Re: [PATCH 3/3] linux-aio: limit the batch size using `aio-max-batch` parameter

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jul 19, 2021 at 12:35:53PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 13, 2021 at 03:58:04PM +0100, Stefan Hajnoczi wrote:
> > On Wed, Jul 07, 2021 at 05:00:19PM +0200, Stefano Garzarella wrote:
> > > @@ -371,7 +375,7 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
> > >      s->io_q.in_queue++;
> > >      if (!s->io_q.blocked &&
> > >          (!s->io_q.plugged ||
> > > -         s->io_q.in_flight + s->io_q.in_queue >= MAX_EVENTS)) {
> > > +         s->io_q.in_queue >= max_batch)) {
> > 
> > Is it safe to drop the MAX_EVENTS case?
> 
> I think it is safe since in ioq_submit() we have this check while
> dequeueing:
> 
>         QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
>             iocbs[len++] = &aiocb->iocb;
>             if (s->io_q.in_flight + len >= MAX_EVENTS) {
>                 break;
>             }
>         }
> 
> But in term of performance, I think is better what you're suggesting,
> because if we have fewer slots available than `max_batch`, here we were
> delaying the call to io_submit().
> 
> > 
> > Perhaps the following can be used:
> > 
> >  int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
> >  max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight + s->io_q.in_queue, max_batch);
> > 
> 
> Since we will compare `in_queue` with `max_batch`, should we remove it from
> this expression?
> 
> I mean:
> 
>   int64_t max_batch = s->aio_context->aio_max_batch ?: DEFAULT_MAX_BATCH;
>   max_batch = MIN_NON_ZERO(MAX_EVENTS - s->io_q.in_flight, max_batch);
> 
> then as it is in this patch:
> 
>   s->io_q.in_queue++;
>   if (!s->io_q.blocked &&
>       (!s->io_q.plugged ||
>        s->io_q.in_queue >= max_batch)) {
>       ioq_submit(s);
>   }

Good.

Stefan

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