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>
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5 changed files with 39 additions and 10 deletions
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@ -128,3 +128,17 @@ Description:
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preferred request size for workloads where sustained
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throughput is desired. If no optimal I/O size is
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reported this file contains 0.
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What: /sys/block/<disk>/queue/nomerges
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Date: January 2010
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Contact:
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Description:
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Standard I/O elevator operations include attempts to
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merge contiguous I/Os. For known random I/O loads these
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attempts will always fail and result in extra cycles
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being spent in the kernel. This allows one to turn off
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this behavior on one of two ways: When set to 1, complex
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merge checks are disabled, but the simple one-shot merges
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with the previous I/O request are enabled. When set to 2,
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all merge tries are disabled. The default value is 0 -
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which enables all types of merge tries.
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@ -25,11 +25,11 @@ size allowed by the hardware.
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nomerges (RW)
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-------------
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This enables the user to disable the lookup logic involved with IO merging
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requests in the block layer. Merging may still occur through a direct
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1-hit cache, since that comes for (almost) free. The IO scheduler will not
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waste cycles doing tree/hash lookups for merges if nomerges is 1. Defaults
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to 0, enabling all merges.
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This enables the user to disable the lookup logic involved with IO
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merging requests in the block layer. By default (0) all merges are
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enabled. When set to 1 only simple one-hit merges will be tried. When
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set to 2 no merge algorithms will be tried (including one-hit or more
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complex tree/hash lookups).
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nr_requests (RW)
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----------------
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@ -189,7 +189,8 @@ static ssize_t queue_nonrot_store(struct request_queue *q, const char *page,
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static ssize_t queue_nomerges_show(struct request_queue *q, char *page)
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{
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return queue_var_show(blk_queue_nomerges(q), page);
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return queue_var_show((blk_queue_nomerges(q) << 1) |
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blk_queue_noxmerges(q), page);
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}
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static ssize_t queue_nomerges_store(struct request_queue *q, const char *page,
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@ -199,10 +200,12 @@ static ssize_t queue_nomerges_store(struct request_queue *q, const char *page,
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ssize_t ret = queue_var_store(&nm, page, count);
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spin_lock_irq(q->queue_lock);
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if (nm)
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queue_flag_clear(QUEUE_FLAG_NOMERGES, q);
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queue_flag_clear(QUEUE_FLAG_NOXMERGES, q);
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if (nm == 2)
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queue_flag_set(QUEUE_FLAG_NOMERGES, q);
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else
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queue_flag_clear(QUEUE_FLAG_NOMERGES, q);
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else if (nm)
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queue_flag_set(QUEUE_FLAG_NOXMERGES, q);
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spin_unlock_irq(q->queue_lock);
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return ret;
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@ -473,6 +473,15 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
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struct request *__rq;
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int ret;
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/*
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* Levels of merges:
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* nomerges: No merges at all attempted
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* noxmerges: Only simple one-hit cache try
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* merges: All merge tries attempted
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*/
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if (blk_queue_nomerges(q))
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return ELEVATOR_NO_MERGE;
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/*
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* First try one-hit cache.
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*/
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@ -484,7 +493,7 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
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}
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}
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if (blk_queue_nomerges(q))
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if (blk_queue_noxmerges(q))
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return ELEVATOR_NO_MERGE;
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/*
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@ -463,6 +463,7 @@ struct request_queue
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#define QUEUE_FLAG_IO_STAT 15 /* do IO stats */
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#define QUEUE_FLAG_CQ 16 /* hardware does queuing */
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#define QUEUE_FLAG_DISCARD 17 /* supports DISCARD */
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#define QUEUE_FLAG_NOXMERGES 18 /* No extended merges */
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#define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) | \
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(1 << QUEUE_FLAG_CLUSTER) | \
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@ -589,6 +590,8 @@ enum {
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#define blk_queue_queuing(q) test_bit(QUEUE_FLAG_CQ, &(q)->queue_flags)
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#define blk_queue_stopped(q) test_bit(QUEUE_FLAG_STOPPED, &(q)->queue_flags)
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#define blk_queue_nomerges(q) test_bit(QUEUE_FLAG_NOMERGES, &(q)->queue_flags)
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#define blk_queue_noxmerges(q) \
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test_bit(QUEUE_FLAG_NOXMERGES, &(q)->queue_flags)
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#define blk_queue_nonrot(q) test_bit(QUEUE_FLAG_NONROT, &(q)->queue_flags)
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#define blk_queue_io_stat(q) test_bit(QUEUE_FLAG_IO_STAT, &(q)->queue_flags)
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#define blk_queue_flushing(q) ((q)->ordseq)
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