Commit ac9fafa1 authored by Alan D. Brunelle's avatar Alan D. Brunelle Committed by Jens Axboe
Browse files

block: 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.

Signed-off-by: default avatarAlan D. Brunelle <alan.brunelle@hp.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent d7e3c324

block/blk-sysfs.c

+26 −0
Original line number Diff line number Diff line
@@ -135,6 +135,25 @@ 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)

{

	return queue_var_show(blk_queue_nomerges(q), 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 +189,12 @@ 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 +202,7 @@ 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
Original line number Diff line number Diff line
@@ -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
Original line number Diff line number Diff line
@@ -408,6 +408,7 @@ 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 */



static inline void queue_flag_set_unlocked(unsigned int flag,

					   struct request_queue *q)
@@ -476,6 +477,7 @@ 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)

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