From: Jason Wang <jasowang@xxxxxxxxxx> Date: Fri, 7 Mar 2014 13:28:27 +0800 > This is because the delay added by htb may lead the delay the finish > of DMAs and cause the pending DMAs for tap0 exceeds the limit > (VHOST_MAX_PEND). In this case vhost stop handling tx request until > htb send some packets. The problem here is all of the packets > transmission were blocked even if it does not go to VM2. Isn't this essentially head of line blocking? > We can solve this issue by relaxing it a little bit: switching to use > data copy instead of stopping tx when the number of pending DMAs > exceed half of the vq size. This is safe because: > > - The number of pending DMAs were still limited (half of the vq size) > - The out of order completion during mode switch can make sure that > most of the tx buffers were freed in time in guest. > > So even if about 50% packets were delayed in zero-copy case, vhost > could continue to do the transmission through data copy in this case. > > Test result: > > Before this patch: > VM1 to VM2 throughput is 9.3Mbit/s > VM1 to External throughput is 40Mbit/s > CPU utilization is 7% > > After this patch: > VM1 to VM2 throughput is 9.3Mbit/s > Vm1 to External throughput is 93Mbit/s > CPU utilization is 16% > > Completed performance test on 40gbe shows no obvious changes in both > throughput and cpu utilization with this patch. > > The patch only solve this issue when unlimited sndbuf. We still need a > solution for limited sndbuf. > > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> > Cc: Qin Chuanyu <qinchuanyu@xxxxxxxxxx> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx> I'd like some vhost experts reviewing this before I apply it. -- 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
