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

[Jens Axboe <axboe@kernel.dk>]

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,
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.

-- 
Jens Axboe