timers and performance measurement under Linux

[James Crotinger]

How important is it that the timer used by the benchmark class measure CPU
time? I'm not sure how we've even been using this class for parallel codes -
in fact, I was thinking that back when we were just testing with threads, we
were measuring wall-clock time. But the default implementation of
Utilities/Clock.h uses the "clock()" call, which is supposed to measure
"elapsed processor time", though I'm not completely sure what that means for
a multithreaded program on a multi-processor. On the SGI we were using the
high-performance timers, which I believe access the CPUs hardware
performance registers, so I would think that that was CPU time as well.
Anyway, the problem is that clock() only has 10 ms granularity under Linux
and that's really crappy if you're trying to measure scaling with problem
size. I've written a version that uses gettimeofday, which has a granularity
of something like 15 microseconds under Linux (may depend on processor
speed?), but this is definitely not a measure of CPU time. For serial
programs this just means that you need to test on an unloaded machine to
have the results make sense. For parallel programs, this means that you need
to interpret the results differently. Anyway, I'm using this and it seems
generally useful, so I'd like to check it in, but was wondering if the
clock-time versus CPU-time measurement was a problem for anyone. I suppose
we could put an accessor on Clock that would allow the user to find out what
type of time was being presented, and Benchmark could examine this and
calculate appropriately, if necessary.

As a side comment, I'm using KCC on one of our 1 GHz PIII Linux boxes.
ABCTest (which is doing a simple non-stencil calculation, so this is as fast
as it gets) is only getting 45 MFlops for the C version, which is the
fastest. This seems pretty pathetic. What have other Linux users seen on
what types of boxes? Also, what is available on Linux for profiling? Is
prof/gprof it? Does KAI sell anything commercial? Does the PIII have any
hardware performance monitoring and is there access to it? Does VTune (under
Windows) inline sufficiently well to do performance testing there (allegedly
good profiling tools, so it might be OK for the serial stuff, but I don't
know if it is capable of "inlining everything"). 

  Jim
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