[PATCH V2 00/20] blk-mq-sched: improve SCSI-MQ performance

Ming Lei
2017-08-05T06:57:19Z

In Red Hat internal storage test wrt. blk-mq scheduler, we
found that I/O performance is much bad with mq-deadline, especially
about sequential I/O on some multi-queue SCSI devcies(lpfc, qla2xxx,
SRP...)

Turns out one big issue causes the performance regression: requests
are still dequeued from sw queue/scheduler queue even when ldd's
queue is busy, so I/O merge becomes quite difficult to make, then
sequential IO degrades a lot.

The 1st five patches improve this situation, and brings back
some performance loss.

But looks they are still not enough. It is caused by
the shared queue depth among all hw queues. For SCSI devices,
.cmd_per_lun defines the max number of pending I/O on one
request queue, which is per-request_queue depth. So during
dispatch, if one hctx is too busy to move on, all hctxs can't
dispatch too because of the per-request_queue depth.

Patch 6 ~ 14 use per-request_queue dispatch list to avoid
to dequeue requests from sw/scheduler queue when lld queue
is busy.

Patch 15 ~20 improve bio merge via hash table in sw queue,
which makes bio merge more efficient than current approch
in which only the last 8 requests are checked. Since patch
6~14 converts to the scheduler way of dequeuing one request
from sw queue one time for SCSI device, and the times of
acquring ctx->lock is increased, and merging bio via hash
table decreases holding time of ctx->lock and should eliminate
effect from patch 14.

With this changes, SCSI-MQ sequential I/O performance is
improved much, for lpfc, it is basically brought back
compared with block legacy path[1], especially mq-deadline
is improved by > X10 [1] on lpfc and by > 3X on SCSI SRP,
For mq-none it is improved by 10% on lpfc, and write is
improved by > 10% on SRP too.

Also Bart worried that this patchset may affect SRP, so provide
test data on SCSI SRP this time:

- fio(libaio, bs:4k, dio, queue_depth:64, 64 jobs)
- system(16 cores, dual sockets, mem: 96G)

|v4.13-rc3 |v4.13-rc3 | v4.13-rc3+patches |
|blk-legacy dd |blk-mq none | blk-mq none |
-----------------------------------------------------------|
read :iops| 587K | 526K | 537K |
randread :iops| 115K | 140K | 139K |
write :iops| 596K | 519K | 602K |
randwrite:iops| 103K | 122K | 120K |

|v4.13-rc3 |v4.13-rc3 | v4.13-rc3+patches
|blk-legacy dd |blk-mq dd | blk-mq dd |
------------------------------------------------------------
read :iops| 587K | 155K | 522K |
randread :iops| 115K | 140K | 141K |
write :iops| 596K | 135K | 587K |
randwrite:iops| 103K | 120K | 118K |

V2:
- dequeue request from sw queues in round roubin's style
as suggested by Bart, and introduces one helper in sbitmap
for this purpose
- improve bio merge via hash table from sw queue
- add comments about using DISPATCH_BUSY state in lockless way,
simplifying handling on busy state,
- hold ctx->lock when clearing ctx busy bit as suggested
by Bart

[1] http://marc.info/?l=linux-block&m=150151989915776&w=2

Ming Lei (20):
blk-mq-sched: fix scheduler bad performance
sbitmap: introduce __sbitmap_for_each_set()
blk-mq: introduce blk_mq_dispatch_rq_from_ctx()
blk-mq-sched: move actual dispatching into one helper
blk-mq-sched: improve dispatching from sw queue
blk-mq-sched: don't dequeue request until all in ->dispatch are
flushed
blk-mq-sched: introduce blk_mq_sched_queue_depth()
blk-mq-sched: use q->queue_depth as hint for q->nr_requests
blk-mq: introduce BLK_MQ_F_SHARED_DEPTH
blk-mq-sched: introduce helpers for query, change busy state
blk-mq: introduce helpers for operating ->dispatch list
blk-mq: introduce pointers to dispatch lock & list
blk-mq: pass 'request_queue *' to several helpers of operating BUSY
blk-mq-sched: improve IO scheduling on SCSI devcie
block: introduce rqhash helpers
block: move actual bio merge code into __elv_merge
block: add check on elevator for supporting bio merge via hashtable
from blk-mq sw queue
block: introduce .last_merge and .hash to blk_mq_ctx
blk-mq-sched: refactor blk_mq_sched_try_merge()
blk-mq: improve bio merge from blk-mq sw queue

block/blk-mq-debugfs.c | 12 ++--
block/blk-mq-sched.c | 187 +++++++++++++++++++++++++++++-------------------
block/blk-mq-sched.h | 23 ++++++
block/blk-mq.c | 133 +++++++++++++++++++++++++++++++---
block/blk-mq.h | 73 +++++++++++++++++++
block/blk-settings.c | 2 +
block/blk.h | 55 ++++++++++++++
block/elevator.c | 93 ++++++++++++++----------
include/linux/blk-mq.h | 5 ++
include/linux/blkdev.h | 5 ++
include/linux/sbitmap.h | 54 ++++++++++----
11 files changed, 504 insertions(+), 138 deletions(-)

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2.9.4