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

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Message-ID: <4EDB8EEB.4070309@redhat.com>
Date:	Sun, 04 Dec 2011 17:16:59 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Sasha Levin <levinsasha928@...il.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 12/04/2011 05:11 PM, Michael S. Tsirkin wrote:
> > There's also the used ring, but that's a
> > mistake if you have out of order completion.  We should have used copying.
>
> Seems unrelated... unless you want used to be written into
> descriptor ring itself?

The avail/used rings are in addition to the regular ring, no?  If you
copy descriptors, then it goes away.

> But, I don't really know why does virtio ring insist on
> making the 3 buffers (avail/used/descriptor)
> physically contigious. Rusty?

Let's drop them instead.

>
> > 16kB worth of descriptors is 1024 entries.  With 4kB buffers, that's 4MB
> > worth of data, or 4 ms at 10GbE line speed.  With 1500 byte buffers it's
> > just 1.5 ms.  In any case I think it's sufficient.
>
> Right. So I think that without indirect, we waste about 3 entries
> per packet for virtio header and transport etc headers.

That does suck.  Are there issues in increasing the ring size?  Or
making it discontiguous?

Can you take a peek at how Xen manages its rings?  They have the same
problems we do.

-- 
error compiling committee.c: too many arguments to function

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