prof question

[Ed Nather]

<>
From:      Doug Gwyn (VLD/VMB) <gwyn at brl-vld>

	Profiling samples one's process every so often (clock tick typically)
	and records where the PC (program counter) is each sample.
             *       *              *          *            *
		- Since the sampling is periodic it may not be representative
		  of the real PC distribution due to beating with code
		  periodicities (especially if code and sampling are driven by
		  the same clock);
		- Because the code is sampled rather than exhaustively traced,
		  some sections can be missed, and in general Poisson sampling
		  statistics apply (i.e. sampling error for a bin is roughly
		  the square root of the bin count).

I must disagree about Poisson statistics -- in fact, the first quoted point
above explains beautifully why they *don't* apply.   The basis of Poisson
statistical description is that the population from which the sample is
drawn (in this case, the times spent by the program in its various parts)
is randomly distributed in the sampled parameter.  If beating occurs this
assumption is seriously violated.  True, the "central limit theorum" says
if you sample non-random stuff for long enough it begins to look random,
but my experiences with "prof" suggest the effects are more erratic than
random.  With random distributions statistical predictions are possible.
With erratic ones, you're on your own -- and good luck!
-- 

                                       Ed Nather
                                       ihnp4!{ut-sally,kpno}!utastro!nather