Re: [RFC 0/2] optimise local-tw task resheduling

Ming Lei
2023-03-16T01:26:34Z

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