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

["Michael S. Tsirkin" <mst@redhat.com>]

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.