On 05/20/2010 01:33 AM, Michael S. Tsirkin wrote: > >> 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. > It's still suboptimal, we have to bounce both the avail/used rings and the descriptor pool, compared to just the descriptor ring with a direct design. Plus we don't need a fancy allocator. When amortizing cachelines, simple data structures win. -- 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
