Skip I/O merges when disabled
PATCH 1/1 Skip I/O merges when disabled
The block I/O + elevator + I/O scheduler code spend a lot of time trying to merge I/Os -- rightfully so under "normal" circumstances. However, if one were to know that the incoming I/O stream was /very/ random in nature, the cycles are wasted.
This patch adds a per-request_queue tunable that (when set) disables merge attempts (beyond the simple one-hit cache check), thus freeing up a non-trivial amount of CPU cycles.
--- block/blk-sysfs.c | 27 +++++++++++++++++++++++++++ block/elevator.c | 3 +++ include/linux/blkdev.h | 2 ++ 3 files changed, 32 insertions(+), 0 deletions(-)
block/blk-sysfs.c +27 -0
@@ -135,6 +135,26 @@ static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)return queue_var_show (max_hw_sectors_kb, (page)); } +static ssize_t queue_nomerges_show (struct request_queue *q,char *page) +{ +int nm =test_bit (QUEUE_FLAG_NOMERGES , &q->queue_flags); +return queue_var_show (nm, page); +} + +static ssize_t queue_nomerges_store (struct request_queue *q,const char *page, +size_t count) +{ +unsigned long nm; +ssize_t ret =queue_var_store (&nm, page, count); + +if (nm) +set_bit (QUEUE_FLAG_NOMERGES , &q->queue_flags); +else +clear_bit (QUEUE_FLAG_NOMERGES , &q->queue_flags); + +return ret; +} +static struct queue_sysfs_entry queue_requests_entry = { .attr = {.name ="nr_requests" , .mode =S_IRUGO |S_IWUSR },
@@ -170,6 +190,12 @@ static struct queue_sysfs_entry queue_hw_sector_size_entry = { .show = queue_hw_sector_size_show, }; +static struct queue_sysfs_entry queue_nomerges_entry = { + .attr = {.name ="nomerges" , .mode =S_IRUGO |S_IWUSR }, + .show = queue_nomerges_show, + .store = queue_nomerges_store, +}; +static struct attribute *default_attrs[] = { &queue_requests_entry.attr, &queue_ra_entry.attr,
@@ -177,6 +203,7 @@ static struct attribute *default_attrs[] = { &queue_max_sectors_entry.attr, &queue_iosched_entry.attr, &queue_hw_sector_size_entry.attr, + &queue_nomerges_entry.attr, NULL, };
block/elevator.c +3 -0
@@ -488,6 +488,9 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio) } } +if (blk_queue_nomerges (q)) +return ELEVATOR_NO_MERGE ; +/* * See if our hash lookup can find a potential backmerge. */
include/linux/blkdev.h +2 -0
@@ -407,6 +407,7 @@ struct request_queue#define QUEUE_FLAG_PLUGGED 7/* queue is plugged */ #define QUEUE_FLAG_ELVSWITCH 8/* don't use elevator, just do FIFO */ #define QUEUE_FLAG_BIDI 9/* queue supports bidi requests */ +#define QUEUE_FLAG_NOMERGES 10/* disable merge attempts */ enum {/*
@@ -451,6 +452,7 @@ enum {#define blk_queue_plugged (q) test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)#define blk_queue_tagged (q) test_bit(QUEUE_FLAG_QUEUED, &(q)->queue_flags)#define blk_queue_stopped (q) test_bit(QUEUE_FLAG_STOPPED, &(q)->queue_flags) +#define blk_queue_nomerges (q) test_bit(QUEUE_FLAG_NOMERGES, &(q)->queue_flags)#define blk_queue_flushing (q) ((q)->ordseq)#define blk_fs_request (rq) ((rq)->cmd_type == REQ_TYPE_FS)
Signature · 1 line
1.5.4.3
Quoted 9 lines · Alan D. Brunelle
The block I/O + elevator + I/O scheduler code spend a lot of time trying to merge I/Os -- rightfully so under "normal" circumstances. However, if one were to know that the incoming I/O stream was /very/ random in nature, the cycles are wasted.
This patch adds a per-request_queue tunable that (when set) disables merge attempts (beyond the simple one-hit cache check), thus freeing up a non-trivial amount of CPU cycles.
Thanks Alan, applied. Made one little change (since I had to hand apply the patch anyway):
block/blk-sysfs.c static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page) · line 142
Quoted 5 lines
block/blk-sysfs.c +5
-0+static ssize_t queue_nomerges_show (struct request_queue *q,char *page) +{ +int nm =test_bit (QUEUE_FLAG_NOMERGES , &q->queue_flags); +return queue_var_show (nm, page); +}
static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
{
return queue_var_show(blk_queue_nomerges(q), page);
}Signature · 1 line
Jens Axboe