On 7/20/20 10:06 AM, Pavel Begunkov wrote: > On 20/07/2020 18:49, Jens Axboe wrote: >> On 7/20/20 9:22 AM, Pavel Begunkov wrote: >>> On 18/07/2020 17:37, Jens Axboe wrote: >>>> On 7/18/20 2:32 AM, Pavel Begunkov wrote: >>>>> For my a bit exaggerated test case perf continues to show high CPU >>>>> cosumption by io_dismantle(), and so calling it io_iopoll_complete(). >>>>> Even though the patch doesn't yield throughput increase for my setup, >>>>> probably because the effect is hidden behind polling, but it definitely >>>>> improves relative percentage. And the difference should only grow with >>>>> increasing number of CPUs. Another reason to have this is that atomics >>>>> may affect other parallel tasks (e.g. which doesn't use io_uring) >>>>> >>>>> before: >>>>> io_iopoll_complete: 5.29% >>>>> io_dismantle_req: 2.16% >>>>> >>>>> after: >>>>> io_iopoll_complete: 3.39% >>>>> io_dismantle_req: 0.465% >>>> >>>> Still not seeing a win here, but it's clean and it _should_ work. For >>>> some reason I end up getting the offset in task ref put growing the >>>> fput_many(). Which doesn't (on the surface) make a lot of sense, but >>>> may just mean that we have some weird side effects. >>> >>> It grows because the patch is garbage, the second condition is always false. >>> See the diff. Could you please drop both patches? >> >> Hah, indeed. With this on top, it looks like it should in terms of >> performance and profiles. > > It just shows, that it doesn't really matters for a single-threaded app, > as expected. Worth to throw some contention though. I'll think about > finding some time to get/borrow a multi-threaded one. But it kind of did here, ended up being mostly a wash in terms of perf here as my testing reported. With the incremental applied, it's up a bit over before the task put batching. >> I can just fold this into the existing one, if you'd like. > > Would be nice. I'm going to double-check the counter and re-measure anyway. > BTW, how did you find it? A tool or a proc file would be awesome. For this kind of testing, I just use t/io_uring out of fio. It's probably the lowest overhead kind of tool: # sudo taskset -c 0 t/io_uring -b512 -p1 /dev/nvme2n1 -- Jens Axboe
