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
