linux-kernel - Re: [PATCH] virtio-ring: Use threshold for switching to indirect descriptors

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Date:	Sun, 4 Dec 2011 18:22:21 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	markmc@...hat.com
Subject: Re: [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@...hat.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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