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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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