Re: [PATCH] virtio-ring: Use threshold for switching to indirectdescriptors

[Sasha Levin]

On Sun, 2011-12-04 at 18:22 +0200, Michael S. Tsirkin wrote:
> 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@xxxxxxxxxx> 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?

The 5% improvement is over the 'regular' indirect on, not over indirect
off. Sorry for the confusion there.

I suggested this change regardless of the outcome of indirect descriptor
threshold discussion, since it would help anyways.

-- 

Sasha.

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