Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index intoring itself

From: Michael S. Tsirkin
Date: Wed May 19 2010 - 18:37:27 EST

On Wed, May 19, 2010 at 11:06:42AM +0300, Avi Kivity wrote:
> 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.

On the other hand, consider that on fast path we are never using all
of the ring. With a good allocator we might be able to keep
reusing only small part of the ring, instead of wrapping around
all of it all of the time.

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