[RFC] virtio: use mandatory barriers for remote processor vdevs
From: benh@kernel.crashing.org (Benjamin Herrenschmidt)
Date: 2011-12-11 22:27:57
Also in:
kvm, lkml, virtualization
On Sun, 2011-12-11 at 14:25 +0200, Michael S. Tsirkin wrote:
Forwarding some results by Amos, who run multiple netperf streams in parallel, from an external box to the guest. TCP_STREAM results were noisy. This could be due to buffering done by TCP, where packet size varies even as message size is constant. TCP_RR results were consistent. In this benchmark, after switching to mandatory barriers, CPU utilization increased by up to 35% while throughput went down by up to 14%. the normalized throughput/cpu regressed consistently, between 7 and 35% The "fix" applied was simply this:
What machine & processor was this ? Cheers, Ben.
quoted hunk ↗ jump to hunk
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 3198f2e..fdccb77 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c@@ -23,7 +23,7 @@ /* virtio guest is communicating with a virtual "device" that actually runs on * a host processor. Memory barriers are used to control SMP effects. */ -#ifdef CONFIG_SMP +#if 0 /* Where possible, use SMP barriers which are more lightweight than mandatory * barriers, because mandatory barriers control MMIO effects on accesses * through relaxed memory I/O windows (which virtio does not use). */