[Beowulf] cpufreq, multiple cores, load

Bill Broadley bill at cse.ucdavis.edu
Tue Mar 9 13:50:05 PST 2010

David Mathog wrote:
> Starting a second cpuburn apparently schedules it
> on one of the cores on the unused second processor, rather than
> on the equally unused, but already sped up, second core on the first
> CPU.

Since that gives the most additional performance that seems a reasonable
default.  So you add an additional cache and memory system.  Numactl or the
related system calls would let you schedule it on the first CPU if desired.

> This flips the remaining two cores also to 2400 MHz, negating any
> further benefit from "ondemand" as more cpuburn processes are added.
> That is, there are 5 states for increasing cpuburn load but, only the
> lowest two have different power consumption for "ondemand" than for
> "performance".

Unless you choose to do otherwise.  If you are willing to ignore the 2nd chips
cache and memory system you could keep power lower until over half the system
is busy.

> Anyway, this is just 4 cores total, but it makes me wonder what happens
> with a system having say, two quad core processors, where if the same
> sort of scheduling/cpufreq logic apply, two CPU saturating jobs (still
> only 1/4 of total available CPU capacity) will effectively negate the
> energy saving modes.  For instance, imagine some future 24 core behemoth
> that acts the same way.  One might almost dispense with power saving
> modes altogether if one CPU intensive job is going to kick the other 23
> cores into a high power state.  Or do the newer CPUs, either AMD's or
> Intel's, allow different frequencies on each core of a CPU?  

I seem to recall that one of the recent AMD tweaks was to allow additional
tweaks.  I forget if it was voltage, or clock speed that can now be controller
per core.  I believe the north bridge and memory bus also can enter a lower
power state when not in use.

Next time I have a nehalem dual socket in my office I'll test it.

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