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

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Message-ID: <87zkfbre9x.fsf@rustcorp.com.au>
Date:	Fri, 02 Dec 2011 11:16:50 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Sasha Levin <levinsasha928@...il.com>
Cc:	Avi Kivity <avi@...hat.com>, 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 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?

Thanks,
Rusty.
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