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

[Pavel Begunkov]

On 3/12/23 15:30, Jens Axboe wrote:
> On 3/11/23 1:45?PM, Pavel Begunkov wrote:
> > 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.
> > >
> > > Applied the two patches, but didn't see much of a change? Performance is
> > > about the same, and cx rate ditto. Confused... As you probably know,
> > > this test waits for 32 ios at the time.
> >
> > If I'd to guess it already has perfect batching, for which case
> > the patch does nothing. Maybe it's due to SSD coalescing +
> > small ro I/O + consistency and small latencies of Optanes,
> > or might be on the scheduling and the kernel side to be slow
> > to react.
> >
> > I was looking at trace_io_uring_local_work_run() while testing,
> > It's always should be @loop=QD (i.e. 32) for the patch, but
> > the guess is it's also 32 with that setup but without patches.
>
> It very well could be that it's just loaded enough that we get perfect
> batching anyway. I'd need to reuse some of your tracing to know for
> sure.

I used existing trace points. If you see a pattern

trace_io_uring_local_work_run()
trace_io_uring_cqring_wait(@count=32)

trace_io_uring_local_work_run()
trace_io_uring_cqring_wait(@count=32)

...

that would mean a perfect batching. Even more so
if @loops=1


> > > Didn't take a closer look just yet, but I grok the concept. One
> > > immediate thing I'd want to change is the FACILE part of it. Let's call
> > > it something a bit more straightforward, perhaps LIGHT? Or LIGHTWEIGHT?
> >
> > I don't really care, will change, but let me also ask why?
> > They're more or less synonyms, though facile is much less
> > popular. Is that your reasoning?
>
> Yep, it's not very common and the name should be self-explanatory
> immediately for most people.

That's exactly the problem. Someone will think that it's
like normal tw but "better" and blindly apply it. Same happened
before with priority tw lists.

--
Pavel Begunkov