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Re: [Xen-devel] CAP and performance problem

On 06/06/2013 06:27 PM, George Dunlap wrote:
On 06/06/13 16:54, Dario Faggioli wrote:
On gio, 2013-06-06 at 17:42 +0200, Dario Faggioli wrote:
Jokes apart, we may want to add a line there, perhaps on the Wiki, but I
honestly don't think this is by any means the cap mechanism's. :-)

What I meant is: I don't think this can be considered cap's fault.

It's certainly not cap's fault, but it wouldn't hurt to just put a comment saying something like,

"NB that 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, a workload that may take 300s with a cap of 100% may take 900s with a cap of 50%, because in addition to being run only 50% of the time, the cpu is running in a lower state and so the 50% you get is less powerful. If this is a problem, look at performance and cpufreq options in our operating system and your BIOS."


Thanks George, this is exactly what I meant and I think it is quite important to put this "warning" in evidence.

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