On Tue, 2011-11-29 at 16:58 +0200, Avi Kivity wrote: > On 11/29/2011 04:54 PM, Michael S. Tsirkin wrote: > > > > > > Which is actually strange, weren't indirect buffers introduced to make > > > the performance *better*? From what I see it's pretty much the > > > same/worse for virtio-blk. > > > > I know they were introduced to allow adding very large bufs. > > See 9fa29b9df32ba4db055f3977933cd0c1b8fe67cd > > Mark, you wrote the patch, could you tell us which workloads > > benefit the most from indirect bufs? > > > > Indirects are really for block devices with many spindles, since there > the limiting factor is the number of requests in flight. Network > interfaces are limited by bandwidth, it's better to increase the ring > size and use direct buffers there (so the ring size more or less > corresponds to the buffer size). > I did some testing of indirect descriptors under different workloads. All tests were on a 2 vcpu guest with vhost on. Simple TCP_STREAM using netperf. Indirect desc off: guest -> host, 1 stream: ~4600mb/s host -> guest, 1 stream: ~5900mb/s guest -> host, 8 streams: ~620mb/s (on average) host -> guest, 8 stream: ~600mb/s (on average) Indirect desc on: guest -> host, 1 stream: ~4900mb/s host -> guest, 1 stream: ~5400mb/s guest -> host, 8 streams: ~620mb/s (on average) host -> guest, 8 stream: ~600mb/s (on average) Which means that for one stream, guest to host gets faster while host to guest gets slower when indirect descriptors are on. -- Sasha. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
