timer_res.tex

CCSM Research Tools: Community Atmosphere Model (CAM)

% $Id: Timer_res.tex,v 1.1.6.1 2002/04/24 03:25:57 erik Exp $
\section{Restrictions}

The underlying library routine used to gather user and system times is
{\tt times()}.  Unfortunately, the behavior of this routine is not consistent
across platforms.  On IBM and Compaq, it is cumulative across threads.  On
SGI and PC machines it is thread-specific.  This behavior will be reflected
in the printed timings.  For example, on a 4-PE dedicated node the user time
for a given routine called from within a threaded region may exceed the
wallclock number by as much as a factor of 4 on IBM and Compaq machines.

Wall clock, on the other hand, does not have this problem.  If you are not on a
dedicated system, however, wall clock will also measure the time of processes
that are interrupting yours.