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

[Avi Kivity <avi@redhat.com>]

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 panic.