On Wed, Dec 19, 2012 at 03:54:03PM +0000, Daniel P. Berrange wrote:
> > Oh, thanks Daniel! I thought the performance would be the same, just
> > with a different cpu name.
> The CPU name is a shorthand for specifying CPU feature flags, which
> in turns controls what CPU features guests can use. For example,
> if your CPU has support for AES hardware encryption, you want to
> be exposing this to your guests, otherwise they'll fallback to
> slower software AES encryption
To put some numbers to it, from testing I did in a virtualized HPC
environment, using the simple kcbench benchmark (which compiles the kernel
in a loop)
on hardware generic flags westmere
runs completed: 103 88 100
ave kcbench score: 41127 34969 38885
ave wall time/run: 24.49s 28.72s 25.77s
This isn't representative of _all_ loads, of course, but I've found it to be
good, simple general case test. Here, with the generic flags, we see
a penalty of around 15%, but with the single change of using the specific
processor flags, it's closer to 5%.
Or to put it another way, setting the flags correctly gave an 11%
performance boost.
--
Matthew Miller ☁☁☁ Fedora Cloud Architect ☁☁☁ <mattdm@xxxxxxxxxxxxxxxxx>
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