Re: [PATCH V5 00/14] blk-mq-sched: improve sequential I/O performance(part 1)

[Ming Lei]

On Sat, Sep 30, 2017 at 06:27:13PM +0800, Ming Lei wrote:
> 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,
> 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.
> 
> This issue becomes one of mains reasons for reverting default SCSI_MQ
> in V4.13.
> 
> The 1st patch takes direct issue in blk_mq_request_bypass_insert(),
> then we can improve dm-mpath's performance in part 2, which will
> be posted out soon.
> 
> The 2nd six patches improve this situation, and brings back
> some 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 performanc
> improvement on lpfc/qla2xx was observed with V1.[1]
> 
> Please consider it for V4.15.
> 
> [1] http://marc.info/?l=linux-block&m=150151989915776&w=2
> [2] https://marc.info/?l=linux-block&m=150217980602843&w=2
> 
> V5:
> 	- 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
> 
> V4:
> 	- add Reviewed-by tag
> 	- some trival change: typo fix in commit log or comment,
> 	variable name, no actual functional change
> 
> V3:
> 	- 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
> 
> 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
> 
> 
> Ming Lei (7):
>   blk-mq: issue rq directly in blk_mq_request_bypass_insert()
>   blk-mq-sched: fix scheduler bad performance
>   sbitmap: introduce __sbitmap_for_each_set()
>   blk-mq: introduce blk_mq_dequeue_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
> 
>  block/blk-core.c        |   3 +-
>  block/blk-mq-debugfs.c  |   1 +
>  block/blk-mq-sched.c    | 104 ++++++++++++++++++++++++++++++++++++-------
>  block/blk-mq.c          | 114 +++++++++++++++++++++++++++++++++++++++---------
>  block/blk-mq.h          |   4 +-
>  drivers/md/dm-rq.c      |   2 +-
>  include/linux/blk-mq.h  |   3 ++
>  include/linux/sbitmap.h |  64 +++++++++++++++++++--------
>  8 files changed, 238 insertions(+), 57 deletions(-)

Oops, the title should have been:

[PATCH V5 0/7] blk-mq-sched: improve sequential I/O performance(part 1)

Sorry for that.

-- 
Ming