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2.5.1 Profiling

Before you start doing any optimization at all, check which parts of your code are slowing the machine down -- there's no point in tweaking, for example, a one-time initialisation routine which takes only a tiny fraction of the program's runtime. You do this by using a profiler.

Different compilers will invoke a profiler (presuming they have one) in different ways. The Sun and Digital Fortran compilers include profiling code if you give the option -pg to the f77 command. Compile and link the program with this option (if you wish, you can compile only those modules you want to profile with the -pg option, but you must include the option in the final link command), and then run it. This run will create a file gmon.out in your current directory. Then you can run gprof. Taking as example the programs in Appendix A.4, we can build and profile them as follows:

% f77 -pg -o timewaster p1.f p2.f p3.f
MAIN silly:
% timewaster
% ls 
gmon.out        p1.o            p2.o            p3.o
p1.f            p2.f            p3.f            timewaster*
% gprof timewaster >timewaster.gprof

The output file timewaster.gprof is many lines long, even though the program is so short! The file contains a great deal of information, but buried amongst it is the following display (from the Sun profiler, trimmed):

called/total       parents 
index  %time    self descendents  called+self    name    	index
called/total       children

0.05        0.64       1/1           main [2]
[1]     97.2    0.05        0.64       1         MAIN_ [1]
0.64        0.00  100000/100000      mkidentity_ [4]
0.00        0.00       1/1           __s_wsle_nv [167]
0.00        0.00       1/1           determinant_ [8]
0.00        0.00       1/1           __do_l_out [157]
0.00        0.00       1/1           __e_wsle [158]
0.64        0.00  100000/100000      MAIN_ [1]
4]     90.1    0.64        0.00  100000         mkidentity_ [4]
This indicates that the subroutine mkidentity was called 100000 times from the main program, and that 90.1% of the time -- a total of 0.64 seconds -- was spent in that routine. Were this a real project, this would indicate that this routine would be a good one to examine for speedups.

This example, and a further one using the tcov utility, were taken from [sunf77], which might be consulted for further details. pixie is the corresponding utility on Compaqs -- see its man-page for details [RW].

gprof is not specific to Sun, but is available for other architectures, and other compilers (including gcc) as well.

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Next: 2.5.2 Optimization
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Theory and Modelling Resources Cookbook
Starlink Cookbook 13
Norman Gray
2 December 2001. Release 2-5. Last updated 10 March 2003