smp cputime issues

From: Steinar Hauan (
Date: Tue Jan 01 2002 - 20:00:43 EST


  we are encountering some weird timing behaviour on our linux cluster.

  specifically: when running 2 copies of selected programs on a
  dual-cpu system, the cputime reported for each process is up to 25%
  higher than when the processes are run on their own. however, if running
  two different jobs on the same machine, both complete with a cputime
  equal to when run individually. sample timing output attached.

  profiling confirms that everything slows down approximately to scale.
  the results reproduce on a range of different machines (see below).

additional specifications:
  - kernel version 2.4.16 (with apic enabled)
  - chipsets: apollo pro 133, apollo pro 266,
              intel i860, serverworks LE
  - all jobs requires less than 1/10 of physical memory
  - no significant disk i/o takes place
  - timing with dtime(), /usr/bin/time and shell built-in time
  - this behavior is NOT seen for all applications. the worst
    "offender" spends most of its time doing linear algebra.

  ideas or info-pointers appreciated. more specs available on request.


  Steinar Hauan, dept of ChemE  --
  Carnegie Mellon University, Pittsburgh PA, USA

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