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Re: [Xen-devel] [PATCH] docs: Make note for the scheduler "cap" option warning about power management effects

On 12/06/13 15:59, Ian Campbell wrote:
On Wed, 2013-06-12 at 09:57 -0400, Konrad Rzeszutek Wilk wrote:

+NB: Many systems have features that will scale down the computing
+power of a cpu that is not 100% utilized.  This can be in the
+operating system, but can also sometimes be below the operating system
+in the BIOS.  If you set a cap such that individual cores are running
+at less than 100%, this may have an impact on the performance of your
+workload over and above the impact of the cap. For example, if your
+processor runs at 2GHz, and you cap a vm at 50%, the power management
+system may also reduce the clock speed to 1GHz; the effect will be
+that your VM gets 25% of the available power (50% of 1GHz) rather than
+50% (50% of 2GHz).  If you are not getting the performance you expect,
+look at performance and cpufreq options in your operating system and
+your BIOS.
Or .. use 'cpufreq=xen:performance' ?

That should set it to the highest P state.
I committed this already. Assuming this is a good suggestion can we get
an incremental patch please.

Might that kind of thing be better on the wiki page?


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