On 8/30/22 21:34, Dylan Yudaken wrote:
On Tue, 2022-08-30 at 21:19 +0800, Hao Xu wrote:
On 8/19/22 20:19, Dylan Yudaken wrote:
Allow deferring async tasks until the user calls io_uring_enter(2)
with
the IORING_ENTER_GETEVENTS flag. Enable this mode with a flag at
io_uring_setup time. This functionality requires that the later
io_uring_enter will be called from the same submission task, and
therefore
restrict this flag to work only when IORING_SETUP_SINGLE_ISSUER is
also
set.
Being able to hand pick when tasks are run prevents the problem
where
there is current work to be done, however task work runs anyway.
For example, a common workload would obtain a batch of CQEs, and
process
each one. Interrupting this to additional taskwork would add
latency but
not gain anything. If instead task work is deferred to just before
more
CQEs are obtained then no additional latency is added.
The way this is implemented is by trying to keep task work local to
a
io_ring_ctx, rather than to the submission task. This is required,
as the
application will want to wake up only a single io_ring_ctx at a
time to
process work, and so the lists of work have to be kept separate.
This has some other benefits like not having to check the task
continually
in handle_tw_list (and potentially unlocking/locking those), and
reducing
locks in the submit & process completions path.
There are networking cases where using this option can reduce
request
latency by 50%. For example a contrived example using [1] where the
client
sends 2k data and receives the same data back while doing some
system
calls (to trigger task work) shows this reduction. The reason ends
up
being that if sending responses is delayed by processing task work,
then
the client side sits idle. Whereas reordering the sends first means
that
the client runs it's workload in parallel with the local task work.
Sorry, seems I misunderstood the purpose of this patchset. Allow me
to
ask a question: "we always first submit sqes then handle task work
(in IORING_SETUP_COOP_TASKRUN mode), how could the sending be
interrupted by task works?"
IORING_SETUP_COOP_TASKRUN causes the task to not be interrupted simply
for task work, however it will still be run on every system call even
if completions are not about to be processed.
gotcha, then sqpoll may not be a tenant of this feature..
IoUring task work (unlike say epoll wakeups) can take a non-trivial
amount of time, and so running them closer to when they are used can
reduce latency of other unrelated operations by not unnecessarily
stalling them.
