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Date: Thu, 25 Sep 2014 21:45:40 +0200
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Subject: Re: [Qemu-devel] [PATCH 2/2] iohandler: Add Linux implementation of
 iohandler GSource
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

Il 25/09/2014 19:21, Fam Zheng ha scritto:
> +    while (true) {
> +        r = epoll_pwait(s->epollfd.fd, events, MAX_EVENTS, 0, &origmask);

You can save a syscall by doing this just once.  You would just get a
readable epoll file descriptor on the next main loop iteration.  Also,
origmask is an input parameter, so you need to use epoll_wait.

Also, as an extra optimization perhaps you can make a second list with
iohandlers that were modified or have a read_poll handler, and only call
iohandler_get_events on that one.

Perhaps this together lowers the cost of epoll enough to more easily
reach the break-even point.

Though I wonder if it would be acceptable to make ioeventfd off by
default for virtio-serial...

Thanks,

Paolo

> +        if (r < 0) {
> +            break;
> +        } else if (r == 0) {
> +            break;
> +        } else {
> +            for (i = 0; i < r; i++) {
> +                iohandler_dispatch_event(s, &events[i]);
> +            }
> +            if (r < MAX_EVENTS) {
> +                break;
> +            }
> +        }
> +    }
> +