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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
