[Beowulf] cpufreq, multiple cores, load

[Peter Kjellstrom]

On Wednesday 10 March 2010, Hearns, John wrote:
...
> Sorry to set off at a slight tangent - on a related matter, does anyone
> have a good writeup on the CPU scaling controls in Linux for Nehalem -
> ie controlling TurboBoost.
> Yes, I have Googled but as above someone here normally had a blindlingly
> good resource.

Turbo mode shows up as its own frequency step (one Mhz over normal max):

 cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
 2268000 2267000 2133000 2000000 1867000 1733000 1600000

2267(Mhz) is the highest "normal" setting on this E5520. 2268(Mhz) is the 
turbo mode setting.

Linux will treat 2268 as a normal available frequency (and since it's the 
highest this is what will get chosen under load).

If you wan't to run without turbo mode just set the frequency statically to 
2267 (of course this will be true even at idle then).

When the system is running at 2268 it's using turbo mode, the actual frequency 
will be somewhere between 2267 and MAX_TURBO. Where it will end up between 
these two depends on power/thermal head room and what MAX_TURBO is depends on 
the specific cpu model.

For E55* MAX_TURBO is "one step up", that is a 2.267 E5520 can go to 2.4 
(normal max of E5530). For X55* it's two steps up (a 2.667 can go to 2.93 
etc.).

> The reason I ask is that we have some Nehalem boxes, which will
> principally run a serial application.
> Could be useful to see what effect turning on turboboost has.
> I also saw there is a Gnome taskbar applet for showing CPU frequencies -
> anyone got recommendations for a good monitor for that sort of thing?
> Gkrellem springs to mind.

Note that when running turbo mode linux will think it's running a normal 
frequency mode one Mhz up (2268 Mhz for the E5520) not at the true frequency.

/Peter

> John Hearns
> McLaren Racing
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