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

Dor Laor
2007-04-04T21:33:42Z

>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.

Maybe you can come up with a patch ;)
If you touch those areas anyway and run several benchmarks, writing a
small piece of code is neglectable effort.

>
>Jun
>---
>Intel Open Source Technology Center