Re: [Qemu-devel] [PATCH 2/2] AioContext: optimize clearing the EventNotifier

[Paolo Bonzini <pbonzini@redhat.com>]

On 20/07/2015 04:27, Fam Zheng wrote:
> For aio-posix, how about keeping the optimization local which doesn't need
> atomic operation? (no idea for win32 :)
> 
> diff --git a/aio-posix.c b/aio-posix.c
> index 5c8b266..7e98123 100644
> --- a/aio-posix.c
> +++ b/aio-posix.c
> @@ -236,6 +236,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
>      int i, ret;
>      bool progress;
>      int64_t timeout;
> +    int aio_notifier_idx = -1;
>  
>      aio_context_acquire(ctx);
>      progress = false;
> @@ -256,11 +257,18 @@ bool aio_poll(AioContext *ctx, bool blocking)
>      assert(npfd == 0);
>  
>      /* fill pollfds */
> +    i = 0;
>      QLIST_FOREACH(node, &ctx->aio_handlers, node) {
>          if (!node->deleted && node->pfd.events) {
>              add_pollfd(node);
> +            if (node->pfd.fd == event_notifier_get_fd(&ctx->notifier)) {
> +                assert(aio_notifier_idx == -1);
> +                aio_notifier_idx = i;
> +            }
> +            i++;
>          }
>      }

That's a good idea.  Since aio_set_fd_handler uses QLIST_INSERT_HEAD,
perhaps we can be sure that aio_notifier_idx is always the last one
(i.e. i-1)?  And the same can be done on Windows, I think.

> +    if (pollfds[aio_notifier_idx].revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) {
> +        event_notifier_test_and_clear(&ctx->notifier);
> +    }

Might as well zero pollfds[aio_notifier_idx].revents after clearing it.

The atomic operation is not too expensive, so it would really be more
about simplicity than about speed.  Not that simplicity is a bad thing!

I'll send v2 of the first patch, you can look at doing the optimization.
 Thanks!

Paolo