Re: High vm-exit latencies during kvm boot-up/shutdown

Avi Kivity
2007-10-23T16:38:38Z

Jan Kiszka wrote:
> It's both: -rt tests were performed with nosmp (-rt locks up under SMP
> here), and the Xenomai tests, including the last instrumentation, ran in
> SMP mode. So I tend to exclude SMP effects.
>
> Do you have some suggestion how to analyse what the guest is executing
> while those IRQs are delayed? As I said, I suspect some interaction of
> the Bochs BIOS is triggering the effect, because it only shows up (with
> a Linux guest) before the BIOS prints the first messages.
>

The guest is probably spinning. Can you try modifying
user/test/bootstrap.S, around the start label, to look like

start: jmp start

? This can then be executed with

user/kvmctl user/test/bootstrap user/test/access.flat

This will give us something consistent to test. The output of kvm_stat
should also be interesting (should show exits and irq_exits advancing at
10K/sec).

(once Linux starts executing, it takes a lot of pagefault exits for the
mmu, and then a lot of hlt exit once it idles).

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.