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(a)fedoraproject.org>