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Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
Date: Wed, 19 May 2010 11:06:42 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

On 05/19/2010 10:39 AM, Rusty Russell wrote:

I think we're talking about the last 2 entries of the avail ring.  That means
the worst case is 1 false bounce every time around the ring.

It's low, but why introduce an inefficiency when you can avoid doing it for the same effort?

I think that's
why we're debating it instead of measuring it :)

Measure before optimize is good for code but not for protocols. Protocols have to be robust against future changes. Virtio is warty enough already, we can't keep doing local optimizations.

Note that this is a exclusive->shared->exclusive bounce only, too.

A bounce is a bounce.

Virtio is already way too bouncy due to the indirection between the avail/used rings and the descriptor pool. A device with out of order completion (like virtio-blk) will quickly randomize the unused descriptor indexes, so every descriptor fetch will require a bounce.

In contrast, if the rings hold the descriptors themselves instead of pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for every descriptor, amortized.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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