Re: virtio performance issue

[Mark McLoughlin <markmc@redhat.com>]

On Tue, 2008-09-16 at 22:24 +0300, Ben-Ami Yassour wrote:
> On Tue, 2008-09-16 at 09:16 -0500, Anthony Liguori wrote:
> > Ben-Ami Yassour wrote:
> > > I am running virtio with the latest KVM code, and see a significant
> > > performance issue.
> > >
> > > Ping to the host (or any other close machine) reports a 4ms delay.
> > >   
> > 
> > What kvm version and what host kernel version?
> > 
> > It's very easy to mistakenly compile qemu without GSO support too.  You 
> > have to make sure that the 2.6.27 if_tun.h is being included by QEMU.
> 
> Is there an option to control GSO support? How?

GSO support is unconditionally enabled with model=virtio if
kvm-userspace is built with the correct kernel headers, the host kernel
supports tun/tap's IFF_VNET_HDR extension and if the guest supports GSO.

> I am using the kernel and userspace that I pulled from the kvm tree
> today.
> 
> Based on your comment, we checked and the build of the userspace does
> not take if_tun.h from the kernel tree, it takes it from the system
> include files.
> The reason was that the file was not copied as part of the userspace
> build.
> 
> To fix this we made the following change:
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 3f5f6da..b81b098 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -53,7 +53,7 @@ T = $(subst -sync,,$@)-tmp
>  header-sync:
>         rm -rf $T
>         rsync -R \
> -            "$(LINUX)"/./include/linux/kvm*.h \
> +            "$(LINUX)"/./include/linux/*.h \
>              "$(LINUX)"/./include/asm-*/kvm*.h \

Ouch, looks like we need a fix like this alright - maybe just copy
if_tun.h and virtio*.h ?

> Even with this change and compiling the userspace with the correct
> if_tun.h the results are the same, ping takes 4ms.

GSO shouldn't affect ping latency - it should only affect throughput.

I'd expect ping latency to be in the range of .15ms and .3ms since we
delay our reply for .15ms currently.

Is this a regression? Have you tried bisecting it?

Cheers,
Mark.