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

Jens Axboe
2022-06-19T16:39:00Z

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

Let's just keep the patch. Ratio of skip to non-skip should still be
very tiny.

--
Jens Axboe