On 14/12/2020 18:23, Keith Busch wrote: > On Mon, Dec 14, 2020 at 05:58:56PM +0000, Pavel Begunkov wrote: >> On 13/12/2020 18:19, Keith Busch wrote: >>> On Fri, Dec 11, 2020 at 12:38:43PM +0000, Pavel Begunkov wrote: >>>> On 11/12/2020 03:37, Keith Busch wrote: >>>>> It sounds like the statistic is using the wrong criteria. It ought to >>>>> use the average time for the next available completion for any request >>>>> rather than the average latency of a specific IO. It might work at high >>>>> depth if the hybrid poll knew the hctx's depth when calculating the >>>>> sleep time, but that information doesn't appear to be readily available. >>>> >>>> It polls (and so sleeps) from submission of a request to its completion, >>>> not from request to request. >>> >>> Right, but the polling thread is responsible for completing all >>> requests, not just the most recent cookie. If the sleep timer uses the >>> round trip of a single request when you have a high queue depth, there >>> are likely to be many completions in the pipeline that aren't getting >>> polled on time. This feeds back to the mean latency, pushing the sleep >>> timer further out. >> >> It rather polls for a particular request and completes others by the way, >> and that's the problem. Completion-to-completion would make much more >> sense if we'd have a separate from waiters poll task. >> >> Or if the semantics would be not "poll for a request", but poll a file. >> And since io_uring IMHO that actually makes more sense even for >> non-hybrid polling. > > The existing block layer polling semantics doesn't poll for a specific > request. Please see the blk_mq_ops driver API for the 'poll' function. > It takes a hardware context, which does not indicate a specific request. > See also the blk_poll() function, which doesn't consider any specific > request in order to break out of the polling loop. Yeah, thanks for pointing out, it's just the users do it that way -- block layer dio and somewhat true for io_uring, and also hybrid part is per request based (and sleeps once per request), that stands out. If would go with coml-to-compl it should be changed. And not to forget that subm-to-compl sometimes is more desirable. -- Pavel Begunkov
