[MITgcm-devel] "nice" levels and CPU speed on Linux laptops

[Ed Hill]

Hi folks,

For those of us using laptops and running recent Linux versions (eg.
Fedora Core 2, 3, & 4) I've noticed an interesting set of circumstances
that you might want to be aware of:

  If you "nice" your processes, you may see a *SIGNIFICANT*
  slowdown!

Why?  Well, recent (2.6.x) Linux kernels support SpeedStep and other
dynamic CPU clock frequency adjustment schemes.  And they are tied to
how "busy" your processor is -- which is in turn scaled by the process
"nice" levels.  So when running processes that have been "nice"-ed my
laptop reports:

  $ renice +19 29070
  ...wait 15--30 seconds...
  $ cat /proc/cpuinfo | grep '^cpu MHz'
  $ cpu MHz         : 599.627

and when running the exact same process (here, its doing APE data post-
processing in MatLAB) at the "normal" nice level:

  $ renice +0 29070
  ...wait 15--30 seconds...
  $ cat /proc/cpuinfo | grep '^cpu MHz'
  $ cpu MHz         : 1798.883

I get the full CPU speed.

This is a neat feature designed to save battery power on laptops.  But
it can be a bit of a surprise when your analysis runs up to 3X slower
than you expected and you've done nothing other than "nice"-ed it to get
some other work done.

Ed

-- 
Edward H. Hill III, PhD
office:  MIT Dept. of EAPS;  Rm 54-1424;  77 Massachusetts Ave.
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