Clean unused entries in the qcow2 L2/refcount cache

After having emptied the cache, the data in the cache tables is no longer useful, so we can tell the kernel that we are done with it. In Linux this frees the resources associated with it.

The effect of this can be seen in the block-commit operation: it moves data from the top to the base image (and fills both caches), then it empties the top image. At this point the data in that cache is no longer needed so it's just wasting memory.

Signed-off-by Alberto Garcia <[email protected]>

---
  block/qcow2-cache.c | 20 ++++++++++++++++++++
  1 file changed, 20 insertions(+)

Quoted 3 lines

After having emptied the cache, the data in the cache tables is no longer useful, so we can tell the kernel that we are done with it. In Linux this frees the resources associated with it.

Looks good, but by applying the same logic, you could do the same call in qcow2_cache_create(). So what about it? :-)

Also note that bdrv_commit() is used only by the HMP commit operation, not by QMP commit.

Quoted 5 lines · Max Reitz

Quoted 3 lines

After having emptied the cache, the data in the cache tables is no longer useful, so we can tell the kernel that we are done with it. In Linux this frees the resources associated with it.

Looks good, but by applying the same logic, you could do the same call in qcow2_cache_create(). So what about it? :-)

But after qcow2_cache_create() the memory is still unused so this doesn't have any affect.

Quoted 2 lines

Also note that bdrv_commit() is used only by the HMP commit operation, not by QMP commit.

Thanks, I should probably clarify that in the commit message.

Berto

This adds a new 'cache-clean-interval' option that cleans all qcow2 cache entries that haven't been used in a certain interval, given in seconds.

This allows setting a large L2 cache size so it can handle scenarios with lots of I/O and at the same time use little memory during periods of inactivity.

This feature currently relies on MADV_DONTNEED to free that memory, so it is not useful in systems that don't follow that behavior.

Signed-off-by Alberto Garcia <[email protected]>

---
  block/qcow2-cache.c  | 35 ++++++++++++++++++++++++++++++++
  block/qcow2.c        | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++
  block/qcow2.h        |  4 ++++
  qapi/block-core.json |  6 +++++-
  4 files changed, 100 insertions(+), 1 deletion(-)

block/qcow2-cache.c +35 -0

index ed14a92..a215f5b 100644
@@ -43,6 +43,7 @@ struct Qcow2Cache {
      bool                    depends_on_flush;
      void                   *table_array;
      uint64_t                lru_counter;
+    uint64_t                cache_clean_lru_counter;
  };
 
  static inline void *qcow2_cache_get_table_addr(BlockDriverState *bs,

@@ -78,6 +79,40 @@ static void qcow2_cache_table_release(BlockDriverState *bs, Qcow2Cache *c,
  #endif
  }
 
+static inline bool can_clean_entry(Qcow2Cache *c, int i)
+{
+    Qcow2CachedTable *t = &c->entries[i];
+    return t->ref == 0 && !t->dirty && t->offset != 0 &&
+        t->lru_counter <= c->cache_clean_lru_counter;
+}
+
+void qcow2_cache_clean_unused(BlockDriverState *bs, Qcow2Cache *c)
+{
+    int i = 0;
+    while (i < c->size) {
+        int to_clean = 0;
+
+        /* Skip the entries that we don't need to clean */
+        while (i < c->size && !can_clean_entry(c, i)) {
+            i++;
+        }
+
+        /* And count how many we can clean in a row */
+        while (i < c->size && can_clean_entry(c, i)) {
+            c->entries[i].offset = 0;
+            c->entries[i].lru_counter = 0;
+            i++;
+            to_clean++;
+        }
+
+        if (to_clean > 0) {
+            qcow2_cache_table_release(bs, c, i - to_clean, to_clean);
+        }
+    }
+
+    c->cache_clean_lru_counter = c->lru_counter;
+}
+
  Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
  {
      BDRVQcowState *s = bs->opaque;

block/qcow2.c +14 -0

index b9a72e3..f6ed39f 100644
@@ -468,6 +468,11 @@ static QemuOptsList qcow2_runtime_opts = {
              .type = QEMU_OPT_SIZE,
              .help = "Maximum refcount block cache size",
          },
+        {
+            .name = QCOW2_OPT_CACHE_CLEAN_INTERVAL,
+            .type = QEMU_OPT_NUMBER,
+            .help = "Clean unused cache entries after this time (in seconds)",
+        },
          { /* end of list */ }
      },
  };

@@ -483,6 +488,49 @@ static const char *overlap_bool_option_names[QCOW2_OL_MAX_BITNR] = {
      [QCOW2_OL_INACTIVE_L2_BITNR]    = QCOW2_OPT_OVERLAP_INACTIVE_L2,
  };
 
+static void cache_clean_timer_cb(void *opaque)
+{
+    BlockDriverState *bs = opaque;
+    BDRVQcowState *s = bs->opaque;
+    qcow2_cache_clean_unused(bs, s->l2_table_cache);
+    qcow2_cache_clean_unused(bs, s->refcount_block_cache);
+    timer_mod(s->cache_clean_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+              s->cache_clean_interval * 1000);
+}

block/qcow2.c +24 -0

+    }
+}
+
+static void cache_clean_timer_del(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    if (s->cache_clean_timer) {
+        timer_del(s->cache_clean_timer);
+        timer_free(s->cache_clean_timer);
+        s->cache_clean_timer = NULL;
+    }
+}
+
+static void qcow2_detach_aio_context(BlockDriverState *bs)
+{
+    cache_clean_timer_del(bs);
+}
+
+static void qcow2_attach_aio_context(BlockDriverState *bs,
+                                     AioContext *new_context)
+{
+    cache_clean_timer_init(bs, new_context);
+}
+
  static void read_cache_sizes(QemuOpts *opts, uint64_t *l2_cache_size,
                               uint64_t *refcount_cache_size, Error **errp)
  {

block/qcow2.c +6 -0

+    cache_clean_timer_init(bs, bdrv_get_aio_context(bs));
 
      s->cluster_cache = g_malloc(s->cluster_size);
      /* one more sector for decompressed data alignment */

@@ -1004,6 +1055,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
      qemu_vfree(s->l1_table);
      /* else pre-write overlap checks in cache_destroy may crash */
      s->l1_table = NULL;
+    cache_clean_timer_del(bs);
      if (s->l2_table_cache) {
          qcow2_cache_destroy(bs, s->l2_table_cache);
      }

@@ -1453,6 +1505,7 @@ static void qcow2_close(BlockDriverState *bs)
          }
      }
 
+    cache_clean_timer_del(bs);
      qcow2_cache_destroy(bs, s->l2_table_cache);
      qcow2_cache_destroy(bs, s->refcount_block_cache);
 

@@ -2964,6 +3017,9 @@ BlockDriver bdrv_qcow2 = {
      .create_opts         = &qcow2_create_opts,
      .bdrv_check          = qcow2_check,
      .bdrv_amend_options  = qcow2_amend_options,
+
+    .bdrv_detach_aio_context  = qcow2_detach_aio_context,
+    .bdrv_attach_aio_context  = qcow2_attach_aio_context,
  };
 
  static void bdrv_qcow2_init(void)

block/qcow2.h +4 -0

index 0076512..2c45eb2 100644
@@ -93,6 +93,7 @@
  #define QCOW2_OPT_CACHE_SIZE "cache-size"
  #define QCOW2_OPT_L2_CACHE_SIZE "l2-cache-size"
  #define QCOW2_OPT_REFCOUNT_CACHE_SIZE "refcount-cache-size"
+#define QCOW2_OPT_CACHE_CLEAN_INTERVAL "cache-clean-interval"
 
  typedef struct QCowHeader {
      uint32_t magic;

@@ -236,6 +237,8 @@ typedef struct BDRVQcowState {
 
      Qcow2Cache* l2_table_cache;
      Qcow2Cache* refcount_block_cache;
+    QEMUTimer *cache_clean_timer;
+    unsigned cache_clean_interval;
 
      uint8_t *cluster_cache;
      uint8_t *cluster_data;

@@ -581,6 +584,7 @@ int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
      Qcow2Cache *dependency);
  void qcow2_cache_depends_on_flush(Qcow2Cache *c);
 
+void qcow2_cache_clean_unused(BlockDriverState *bs, Qcow2Cache *c);
  int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c);
 
  int qcow2_cache_get(BlockDriverState *bs, Qcow2Cache *c, uint64_t offset,

qapi/block-core.json +5 -1

index 863ffea..f42338d 100644
@@ -1538,6 +1538,9 @@
  # @refcount-cache-size:   #optional the maximum size of the refcount block cache
  #                         in bytes (since 2.2)
  #
+# @cache-clean-interval:  #optional clean unused entries in the L2 and refcount
+#                         caches. The interval is in seconds (since 2.4)
+#
  # Since: 1.7
  ##
  { 'struct': 'BlockdevOptionsQcow2',

@@ -1549,7 +1552,8 @@
              '*overlap-check': 'Qcow2OverlapChecks',
              '*cache-size': 'int',
              '*l2-cache-size': 'int',
-            '*refcount-cache-size': 'int' } }
+            '*refcount-cache-size': 'int',
+            '*cache-clean-interval': 'int' } }
 
 
  ##

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Signed-off-by Alberto Garcia <[email protected]>

---
  block/qcow2-cache.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

block/qcow2-cache.c +0 -0

index a215f5b..43590ff 100644

Quoted 1 line · Alberto Garcia

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Hm, not seven?

Quoted 5 lines

Signed-off-by Alberto Garcia <[email protected]>

---
  block/qcow2-cache.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

block/qcow2-cache.c line 37

Quoted 13 lines

block/qcow2-cache.c +1 -1

index a215f5b..43590ff 100644
@@ -31,9 +31,9 @@
    typedef struct Qcow2CachedTable {
      int64_t  offset;
-    bool     dirty;
      uint64_t lru_counter;
      int      ref;
+    bool     dirty;
  } Qcow2CachedTable;
    struct Qcow2Cache {

With "7" above, or an explanation why it actually is 8:

Reviewed-by Max Reitz <[email protected]>

Quoted 9 lines · Max Reitz

Quoted 1 line · Alberto Garcia

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Hm, not seven?

Quoted 5 lines

Signed-off-by Alberto Garcia <[email protected]>

---
   block/qcow2-cache.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

Quoted 14 lines

block/qcow2-cache.c line 37

Quoted 13 lines

block/qcow2-cache.c +1 -1

index a215f5b..43590ff 100644
@@ -31,9 +31,9 @@
     typedef struct Qcow2CachedTable {
       int64_t  offset;
-    bool     dirty;
       uint64_t lru_counter;
       int      ref;
+    bool     dirty;
   } Qcow2CachedTable;
     struct Qcow2Cache {

With "7" above, or an explanation why it actually is 8:

Old layout:

0-7   offset
8     dirty
9-15  padding

16-23 lru_counter
24-27 dirty
28-31 padding

New layout:

0-7 offset
8-15 lru_counter
16-19 ref
20 dirty
21-23 padding

It indeed saves 8 bytes.

Quoted 3 lines

Reviewed-by Max Reitz <[email protected]>

Quoted 1 line · Alberto Garcia

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Hm, not seven?

block/qcow2-cache.c line 37

Quoted 7 lines

  typedef struct Qcow2CachedTable {
      int64_t  offset;

block/qcow2-cache.c +1 -1

-    bool     dirty;
      uint64_t lru_counter;
      int      ref;
+    bool     dirty;
  } Qcow2CachedTable;

Changing the current ordering saves 8 bytes per cache entry in x86_64.

Signed-off-by Alberto Garcia <[email protected]>

---
 block/qcow2-cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)