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
