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SubjectRe: [PATCH] virtio-ring: Use threshold for switching to indirect descriptors
On Sun, Dec 04, 2011 at 02:13:51PM +0200, Sasha Levin wrote:
> On Sun, 2011-12-04 at 13:52 +0200, Avi Kivity wrote:
> > On 12/03/2011 01:50 PM, Sasha Levin wrote:
> > > On Fri, 2011-12-02 at 11:16 +1030, Rusty Russell wrote:
> > > > On Thu, 1 Dec 2011 12:26:42 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > > > On Thu, Dec 01, 2011 at 10:09:37AM +0200, Sasha Levin wrote:
> > > > > > On Thu, 2011-12-01 at 09:58 +0200, Michael S. Tsirkin wrote:
> > > > > > > We'll presumably need some logic to increment is back,
> > > > > > > to account for random workload changes.
> > > > > > > Something like slow start?
> > > > > >
> > > > > > We can increment it each time the queue was less than 10% full, it
> > > > > > should act like slow start, no?
> > > > >
> > > > > No, we really shouldn't get an empty ring as long as things behave
> > > > > well. What I meant is something like:
> > > >
> > > > I was thinking of the network output case, but you're right. We need to
> > > > distinguish between usually full (eg. virtio-net input) and usually
> > > > empty (eg. virtio-net output).
> > > >
> > > > The signal for "we to pack more into the ring" is different. We could
> > > > use some hacky heuristic like "out == 0" but I'd rather make it explicit
> > > > when we set up the virtqueue.
> > > >
> > > > Our other alternative, moving the logic to the driver, is worse.
> > > >
> > > > As to fading the effect over time, that's harder. We have to deplete
> > > > the ring quite a few times before it turns into always-indirect. We
> > > > could back off every time the ring is totally idle, but that may hurt
> > > > bursty traffic. Let's try simple first?
> > >
> > > I tried to take a different approach, and tried putting the indirect
> > > descriptors in a kmem_cache as Michael suggested. The benchmarks showed
> > > that this way virtio-net actually worked faster with indirect on even in
> > > a single stream.
> >
> > How much better?
>
> host->guest was same with both indirect on and off, and guest->host went
> up by 5% with indirect on.
>
> This was just a simple 1 TCP stream test.

I'm confused. didn't you see a bigger benefit for guest->host by
switching indirect off?

> >
> > I think that if indirects benefit networking, then we're doing something
> > wrong. What's going on? Does the ring get filled too early? If so we
> > should expand it.
> >
>
> --
>
> Sasha.


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