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(python2.1-lib.info)Limitations

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Limitations
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There are two fundamental limitations on this profiler.  The first is
that it relies on the Python interpreter to dispatch "call", "return",
and "exception" events.  Compiled C code does not get interpreted, and
hence is "invisible" to the profiler.  All time spent in C code
(including built-in functions) will be charged to the Python function
that invoked the C code.  If the C code calls out to some native Python
code, then those calls will be profiled properly.

The second limitation has to do with accuracy of timing information.
There is a fundamental problem with deterministic profilers involving
accuracy.  The most obvious restriction is that the underlying "clock"
is only ticking at a rate (typically) of about .001 seconds.  Hence no
measurements will be more accurate that that underlying clock.  If
enough measurements are taken, then the "error" will tend to average
out. Unfortunately, removing this first error induces a second source
of error...

The second problem is that it "takes a while" from when an event is
dispatched until the profiler's call to get the time actually _gets_
the state of the clock.  Similarly, there is a certain lag when exiting
the profiler event handler from the time that the clock's value was
obtained (and then squirreled away), until the user's code is once
again executing.  As a result, functions that are called many times, or
call many functions, will typically accumulate this error.  The error
that accumulates in this fashion is typically less than the accuracy of
the clock (i.e., less than one clock tick), but it _can_ accumulate and
become very significant.  This profiler provides a means of calibrating
itself for a given platform so that this error can be probabilistically
(i.e., on the average) removed.  After the profiler is calibrated, it
will be more accurate (in a least square sense), but it will sometimes
produce negative numbers (when call counts are exceptionally low, and
the gods of probability work against you :-). )  Do _not_ be alarmed by
negative numbers in the profile.  They should _only_ appear if you have
calibrated your profiler, and the results are actually better than
without calibration.


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