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

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Message-ID: <20111204151148.GA21851@redhat.com>
Date:	Sun, 4 Dec 2011 17:11:49 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...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 Sun, Dec 04, 2011 at 02:06:34PM +0200, Avi Kivity wrote:
> On 12/04/2011 02:01 PM, Michael S. Tsirkin wrote:
> > > 
> > > How much better?
> > > 
> > > 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.
> >
> > The ring is physically contigious.
> > With 256 entries and 64 bytes each, that's already 16K.
> 
> A descriptor is just 16 bytes.

Right. Not sure where did I get 64.

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

> 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.

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