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, 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. -- Jens Axboe
