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

Pavel Begunkov
2023-03-16T12:27:03Z

On 3/11/23 17:24, Jens Axboe wrote:
> On 3/10/23 12:04?PM, 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.
>>
>> Not for inclusion yet, I want to add an optimisation for when
>> waiting for 1 CQE.
>
> Ran this on the usual peak benchmark, using IRQ. IOPS is around ~70M for
> that, and I see context rates of around 8.1-8.3M/sec with the current
> kernel.

Tried it out. No difference with bs=512, qd=4 is completed before
it gets to schedule() in io_cqring_wait(). With QD32, it's local tw run
__io_run_local_work() spins 2 loops, and QD=8 somewhat in the middle
with rare extra sched.

For bs=4096 QD=8 I see a lot of:

io_cqring_wait() @min_events=8
schedule()
__io_run_local_work() nr=4
schedule()
__io_run_local_work() nr=4

And if we benchmark without and with the patch there is a nice
CPU util reduction.

CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
0 1.18 0.00 19.24 0.00 0.00 0.00 0.00 0.00 0.00 79.57
0 1.63 0.00 29.38 0.00 0.00 0.00 0.00 0.00 0.00 68.98

--
Pavel Begunkov