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

Pavel Begunkov
2022-06-19T16:19:58Z

On 6/19/22 17:17, Jens Axboe wrote:
> 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?

Apart from ifs in the hot path with no understanding whether
it helps anything, no

--
Pavel Begunkov