cpu feature flags matter [was Re: QEMU Virtual CPU version]

[Matthew Miller]

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 at fedoraproject.org>