On 12/20/22 18:10, Jens Axboe wrote:
On 12/20/22 11:06 AM, Pavel Begunkov wrote:
On 12/20/22 17:58, Pavel Begunkov wrote:
NOT FOR INCLUSION, needs some ring poll workarounds
Flush completions is done either from the submit syscall or by the
task_work, both are in the context of the submitter task, and when it
goes for a single threaded rings like implied by ->task_complete, there
won't be any waiters on ->cq_wait but the master task. That means that
there can be no tasks sleeping on cq_wait while we run
__io_submit_flush_completions() and so waking up can be skipped.
Not trivial to benchmark as we need something to emulate a task_work
coming in the middle of waiting. I used the diff below to complete nops
in tw and removed preliminary tw runs for the "in the middle of waiting"
part. IORING_SETUP_SKIP_CQWAKE controls whether we use optimisation or
not.
It gets around 15% more IOPS (6769526 -> 7803304), which correlates
to 10% of wakeup cost in profiles. Another interesting part is that
waitqueues are excessive for our purposes and we can replace cq_wait
with something less heavier, e.g. atomic bit set
I was thinking something like that the other day, for most purposes
the wait infra is too heavy handed for our case. If we exclude poll
for a second, everything else is internal and eg doesn't need IRQ
safe locking at all. That's just one part of it. But I didn't have
Ring polling? We can move it to a separate waitqueue, probably with
some tricks to remove extra ifs from the hot path, which I'm
planning to add in v2.
a good idea for the poll() side of things, which would be required
to make some progress there.
I'll play with replacing waitqueues with a bitops, should save some
extra ~5% with the benchmark I used.
--
Pavel Begunkov
