Floating point vs Integer performance

Floating point vs Integer performance

[Benjamin Prosnitz]

I ran some benchmarks inside of KVM-12 on my system and outside of KVM
(no virtualization on my system).  I found that floating point
performance when running both with and without KVM was approximately the
same, but integer performance in KVM was about 25% of what I get when
running the test outside of KVM. These were tests using Dhrystone 2 and
Double Precision Whetstone on a Core 2 Duo T7200 processor.

Any thoughts on why the integer and floating point performance appears
to differ so much?

Benjamin Prosnitz


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Re: Floating point vs Integer performance

[Avi Kivity]

Benjamin Prosnitz wrote:
> I ran some benchmarks inside of KVM-12 on my system and outside of KVM
> (no virtualization on my system).  I found that floating point
> performance when running both with and without KVM was approximately the
> same, but integer performance in KVM was about 25% of what I get when
> running the test outside of KVM. These were tests using Dhrystone 2 and
> Double Precision Whetstone on a Core 2 Duo T7200 processor.
>
> Any thoughts on why the integer and floating point performance appears
> to differ so much?
>
>   

Perhaps the integer benchmarks thrash the mmu cache.  Try raising 
KVM_NUM_MMU_PAGES to 1024.

4MB per VM lowmem is a lot, but if it helps many workloads, it's worth it.


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


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Re: Floating point vs Integer performance

[Michael Riepe]

Hi!

Benjamin Prosnitz wrote:
> I ran some benchmarks inside of KVM-12 on my system and outside of KVM
> (no virtualization on my system).  I found that floating point
> performance when running both with and without KVM was approximately the
> same, but integer performance in KVM was about 25% of what I get when
> running the test outside of KVM. These were tests using Dhrystone 2 and
> Double Precision Whetstone on a Core 2 Duo T7200 processor.
> 
> Any thoughts on why the integer and floating point performance appears
> to differ so much?

I guess it's the benchmark. In contrast to its FP counterpart, Dhrystone
doesn't crunch numbers most of the time. It also performs lots of string
operations, for example (memory access == slow).

-- 
Michael "Tired" Riepe <michael-0QoEqw4nQxo@public.gmane.org>
X-Tired: Each morning I get up I die a little

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