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

Paolo Valente
2017-08-08T08:10:01Z

> Il giorno 05 ago 2017, alle ore 08:56, Ming Lei <ming.lei@redhat.com> =
ha scritto:
>=20
> 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...)
>=20
> 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.
>=20
> The 1st five patches improve this situation, and brings back
> some performance loss.
>=20
> 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.
>=20
> Patch 6 ~ 14 use per-request_queue dispatch list to avoid
> to dequeue requests from sw/scheduler queue when lld queue
> is busy.
>=20
> 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.=20
>=20
> 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.
>=20
> Also Bart worried that this patchset may affect SRP, so provide
> test data on SCSI SRP this time:
>=20
> - fio(libaio, bs:4k, dio, queue_depth:64, 64 jobs)
> - system(16 cores, dual sockets, mem: 96G)
>=20
> |v4.13-rc3 |v4.13-rc3 | v4.13-rc3+patches |
> |blk-legacy dd |blk-mq none | blk-mq none |
> -----------------------------------------------------------| =20
> read :iops| 587K | 526K | 537K |
> randread :iops| 115K | 140K | 139K |
> write :iops| 596K | 519K | 602K |
> randwrite:iops| 103K | 122K | 120K |
>=20
>=20
> |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 |
>=20
> 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
>=20
>=20

Hi,
I've performance-tested Ming's patchset with the dbench4 test in
MMTests, and with the mq-deadline and bfq schedulers. Max latencies,
have decreased dramatically: up to 32 times. Very good results for
average latencies as well.

For brevity, here are only results for deadline. You can find full
results with bfq in the thread that triggered my testing of Ming's
patches [1].

MQ-DEADLINE WITHOUT MING'S PATCHES

Operation Count AvgLat MaxLat
--------------------------------------------------
Flush 13760 90.542 13221.495
Close 137654 0.008 27.133
LockX 640 0.009 0.115
Rename 8064 1.062 246.759
ReadX 297956 0.051 347.018
WriteX 94698 425.636 15090.020
Unlink 35077 0.580 208.462
UnlockX 640 0.007 0.291
FIND_FIRST 66630 0.566 530.339
SET_FILE_INFORMATION 16000 1.419 811.494
QUERY_FILE_INFORMATION 30717 0.004 1.108
QUERY_PATH_INFORMATION 176153 0.182 517.419
QUERY_FS_INFORMATION 30857 0.018 18.562
NTCreateX 184145 0.281 582.076

Throughput 8.93961 MB/sec 64 clients 64 procs max_latency=3D15090.026 =
ms

MQ-DEADLINE WITH MING'S PATCHES

Operation Count AvgLat MaxLat
--------------------------------------------------
Flush 13760 48.650 431.525
Close 144320 0.004 7.605
LockX 640 0.005 0.019
Rename 8320 0.187 5.702
ReadX 309248 0.023 216.220
WriteX 97176 338.961 5464.995
Unlink 39744 0.454 315.207
UnlockX 640 0.004 0.027
FIND_FIRST 69184 0.042 17.648
SET_FILE_INFORMATION 16128 0.113 134.464
QUERY_FILE_INFORMATION 31104 0.004 0.370
QUERY_PATH_INFORMATION 187136 0.031 168.554
QUERY_FS_INFORMATION 33024 0.009 2.915
NTCreateX 196672 0.152 163.835

Thanks,
Paolo

[1] https://lkml.org/lkml/2017/8/3/157

> [1] http://marc.info/?l=3Dlinux-block&m=3D150151989915776&w=3D2
>=20
> 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
>=20
> 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(-)
>=20
> --=20
> 2.9.4
>=20