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: Alan D. Brunelle <alan.brunelle@hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
Alan D. Brunelle 2010-01-29 09:04:08 +01:00 committed by Jens Axboe
parent 47483e2520
commit 488991e28e
5 changed files with 39 additions and 10 deletions

View File

@ -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.

View File

@ -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)
----------------

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@ -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;

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@ -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;
/*

View File

@ -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)