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

[Ming Lei]

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.

But ublk applies batch schedule similar with io_uring before calling
io_uring_cmd_complete_in_task().

thanks,
Ming