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Subject[PATCH V7 0/6] blk-mq-sched: improve sequential I/O performance
Hi Jens,

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,

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.

This issue becomes one of mains reasons for reverting default SCSI_MQ
in V4.13.

This six patches improve this situation, and brings back
performance loss.

With this change, SCSI-MQ sequential I/O performance is
improved much, Paolo reported that mq-deadline performance
improved much[2] in his dbench test wrt V2. Also performance
improvement on lpfc/qla2xx was observed with V1.[1]



- introduce .get_budget/.put_budget, and IO merge gets improved
compared with V6, and in theory this approach is better than the way
taken in block legacy

- drop patch of 'blk-mq-sched: don't dequeue request until all in ->dispatch are flushed'

- address comments from Christoph
- drop the 1st patch which changes blk_mq_request_bypass_insert(),
which should belong to dm-mpath's improvement
- move ' blk-mq-sched: move actual dispatching into one helper'
as 2nd patch, and use the introduced helper to simplify dispatch
- merge two previous patches into one for improving dispatch from sw queue
- make comment/commit log line width as ~70, as suggested by

- address some comments from Omar
- add Tested-by & Reveiewed-by tag
- use direct issue for blk_mq_request_bypass_insert(), and
start to consider to improve sequential I/O for dm-mpath
- only include part 1(the original patch 1 ~ 6), as suggested
by Omar

- add Reviewed-by tag
- some trival change: typo fix in commit log or comment,
variable name, no actual functional change

- totally round robin for picking req from ctx, as suggested
by Bart
- remove one local variable in __sbitmap_for_each_set()
- drop patches of single dispatch list, which can improve
performance on mq-deadline, but cause a bit degrade on
none because all hctxs need to be checked after ->dispatch
is flushed. Will post it again once it is mature.
- rebase on v4.13-rc6 with block for-next

- 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

Ming Lei (6):
blk-mq-sched: fix scheduler bad performance
blk-mq-sched: move actual dispatching into one helper
sbitmap: introduce __sbitmap_for_each_set()
blk-mq: introduce .get_budget and .put_budget in blk_mq_ops
blk-mq-sched: improve dispatching from sw queue
SCSI: implement .get_budget and .put_budget for blk-mq

block/blk-mq-sched.c | 121 +++++++++++++++++++++++++++++++++++++++++-------
block/blk-mq.c | 74 +++++++++++++++++++++++++++--
block/blk-mq.h | 4 +-
drivers/scsi/scsi_lib.c | 42 +++++++++++++----
include/linux/blk-mq.h | 12 +++++
include/linux/sbitmap.h | 64 ++++++++++++++++++-------
6 files changed, 268 insertions(+), 49 deletions(-)


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