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

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Message-ID: <4EDB624A.3030403@redhat.com>
Date:	Sun, 04 Dec 2011 14:06:34 +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 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.  There's also the used ring, but that's a
mistake if you have out of order completion.  We should have used copying.

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.

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

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