hi, > On 10/28/21 6:28?AM, Xiaoguang Wang wrote: >> Recently I spend time to research io_uring's fast-poll and multi-shot's >> performance using network echo-server model. Previously I always thought >> fast-poll is better than multi-shot and will give better performance, >> but indeed multi-shot is almost always better than fast-poll in real >> test, which is very interesting. I use ebpf to have some measurements, >> it shows that whether fast-poll is excellent or not depends entirely on >> that the first nowait try in io_issue_sqe() succeeds or fails. Take >> io_recv operation as example(recv buffer is 16 bytes): >> 1) the first nowait succeeds, a simple io_recv() is enough. >> In my test machine, successful io_recv() consumes 1110ns averagely. >> >> 2) the first nowait fails, then we'll have some expensive work, which >> contains failed io_revc(), apoll allocations, vfs_poll(), miscellaneous >> initializations anc check in __io_arm_poll_handler() and a final >> successful io_recv(). Among then: >> failed io_revc() consumes 620ns averagely. >> vfs_poll() consumes 550ns averagely. >> I don't measure other overhead yet, but we can see if the first nowait >> try fails, we'll need at least 2290ns(620 + 550 + 1110) to complete it. >> In my echo server tests, 40% of first nowait io_recv() operations fails. >> >> From above measurements, it can explain why mulit-shot is better than >> multi-shot, mulit-shot can ensure the first nowait try succeed. >> >> Based on above measurements, I try to improve fast-poll a bit: >> Introduce fix poll support, currently it only works in file registered >> mode. With this feature, we can get rid of various repeated operations >> in io_arm_poll_handler(), contains apoll allocations, and miscellaneous >> initializations anc check. > I was toying with an idea on how to do persistent poll support, > basically moving the wait_queue_entry out of io_poll and hence detaching > it from the io_kiocb. That would allow a per-file (and type) poll entry > to remain persistent in the kernel rather than needing to do this > expensive work repeatedly. Pavel kindly reminded me of your work, which > unfortunately I had totally forgotten. > > Did you end up taking this further? My idea was to make it work > independently of fixed files, but I also don't want to reinvent the > wheel if you ended up with something like this. I haven't continued to work on this work since last patch set and currently I don't have time for myself to continue working on this job, sorry. It'll be great if we can add similar fixed poll for fast-poll feature, or if we can eliminate the possible failed first no-wait submit overhead. Recently, aone of our clients also wants to use asio(with io_uring enabled), seems that asio(use io_uring fast-poll) does not perform better than asio(epoll), I need to figure that out firstly. asio: https://github.com/chriskohlhoff/asio.git Regards, Xiaoguang Wang >
