On Tue, May 22, 2012 at 09:41:41PM +1000, Benjamin Herrenschmidt wrote: > On Tue, 2012-05-22 at 14:14 +0300, Michael S. Tsirkin wrote: > > > The baseline is the laptop without kvm talking to the server. The > > > TCP_STREAM test results are: > > > > It's not a good test. The thing most affecting throughput results is > > how > > much CPU does you guest get. So as a minumum you need to measure CPU > > utilization on the host and divide by that. > > The simple fact that we don't reach the baseline while in qemu seems to > be a reasonably good indication that we tend to be CPU bound already so > it's not -that- relevant. It would be if we were saturating the network. > > But yes, I can try to do more tests tomorrow, it would be nice if you > could contribute a proper test protocol (or even test on some machines) > since you seem to be familiar with those measurements (and I have a very > limited access to x86 gear ... basically just my laptop). > > Cheers, > Ben.
I have a deja vu. Amos sent perf results when you argued about exactly the same issue in guest virtio. Delta was small but measureable. At the moment I have no free time or free hardware to redo the same work all over again. It's a well known fact that actual memory barrier is slow on x86 CPUs. You can't see it with network on your laptop? Write a microbenchmark. -- MST