Re: [PATCH for-next 5/7] io_uring: remove ->flush_cqes optimisation

Pavel Begunkov
2022-06-19T16:17:29Z

On 6/19/22 16:52, Jens Axboe wrote:
> On 6/19/22 8:52 AM, Pavel Begunkov wrote:
>> On 6/19/22 14:31, Jens Axboe wrote:
>>> On 6/19/22 5:26 AM, Pavel Begunkov wrote:
>>>> It's not clear how widely used IOSQE_CQE_SKIP_SUCCESS is, and how often
>>>> ->flush_cqes flag prevents from completion being flushed. Sometimes it's
>>>> high level of concurrency that enables it at least for one CQE, but
>>>> sometimes it doesn't save much because nobody waiting on the CQ.
>>>>
>>>> Remove ->flush_cqes flag and the optimisation, it should benefit the
>>>> normal use case. Note, that there is no spurious eventfd problem with
>>>> that as checks for spuriousness were incorporated into
>>>> io_eventfd_signal().
>>>
>>> Would be note to quantify, which should be pretty easy. Eg run a nop
>>> workload, then run the same but with CQE_SKIP_SUCCESS set. That'd take
>>> it to the extreme, and I do think it'd be nice to have an understanding
>>> of how big the gap could potentially be.
>>>
>>> With luck, it doesn't really matter. Always nice to kill stuff like
>>> this, if it isn't that impactful.
>>
>> Trying without this patch nops32 (submit 32 nops, complete all, repeat).
>>
>> 1) all CQE_SKIP:
>> ~51 Mreqs/s
>> 2) all CQE_SKIP but last, so it triggers locking + *ev_posted()
>> ~49 Mreq/s
>> 3) same as 2) but another task waits on CQ (so we call wake_up_all)
>> ~36 Mreq/s
>>
>> And that's more or less expected. What is more interesting for me
>> is how often for those using CQE_SKIP it helps to avoid this
>> ev_posted()/etc. They obviously can't just mark all requests
>> with it, and most probably helping only some quite niche cases.
>
> That's not too bad. But I think we disagree on CQE_SKIP being niche,

I wasn't talking about CQE_SKIP but rather cases where that
->flush_cqes actually does anything. Consider that when at least
one of the requests queued for inline completion is not CQE_SKIP
->flush_cqes is effectively disabled.

> there are several standard cases where it makes sense. Provide buffers
> is one, though that one we have a better solution for now. But also eg
> OP_CLOSE is something that I'd personally use CQE_SKIP with always.
>
> Hence I don't think it's fair or reasonable to call it "quite niche" in
> terms of general usability.
>
> But if this helps in terms of SINGLE_ISSUER, then I think it's worth it
> as we'll likely see more broad appeal from that.

It neither conflicts with the SINGLE_ISSUER locking optimisations
nor with the meantioned mb() optimisation. So, if there is a good
reason to leave ->flush_cqes alone we can drop the patch.

--
Pavel Begunkov