Re: [Bug #12465] KVM guests stalling on 2.6.28 (bisected)

Ingo Molnar
2009-01-20T18:42:49Z

* Kevin Shanahan <kmshanah-biM/RbsGxha6c6uEtOJ/EA@public.gmane.org> wrote:

> Running the ping test with without apache2 running in the guest:
>
> --- hermes-old.wumi.org.au ping statistics ---
> 900 packets transmitted, 900 received, 0% packet loss, time 902740ms
> rtt min/avg/max/mdev = 0.568/3.745/272.558/16.990 ms
>
> And with apache2 running:
>
> --- hermes-old.wumi.org.au ping statistics ---
> 900 packets transmitted, 900 received, 0% packet loss, time 902758ms
> rtt min/avg/max/mdev = 0.625/25.634/852.739/76.586 ms
>
> In both cases it's quite variable, but the max latency is still not as
> bad as when running with the irq chip enabled.

So the worst-case ping latency is more than 10 times lower?

I'd say this points in the direction of some sort of KVM-internal
wakeup/signalling latency that happens if KVM does not deschedule. For
example it could be a bug like this: if a guest image runs at 100% CPU
time for a long time, IRQ injections might not propagate up until the
preemption callbacks run. (but i'm just speculating here)

Ingo