On Wed, Mar 15, 2023 at 04:53:09PM +0000, Pavel Begunkov wrote: > On 3/15/23 02:35, Ming Lei wrote: > > Hi Pavel > > > > On Fri, Mar 10, 2023 at 07:04:14PM +0000, Pavel Begunkov wrote: > > > io_uring extensively uses task_work, but when a task is waiting > > > for multiple CQEs it causes lots of rescheduling. This series > > > is an attempt to optimise it and be a base for future improvements. > > > > > > For some zc network tests eventually waiting for a portion of > > > buffers I've got 10x descrease in the number of context switches, > > > which reduced the CPU consumption more than twice (17% -> 8%). > > > It also helps storage cases, while running fio/t/io_uring against > > > a low performant drive it got 2x descrease of the number of context > > > switches for QD8 and ~4 times for QD32. > > > > ublk uses io_uring_cmd_complete_in_task()(io_req_task_work_add()) > > heavily. So I tried this patchset, looks not see obvious change > > on both IOPS and context switches when running 't/io_uring /dev/ublkb0', > > and it is one null ublk target(ublk add -t null -z -u 1 -q 2), IOPS > > is ~2.8M. > > Hi Ming, > > It's enabled for rw requests and send-zc notifications, but > io_uring_cmd_complete_in_task() is not covered. I'll be enabling > it for more cases, including pass through. > > > But ublk applies batch schedule similar with io_uring before calling > > io_uring_cmd_complete_in_task(). > > The feature doesn't tolerate tw that produce multiple CQEs, so > it can't be applied to this batching and the task would stuck > waiting. > > btw, from a quick look it appeared that ublk batching is there > to keep requests together but not to improve batching. And if so, > I think we can get rid of it, rely on io_uring batching and > let ublk to gather its requests from tw list, which sounds > cleaner. I'll elaborate on that later Yeah, the ublk batching can be removed since __io_req_task_work_add already does it, and it is kept just for micro optimization of calling less io_uring_cmd_complete_in_task(), but I think we can get bigger improvement with your tw optimization. Thanks, Ming
