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Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode
Date: Tue, 15 Nov 2016 10:32:02 +0000
User-agent: Mutt/1.7.1 (2016-10-04)

On Mon, Nov 14, 2016 at 09:52:00PM +0100, Paolo Bonzini wrote:
> On 14/11/2016 21:12, Karl Rister wrote:
> >                  256    46,929
> >                  512    35,627
> >                1,024    46,477
> >                2,000    35,247
> >                2,048    46,322
> >                4,000    46,540
> >                4,096    46,368
> >                8,000    47,054
> >                8,192    46,671
> >               16,000    46,466
> >               16,384    32,504
> >               32,000    20,620
> >               32,768    20,807
> 
> Huh, it breaks down exactly when it should start going faster
> (10^9/46000 = ~21000).

Could it be because we're not breaking the polling loop for BHs, new
timers, or aio_notify()?

Once that is fixed polling should achieve maximum performance when
QEMU_AIO_MAX_POLL_NS is at least as long as the duration of a request.

This is logical if there are enough pinned CPUs so the polling thread
can run flat out.

Stefan

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