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

Jens Axboe
2022-06-19T16:17:24Z

On 6/19/22 10:15 AM, Pavel Begunkov wrote:
> 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.

Let me flip that around - is there a good reason NOT to leave the
optimization in there then?

--
Jens Axboe