Commit 488991e2 authored by Alan D. Brunelle's avatar Alan D. Brunelle Committed by Jens Axboe
Browse files

block: Added in stricter no merge semantics for block I/O 



Updated 'nomerges' tunable to accept a value of '2' - indicating that _no_
merges at all are to be attempted (not even the simple one-hit cache).

The following table illustrates the additional benefit - 5 minute runs of
a random I/O load were applied to a dozen devices on a 16-way x86_64 system.

nomerges        Throughput      %System         Improvement (tput / %sys)
--------        ------------    -----------     -------------------------
0               12.45 MB/sec    0.669365609
1               12.50 MB/sec    0.641519199     0.40% / 2.71%
2               12.52 MB/sec    0.639849750     0.56% / 2.96%

Signed-off-by: default avatarAlan D. Brunelle <alan.brunelle@hp.com>
Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
parent 47483e25

Documentation/ABI/testing/sysfs-block

+14 −0
Original line number Diff line number Diff line
@@ -128,3 +128,17 @@ Description: 
		preferred request size for workloads where sustained

		throughput is desired.  If no optimal I/O size is

		reported this file contains 0.



What:		/sys/block/<disk>/queue/nomerges

Date:		January 2010

Contact:

Description:

		Standard I/O elevator operations include attempts to

		merge contiguous I/Os. For known random I/O loads these

		attempts will always fail and result in extra cycles

		being spent in the kernel. This allows one to turn off

		this behavior on one of two ways: When set to 1, complex

		merge checks are disabled, but the simple one-shot merges

		with the previous I/O request are enabled. When set to 2,

		all merge tries are disabled. The default value is 0 -

		which enables all types of merge tries.

Documentation/block/queue-sysfs.txt

+5 −5
Original line number Diff line number Diff line
@@ -25,11 +25,11 @@ size allowed by the hardware. 


nomerges (RW)

-------------

This enables the user to disable the lookup logic involved with IO merging

requests in the block layer. Merging may still occur through a direct

1-hit cache, since that comes for (almost) free. The IO scheduler will not

waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults

to 0, enabling all merges.

This enables the user to disable the lookup logic involved with IO

merging requests in the block layer. By default (0) all merges are

enabled. When set to 1 only simple one-hit merges will be tried. When

set to 2 no merge algorithms will be tried (including one-hit or more

complex tree/hash lookups).



nr_requests (RW)

----------------

block/blk-sysfs.c

+7 −4
Original line number Diff line number Diff line
@@ -189,7 +189,8 @@ static ssize_t queue_nonrot_store(struct request_queue *q, const char *page, 


static ssize_t queue_nomerges_show(struct request_queue *q, char *page)

{

	return queue_var_show(blk_queue_nomerges(q), page);

	return queue_var_show((blk_queue_nomerges(q) << 1) |

			       blk_queue_noxmerges(q), page);

}



static ssize_t queue_nomerges_store(struct request_queue *q, const char *page,
@@ -199,10 +200,12 @@ static ssize_t queue_nomerges_store(struct request_queue *q, const char *page, 
	ssize_t ret = queue_var_store(&nm, page, count);



	spin_lock_irq(q->queue_lock);

	if (nm)

		queue_flag_set(QUEUE_FLAG_NOMERGES, q);

	else

	queue_flag_clear(QUEUE_FLAG_NOMERGES, q);

	queue_flag_clear(QUEUE_FLAG_NOXMERGES, q);

	if (nm == 2)

		queue_flag_set(QUEUE_FLAG_NOMERGES, q);

	else if (nm)

		queue_flag_set(QUEUE_FLAG_NOXMERGES, q);

	spin_unlock_irq(q->queue_lock);



	return ret;

block/elevator.c

+10 −1
Original line number Diff line number Diff line
@@ -473,6 +473,15 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio) 
	struct request *__rq;

	int ret;



	/*

	 * Levels of merges:

	 * 	nomerges:  No merges at all attempted

	 * 	noxmerges: Only simple one-hit cache try

	 * 	merges:	   All merge tries attempted

	 */

	if (blk_queue_nomerges(q))

		return ELEVATOR_NO_MERGE;



	/*

	 * First try one-hit cache.

	 */
@@ -484,7 +493,7 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio) 
		}

	}



	if (blk_queue_nomerges(q))

	if (blk_queue_noxmerges(q))

		return ELEVATOR_NO_MERGE;



	/*

include/linux/blkdev.h

+3 −0
Original line number Diff line number Diff line
@@ -463,6 +463,7 @@ struct request_queue 
#define QUEUE_FLAG_IO_STAT     15	/* do IO stats */

#define QUEUE_FLAG_CQ	       16	/* hardware does queuing */

#define QUEUE_FLAG_DISCARD     17	/* supports DISCARD */

#define QUEUE_FLAG_NOXMERGES   18	/* No extended merges */



#define QUEUE_FLAG_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\

				 (1 << QUEUE_FLAG_CLUSTER) |		\
@@ -589,6 +590,8 @@ enum { 
#define blk_queue_queuing(q)	test_bit(QUEUE_FLAG_CQ, &(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_noxmerges(q)	\

	test_bit(QUEUE_FLAG_NOXMERGES, &(q)->queue_flags)

#define blk_queue_nonrot(q)	test_bit(QUEUE_FLAG_NONROT, &(q)->queue_flags)

#define blk_queue_io_stat(q)	test_bit(QUEUE_FLAG_IO_STAT, &(q)->queue_flags)

#define blk_queue_flushing(q)	((q)->ordseq)

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