Re: KVM performance vs. Xen

[Avi Kivity <avi@redhat.com>]

Andrew Theurer wrote:
>>
>> If the overhead is dominated by copying, then you won't see the 
>> difference.  Once the copying is eliminated, the comparison may yield 
>> different results.  We should certainly see a difference in context 
>> switches.
> I would like to test this the proper way.  What do I need to do to 
> ensure these copies are eliminated?  I am on a 2.6.27 kernel, am I 
> missing anything there?  Anthony, would you be willing to provide a 
> patch to support the changes in the block API?

You need a 2.6.30 host kernel plus a libc patch.  Or the linux-aio qemu 
patch.

>>
>> One cause of context switches won't be eliminated - the 
>> non-saturating workload causes us to switch to the idle thread, which 
>> incurs a heavyweight exit.  This doesn't matter since we're idle 
>> anyway, but when we switch back, we incur a heavyweight entry.
> I have not looked at the schedstat or ftrace yet, but will soon.  
> Maybe it will tell us a little more about the context switches.
>
> Here's a sample of the kvm_stat:

We have about 120K host_state_reloads/sec, 70K pio/sec, and 35K 
interrupts/sec.

That corresponds to 35K virtio notifications/sec (reasonable for 8 
cores), and 85K excess context switches/sec.  These can probably be 
eliminated by using linux-aio, except those due to idling.


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