How To Profile A Program

[Paul E. Johnson]

Darren Schreiber wrote:
> 
> Thanks for the help Marcus.  I haven't gotten to sit down with that code
> since that version went up.  I am planning to return to work on it this
> summer.  So, your pointers are a good head start.  I do think there are
> some serious questions as to why the messages go to nil.  I found that this
> happened usually a number of iterations into the model, and at the time
> didn't have time to track them down.
> 
> I do wonder if the more experienced developers could offer their
> suggestions on what to do in order to clean code.  Obviously, you want to
> remove unneeded variables and add comments to explain things better.
> 
> But, I have to admit I've never learned how to:
> 
> 1.  "profile" a program -- I've seen people use this term here and am not
> sure I know precisely what it means.
> 2.  check for memory leaks -- I believe I have one in my most recent
> Schelling model, but don't know how to find them
> 3.  figure out where the biggest slow downs in the program are -- (I think
> this is what profiling can help with, right?)
> 4.  think systematically about how to improve code

> 
Agreed. Somebody just has to do the work of writing it down :)

To use profiling, the GNU program called gprof must be installed.  I see
it is in the Swarm-2.1 distribution for windows and it is installed by
default in most unix/linux systems I've seen.  My RedHat system says
gprof comes from the binutils package.

To build a profile, you compile your programs with the -pg compiler
flag. The way this is done in Swarm has changed now and then, but
currently you can get it with

make EXTRAOBJCFLAGS=-pg EXTRALDFLAGS=-pg

Then after you run your program a while--long enough for it to build up
some stats on it--you stop it and you should see a new file called
"gmon.out".  

The gprof command produces a readable interpretation of the gmon.out
file. 
In unix systems, the swarm executable is in that hidden directory .lib,
so you tell the gprof program to use that, as in:

gprof .lib/program-name-here

In windows, you do not need the .lib directory in there.

It will whir by on the screen quickly, but you can always ">" it into a
file or pipe to less, "| less"


As for the question of optimization, I can't say there are many general
rules.  I've been nagging people for them for a long time, and the best
answer I get is "don't write programs that waste time".  Don't
recaculate things if you can save results and reuse them is a common
tidbit.  However, one of the authors of XFree86 recently told me this is
subject to qualification.  Since the CPU is faster than RAM access, it
may save time to recalcuate than to store something large.

-- 
Paul E. Johnson                         email: address@hidden
Dept. of Political Science              http://lark.cc.ukans.edu/~pauljohn
University of Kansas                    Office: (785) 864-9086
Lawrence, Kansas 66045                  FAX: (785) 864-5700

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