Re: QEMU PIC indirection patch for in-kernel APIC work

Nakajima, Jun
2007-04-04T17:09:42Z

Avi Kivity wrote:
> Nakajima, Jun wrote:
>> I compared the performance on Xen and KVM for kernel build using the
>> same guest image. Looks like KVM was (kvm-17) three times slower as
>> far as we tested, and that high load of qemu was one of the
>> symptoms. We are looking at the shadow code, but the load of qemu
>> looks very high. I remember we had similar problems in Xen before,
>> but those were fixed. Someone should take a look at the qemu side.
>>
>
> I'd expect the following issues to dominate:
>
> - the shadow cache is quite small at 256 pages. Increasing it may
> increase performance.

Yes, we are aware of this.

>
> - we haven't yet taught the scheduler that migrating vcpus is
> expensive due to the IPI needed to fetch the vmcs. Maybe running
> with 'taskset 1' would help
>
> - shadow eviction policy is FIFO, not LRU, which probably causes many
> page faults.

This may explain that the performance gets worse as we repeat kernel
build, at least, the second run is slower than the first one.

>
> Running kvm_stat can help show what's going on.

Thanks for the good insights. We'll come back with some analysis.

Jun
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