Re: performance tuning, problem with paravirtualized clock

[Avi Kivity <avi@qumranet.com>]

Nikola Ciprich wrote:
> Hi,
> I spent some time trying to tune performance of KVM guest using kernel 
> compilation as a kind of benchmark (I'm using virtual machines for 
> compiling a lot, so it's good benchmark for me in general)
>
> Host machine: 2x quad core XEON E5420  @ 2.50GHz, 4GB RAM, 2.6.24 + kvm-64
> guest configuration: all 8 cores available, 2GB  RAM, 2.6.24 or latest GIT 
> + kvm-64
>
> some results:
> - compilation in KVM guest is roughly 2x slower than on bare metal.
>   

50% scaling is actually quite good for 8-way.

What do you get for 4-way guests?

> - enabling CONFIG_KVM_GUEST under latest GIT with kvm-64 patch applied 
> decreased compile time by about 10%, which is nice!
>
>   

We expect to improve this some more as paravirt_ops improves.

> Is there currently anything more I could do to improve performance? I'm 
> wandering what is slowing compilation, if I compare some CPU intensive 
> application (ie bzip2), it seems to run in nearly native speed, but kernel 
> compilation is much slower even if run from ramdisk, maybe it could be 
> improved further by tunning scheduler etc?
>
>   

As others mentioned, large pages may help somewhat, as well as newer 
hardware.  kvm-65 also improves scalability.

How much cpu does the qemu process consume?  Perfect utilization would 
be around 800%.  What's the system/user ratio (I think you need to use 
2.6.25-rc to get accurate results for this)?

Can you provide the result of 'kvm_stat -1' taken a few times during a 
compile run?

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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