Re: [PATCH] io_uring/sqpoll: Increase task_work submission batch size

[Gabriel Krisman Bertazi <krisman@suse.de>]

Gabriel Krisman Bertazi <krisman@suse.de> writes:

> Our QA team reported a 10%-23%, throughput reduction on an io_uring
> sqpoll testcase doing IO to a null_blk, that I traced back to a
> reduction of the device submission queue depth utilization. It turns out
> that, after commit af5d68f8892f ("io_uring/sqpoll: manage task_work
> privately"), we capped the number of task_work entries that can be
> completed from a single spin of sqpoll to only 8 entries, before the
> sqpoll goes around to (potentially) sleep.  While this cap doesn't drive
> the submission side directly, it impacts the completion behavior, which
> affects the number of IO queued by fio per sqpoll cycle on the
> submission side, and io_uring ends up seeing less ios per sqpoll cycle.
> As a result, block layer plugging is less effective, and we see more
> time spent inside the block layer in profilings charts, and increased
> submission latency measured by fio.

Uh, this should have been tagged as v2.  The original submission and
some discussion available at:

  https://lore.kernel.org/all/94da2142-d7c1-46bb-bc35-05d0d1c28182@kernel.dk/

-- 
Gabriel Krisman Bertazi