiov, iov, out_num);
+            trace_virtio_blk_handle_write(req, sector_num, req->qiov.size / BDRV_SECTOR_SIZE);
+        } else {
+            qemu_iovec_init_external(&req->qiov, in_iov, in_num);
+            trace_virtio_blk_handle_read(req, sector_num, req->qiov.size / BDRV_SECTOR_SIZE);
+        }
+
+        nb_sectors = req->qiov.size / BDRV_SECTOR_SIZE;
+#ifdef DEBUG_MULTIREQ
+        printf("virtio_blk_handle_request p %p sector_num %ld nb_sectors %d is_write %d\n",
+               req, sector_num, nb_sectors, is_write);

hw/block/virtio-blk.c void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb) · line 446

Quoted 10 lines

hw/block/virtio-blk.c +10 -0

+#endif
+
+        block_acct_start(blk_get_stats(req->dev->blk), &req->acct, req->qiov.size,
+                         is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ);
+
+        /* merge would exceed maximum number of requests or IOVs */
+        if (mrb->num_reqs == MAX_MERGE_REQS ||
+            mrb->qiov.niov + req->qiov.niov + 1 > IOV_MAX) {
+            merge = 0;
+        }

Again with your latest patch applied on top:

You're abusing mrb->qiov.niov while gathering requests. mrb->qiov is never initialised and you don't actually add iovs anywhere, but you just increase qiov.niov without the associated data.

Why don't you make it in simple int instead of QEMUIOVector?

Quoted 26 lines

hw/block/virtio-blk.c +25 -0

+
+        /* merge would exceed maximum transfer length of backend device */
+        if (bs->bl.max_transfer_length &&
+            mrb->nb_sectors + nb_sectors > bs->bl.max_transfer_length) {
+            merge = 0;
+        }
+
+        /* requests are not sequential */
+        if (mrb->num_reqs && mrb->sector_num + mrb->nb_sectors != sector_num) {
+            merge = 0;
+        }
+
+        if (!merge) {
+            virtio_submit_multireq(req->dev->blk, mrb);
+        }
+
+        if (mrb->num_reqs == 0) {
+            qemu_iovec_init(&mrb->qiov, MAX_MERGE_REQS);
+            mrb->sector_num = sector_num;
+            mrb->nb_sectors = 0;
+        }
+
+        qemu_iovec_concat(&mrb->qiov, &req->qiov, 0, req->qiov.size);
+        mrb->nb_sectors += req->qiov.size / BDRV_SECTOR_SIZE;
+

hw/block/virtio-blk.c void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb) · line 478

Quoted 7 lines

hw/block/virtio-blk.c +4 -0

+        assert(mrb->nb_sectors == mrb->qiov.size / BDRV_SECTOR_SIZE);
+
+        mrb->reqs[mrb->num_reqs] = req;
+        mrb->num_reqs++;
      } else {
          virtio_blk_req_complete(req, VIRTIO_BLK_S_UNSUPP);
          virtio_blk_free_request(req);

Otherwise it looks as if it could work.

Quoted 103 lines

Am 02.12.2014 um 15:33 hat Peter Lieven geschrieben:

Quoted 96 lines

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

         flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
         flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

         flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
         flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

         flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
         flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

         flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
         flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Signed-off-by Peter Lieven <[email protected]>

---
 hw/block/dataplane/virtio-blk.c |   10 +-
 hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
 include/hw/virtio/virtio-blk.h  |   23 ++--
 3 files changed, 156 insertions(+), 99 deletions(-)

hw/block/dataplane/virtio-blk.c +5 -5

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
     event_notifier_test_and_clear(&s->host_notifier);
     blk_io_plug(s->conf->conf.blk);
     for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
         int ret;
         /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
                                                         req->elem.in_num,
                                                         req->elem.index);
-            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
         }
-        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
         if (likely(ret == -EAGAIN)) { /* vring emptied */
             /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +0 -0

index 490f961..f522bfc 100644

hw/block/virtio-blk.c line 25

Quoted 4 lines

hw/block/virtio-blk.c +1 -0

@@ -22,12 +22,15 @@
 #include "dataplane/virtio-blk.h"
 #include "migration/migration.h"
 #include "block/scsi.h"
+#include "block/block_int.h"

No. :-)

Quoted 3 lines

We could either expose the information that you need through BlockBackend, or wait until your automatic request splitting logic is implemented in block.c and the information isn't needed here any more.

I will add sth to block.c and follow-up with the request splitting logic later. It will still make sense to now the limit here. Otherwise we merge something that we split again afterwards.

Quoted 10 lines

Quoted 10 lines

hw/block/virtio-blk.c +2 -0

 #ifdef __linux__
 # include <scsi/sg.h>
 #endif
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-access.h"
+/* #define DEBUG_MULTIREQ */
+
 VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
 {

Quoted 8 lines

hw/block/virtio-blk.c static void virtio_blk_rw_complete(void *opaque, int ret) · line 95

Quoted 7 lines

hw/block/virtio-blk.c +4 -0

     VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

@@ -88,6 +91,11 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
     trace_virtio_blk_rw_complete(req, ret);
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_blk_rw_complete p %p ret %d\n",
+           req, ret);
+#endif

In the final version, please use tracepoints instead of printfs.

of course. This is just my debug code.

Quoted 57 lines

Quoted 44 lines

hw/block/virtio-blk.c +25 -9

     if (ret) {
         int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
         bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
     virtio_blk_free_request(req);
 }
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
 {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
-    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
         return;
     }
-    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
-                virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 304

Quoted 10 lines

hw/block/virtio-blk.c +9 -0

+                          virtio_blk_rw_complete, mrb0->reqs[0]);
         }
+        qemu_iovec_destroy(&mrb0->qiov);
+        mrb0->num_reqs = 0;
+        return;
+    }
+
+    mrb = g_malloc(sizeof(MultiReqBuffer));
+    memcpy(mrb, mrb0, sizeof(MultiReqBuffer));
+    mrb0->num_reqs = 0;

We probably want to avoid allocations as much as we can. If at all possible, we don't want to memcpy() either.

Quoted 8 lines

Most parts of MultiReqBuffer don't seem to be actually needed during the request and in the callback any more. Essentially, all that is needed is the qiov and a way to find all the requests that belong to the same MultiReqBuffer, so that they can all be completed.

We could create a linked list by having a VirtIOBlockReq *next in the request struct. For the qiov we can probably just use the field in the first request (i.e. we extend the qiov of the first request).

Thats a good idea and should work. I will look into this. But where would you store the merge qiov?

It looks like I would need a small struct which holds the qiov and the pointer to req[0]. This can then be used as opaque value to the callback.

Quoted 13 lines

With these requirements removed, MultiReqBuffer can live on the stack and doesn't need to be copied to the heap.

With the additional patch that you send me, the next few lines read:

hw/block/virtio-blk.c +6 -0

+    qemu_iovec_init(&mrb->qiov, mrb->num_reqs);
+
+    for (i = 0; i < mrb->num_reqs; i++) {
+        qemu_iovec_concat(&mrb->qiov, &mrb->reqs[i]->qiov, 0, mrb->reqs[i]->qiov.size);
+    }
+    assert(mrb->nb_sectors == mrb->qiov.size / BDRV_SECTOR_SIZE);

Quoted 5 lines

The long line is more than 80 characters, but probably more importantly, mrb->num_reqs is a really bad estimation for niov. If the guest uses any vectored I/O, we're sure to get reallocations. What you really want to use here is the old mrb->qiov.niov (which is an abuse to store in the qiov, see below).

Of course, my fault. I was so happy to have an exact value for the number of niov that I put in the wrong variable ;-)

Quoted 14 lines

(I also wonder whether it would make sense to keep qiovs allocated when putting VirtIOBlockReqs into the pool, but that's not for this series.)

Quoted 11 lines

hw/block/virtio-blk.c +11 -0

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,

Quoted 8 lines

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 327

Quoted 8 lines

hw/block/virtio-blk.c +4 -1

+                      virtio_multireq_cb, mrb);
     }
-    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
 }
 static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

Am 04.12.2014 um 15:42 hat Peter Lieven geschrieben:

Quoted 104 lines · Kevin Wolf

Quoted 103 lines

Am 02.12.2014 um 15:33 hat Peter Lieven geschrieben:

Quoted 96 lines

this patch finally introduce multiread support to virtio-blk while multiwrite support was there for a long time read support was missing.

To achieve this the patch does serveral things which might need futher explaination:

- the whole merge and multireq logic is moved from block.c into virtio-blk. This is move is a preparation for directly creating a coroutine out of virtio-blk.

- requests are only merged if they are strictly sequential and no longer sorted. This simplification decreases overhead and reduces latency. It will also merge some requests which were unmergable before.

The old algorithm took up to 32 requests sorted them and tried to merge them. The outcome was anything between 1 and 32 requests. In case of 32 requests there were 31 requests unnecessarily delayed.

On the other hand lets imagine e.g. 16 unmergeable requests followed by 32 mergable requests. The latter 32 requests would have been split into two 16 byte requests.

Last the simplified logic allows for a fast path if we have only a single request in the multirequest. In this case the request is sent as ordinary request without mulltireq callbacks.

As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of merged requests is in the same order while the latency is slightly decreased. One should not stick to much to the numbers because the number of wr_requests are highly fluctuant. I hope the numbers show that this patch is at least not causing to big harm:

Cmdline:
qemu-system-x86_64 -m 1024 -smp 2 -enable-kvm -cdrom ubuntu-14.04.1-server-amd64.iso \
-drive if=virtio,file=/tmp/ubuntu.raw -monitor stdio

Before:
virtio0: rd_bytes=150979072 wr_bytes=2591138816 rd_operations=18475 wr_operations=69216

          flush_operations=15343 wr_total_time_ns=26969283701 rd_total_time_ns=1018449432
          flush_total_time_ns=562596819 rd_merged=0 wr_merged=10453

After:
virtio0: rd_bytes=148112896 wr_bytes=2845834240 rd_operations=17535 wr_operations=72197

          flush_operations=15760 wr_total_time_ns=26104971623 rd_total_time_ns=1012926283
          flush_total_time_ns=564414752 rd_merged=517 wr_merged=9859

Some first numbers of improved read performance while booting:

The Ubuntu 14.04.1 vServer from above:
virtio0: rd_bytes=86158336 wr_bytes=688128 rd_operations=5851 wr_operations=70

          flush_operations=4 wr_total_time_ns=10886705 rd_total_time_ns=416688595
          flush_total_time_ns=288776 rd_merged=1297 wr_merged=2

Windows 2012R2:
virtio0: rd_bytes=176559104 wr_bytes=61859840 rd_operations=7200 wr_operations=360

          flush_operations=68 wr_total_time_ns=34344992718 rd_total_time_ns=134386844669
          flush_total_time_ns=18115517 rd_merged=641 wr_merged=216

Signed-off-by Peter Lieven <[email protected]>

---
  hw/block/dataplane/virtio-blk.c |   10 +-
  hw/block/virtio-blk.c           |  222 ++++++++++++++++++++++++---------------
  include/hw/virtio/virtio-blk.h  |   23 ++--
  3 files changed, 156 insertions(+), 99 deletions(-)

hw/block/dataplane/virtio-blk.c +5 -5

index 1222a37..aa4ad91 100644
@@ -96,9 +96,8 @@ static void handle_notify(EventNotifier *e)
      event_notifier_test_and_clear(&s->host_notifier);
      blk_io_plug(s->conf->conf.blk);
      for (;;) {
-        MultiReqBuffer mrb = {
-            .num_writes = 0,
-        };
+        MultiReqBuffer mrb_rd = {};
+        MultiReqBuffer mrb_wr = {.is_write = 1};
          int ret;
          /* Disable guest->host notifies to avoid unnecessary vmexits */

@@ -117,10 +116,11 @@ static void handle_notify(EventNotifier *e)
                                                          req->elem.in_num,
                                                          req->elem.index);
-            virtio_blk_handle_request(req, &mrb);
+            virtio_blk_handle_request(req, &mrb_wr, &mrb_rd);
          }
-        virtio_submit_multiwrite(s->conf->conf.blk, &mrb);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_wr);
+        virtio_submit_multireq(s->conf->conf.blk, &mrb_rd);
          if (likely(ret == -EAGAIN)) { /* vring emptied */
              /* Re-enable guest->host notifies and stop processing the vring.

hw/block/virtio-blk.c +0 -0

index 490f961..f522bfc 100644

hw/block/virtio-blk.c line 25

Quoted 4 lines

hw/block/virtio-blk.c +1 -0

@@ -22,12 +22,15 @@
  #include "dataplane/virtio-blk.h"
  #include "migration/migration.h"
  #include "block/scsi.h"
+#include "block/block_int.h"

No. :-)

Quoted 6 lines

Quoted 3 lines

We could either expose the information that you need through BlockBackend, or wait until your automatic request splitting logic is implemented in block.c and the information isn't needed here any more.

I will add sth to block.c and follow-up with the request splitting logic later. It will still make sense to now the limit here. Otherwise we merge something that we split again afterwards.

Yup, but then make it an official block layer interface instead of including block_int.h in the device emulation.

Quoted 80 lines

Quoted 9 lines

Quoted 9 lines

hw/block/virtio-blk.c +2 -0

  #ifdef __linux__
  # include <scsi/sg.h>
  #endif
  #include "hw/virtio/virtio-bus.h"
  #include "hw/virtio/virtio-access.h"
+/* #define DEBUG_MULTIREQ */
+
  VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s)
  {

Quoted 8 lines

hw/block/virtio-blk.c static void virtio_blk_rw_complete(void *opaque, int ret) · line 95

Quoted 7 lines

hw/block/virtio-blk.c +4 -0

      VirtIOBlockReq *req = g_slice_new(VirtIOBlockReq);

@@ -88,6 +91,11 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
      trace_virtio_blk_rw_complete(req, ret);
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_blk_rw_complete p %p ret %d\n",
+           req, ret);
+#endif

In the final version, please use tracepoints instead of printfs.

of course. This is just my debug code.

Quoted 61 lines

Quoted 43 lines

hw/block/virtio-blk.c +25 -9

      if (ret) {
          int p = virtio_ldl_p(VIRTIO_DEVICE(req->dev), &req->out.type);
          bool is_read = !(p & VIRTIO_BLK_T_OUT);

@@ -257,24 +265,63 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
      virtio_blk_free_request(req);
  }
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb)
+static void virtio_multireq_cb(void *opaque, int ret)
+{
+    MultiReqBuffer *mrb = opaque;
+    int i;
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_multireq_cb: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+    for (i = 0; i < mrb->num_reqs; i++) {
+        virtio_blk_rw_complete(mrb->reqs[i], ret);
+    }
+
+    qemu_iovec_destroy(&mrb->qiov);
+    g_free(mrb);
+}
+
+void virtio_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb0)
  {
-    int i, ret;
+    MultiReqBuffer *mrb = NULL;
-    if (!mrb->num_writes) {
+    if (!mrb0->num_reqs) {
          return;
      }
-    ret = blk_aio_multiwrite(blk, mrb->blkreq, mrb->num_writes);
-    if (ret != 0) {
-        for (i = 0; i < mrb->num_writes; i++) {
-            if (mrb->blkreq[i].error) {
-                virtio_blk_rw_complete(mrb->blkreq[i].opaque, -EIO);
-            }
+    if (mrb0->num_reqs == 1) {
+        if (mrb0->is_write) {
+            blk_aio_writev(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,
+                           virtio_blk_rw_complete, mrb0->reqs[0]);
+        } else {
+            blk_aio_readv(blk, mrb0->sector_num, &mrb0->reqs[0]->qiov, mrb0->nb_sectors,

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 304

Quoted 10 lines

hw/block/virtio-blk.c +9 -0

+                          virtio_blk_rw_complete, mrb0->reqs[0]);
          }
+        qemu_iovec_destroy(&mrb0->qiov);
+        mrb0->num_reqs = 0;
+        return;
+    }
+
+    mrb = g_malloc(sizeof(MultiReqBuffer));
+    memcpy(mrb, mrb0, sizeof(MultiReqBuffer));
+    mrb0->num_reqs = 0;

We probably want to avoid allocations as much as we can. If at all possible, we don't want to memcpy() either.

Most parts of MultiReqBuffer don't seem to be actually needed during the request and in the callback any more. Essentially, all that is needed is the qiov and a way to find all the requests that belong to the same MultiReqBuffer, so that they can all be completed.

Quoted 8 lines

Quoted 3 lines

We could create a linked list by having a VirtIOBlockReq *next in the request struct. For the qiov we can probably just use the field in the first request (i.e. we extend the qiov of the first request).

Thats a good idea and should work. I will look into this. But where would you store the merge qiov?

It looks like I would need a small struct which holds the qiov and the pointer to req[0]. This can then be used as opaque value to the callback.

My thought was that you replace the original qiov of the first request with the merged one. I don't think you need the original one any more after merging the requests. Alternatively, you could use a separate field in VirtIOBlockReq which stays unused in all but the first request.

In any case the goal is to get rid of a dynamically allocated struct, no matter how small, so opaque must be directly pointing to a request in the end.

Quoted 14 lines

Quoted 14 lines

With these requirements removed, MultiReqBuffer can live on the stack and doesn't need to be copied to the heap.

With the additional patch that you send me, the next few lines read:

hw/block/virtio-blk.c +6 -0

+    qemu_iovec_init(&mrb->qiov, mrb->num_reqs);
+
+    for (i = 0; i < mrb->num_reqs; i++) {
+        qemu_iovec_concat(&mrb->qiov, &mrb->reqs[i]->qiov, 0, mrb->reqs[i]->qiov.size);
+    }
+    assert(mrb->nb_sectors == mrb->qiov.size / BDRV_SECTOR_SIZE);

Quoted 7 lines

Quoted 5 lines

The long line is more than 80 characters, but probably more importantly, mrb->num_reqs is a really bad estimation for niov. If the guest uses any vectored I/O, we're sure to get reallocations. What you really want to use here is the old mrb->qiov.niov (which is an abuse to store in the qiov, see below).

Of course, my fault. I was so happy to have an exact value for the number of niov that I put in the wrong variable ;-)

Happens. :-)

Quoted 14 lines

Quoted 14 lines

(I also wonder whether it would make sense to keep qiovs allocated when putting VirtIOBlockReqs into the pool, but that's not for this series.)

Quoted 11 lines

hw/block/virtio-blk.c +11 -0

+
+#ifdef DEBUG_MULTIREQ
+    printf("virtio_submit_multireq: p %p sector_num %ld nb_sectors %d is_write %d num_reqs %d\n",
+           mrb, mrb->sector_num, mrb->nb_sectors, mrb->is_write, mrb->num_reqs);
+#endif
+
+    if (mrb->is_write) {
+        blk_aio_writev(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,
+                       virtio_multireq_cb, mrb);
+    } else {
+        blk_aio_readv(blk, mrb->sector_num, &mrb->qiov, mrb->nb_sectors,

Quoted 8 lines

Quoted 8 lines

hw/block/virtio-blk.c static void virtio_blk_handle_scsi(VirtIOBlockReq *req) · line 327

Quoted 8 lines

hw/block/virtio-blk.c +4 -1

+                      virtio_multireq_cb, mrb);
      }
-    mrb->num_writes = 0;
+    block_acct_merge_done(blk_get_stats(blk),
+                          mrb->is_write ? BLOCK_ACCT_WRITE : BLOCK_ACCT_READ,
+                          mrb->num_reqs - 1);
  }
  static void virtio_blk_handle_flush(VirtIOBlockReq *req, MultiReqBuffer *mrb)

Kevin