Re: virtio performance issue

[Bernhard Schmidt <berni@birkenwald.de>]

Ben-Ami Yassour <benami@il.ibm.com> wrote:

Hello Ben,

> 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.
>
> In the same setup with an e1000 emulation (just changing model=virtio to
> model=e1000 in the KVM command line), ping reports 0.177ms delay.
>
> BTW, initially I saw that the throughput when using netperf is very low,
> even from guest to host, even though the CPU utilization is low.
>
> What might be the problem?

I had exactly the same issue (with an 2.6.26 kernel and kvm-70 though).
In an attempt to make the guest kernel (w/o modules) as small as
possible I had disabled ACPI in its config. Which works, but introduced
the very same 4ms delay you are seeing and made the VM clock go wild
(even worse with KVM_CLOCK). I did not see other speed issues, but I
have to admit I never benchmarked it.

After enabling ACPI the 4ms delay baseline disappeared and the clock in
the guest is now perfectly in sync with the host.

Regards,
Bernhard