[CentOS] Questions on cpu frequency scaling AMD vs. Intel

Sun Aug 3 15:59:49 UTC 2008
Kai Schaetzl <maillists at conactive.com>

Ned Slider wrote on Sun, 03 Aug 2008 16:16:25 +0100:

> acpi-cpufreq was 
> autoloaded in response to enabling C1E and EIST features in the BIOS 
> (which one is responsible I don't know as I enabled both together).

Ah, it must have been enabled by C1E. I don't know if I have that or can 
enable in BIOS. But it should have nothing to do with the scaling.

> Yes, the frequency does scale up under load. I tested by launching a 
> scientific app that loads all 4 cores at 100%. As fast as I could 
> manually start the app and check the freq, it reported at 2.4GHz. I 
> don't know at what point or under what load it will scale back up, and 
> if scaling is done on a core by core basis, but it does scale back up 
> under full load.

Thanks for confirmation. I tested now with two and four gzip parallel. 
That sends the CPU to 0% idle, but doesn't change the behavior. The quad 
core Xeon shows running at 2000, although it should go up to 2500. And the 
dual core shows 2333 for current frequency and 2000 for current scaling 
frequency - which can't be right. If I recall right, with the original 
CentOS Xen hypervisor kernel it showed both with 2000 all the time. I had 
not noticed that the dual core machine had the Xen 3.2 kernel installed, 
but was not booting with it. I just changed that. So, at least part of the 
behavior is Xen-kernel related.

5 minutes later: oh, yes, it does! Now I got it to 0% idle and current 
frequency jumped to 2333000 (although current scaling frequency was still 
shown at 2000000, on AMDs both figures rise).Looks like a clear bug in the 
centrino kernel module to me. It scales only up if the overall threshold 
is reached


Kai Schätzl, Berlin, Germany
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