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RE: [Xen-devel] Performance difference between Xen versions

> From: John Weekes [mailto:lists.xen@xxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, May 03, 2011 11:40 AM
> On 5/2/2011 8:04 PM, Tian, Kevin wrote:
> >
> > you won't know the exact frequency bumped up in the turbo mode, as
> > it's all handled by the CPU itself. what xen can do is just to tell
> > the cpu now I'm OK to enter turbo mode, which is 2268000 (1M higher
> > than normal P0). Then CPU will decide whether current code can be
> > overclocked based on various conditions, such as TDP, other core activities 
> > in
> the same package, ...
> >
> > One possibility to verify that turbo mode does work is to run a CPU
> > intensive workload on one core, while keeping other cores mostly idle.
> > Then choose cpufreq governor to be performance, and then compare your
> > benchmark when BIOS turbo mode is on/off. This should give you some feeling
> whether turbo mode works on your platform.
> Thanks for the response, Kevin.
> It's good to know that I can check turbo by looking to see if it's in the
> 1000hz-higher mode. It's a little strange to me that the true MHz level of the
> turbo wouldn't be known/shown, but I can live with that, as the actual
> performance is what counts. I'll run those benches.

the true HZ is unknown because it's all dynamic. there may be several stepping 
which may be overclocked in the turbo mode, and the selection is dynamically
done according to the core/package condition. there may be no boost, or the
freq may bump among different levels. the best info we can know is the average
freq in a given window, by accessing aperf/mperf MSR. This info has been 
by the ondemand governor, but I'm not sure whether those MSRs are exposed to 
the user level (possibly yes). But yes as you said the actual performance does 
matter. :-)


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