The Python Profilers

Copyright © 1994, by InfoSeek Corporation, all rights reserved.

Written by James Roskind. [1]

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The profiler was written after only programming in Python for 3 weeks. As a result, it is probably clumsy code, but I don't know for sure yet 'cause I'm a beginner :-). I did work hard to make the code run fast, so that profiling would be a reasonable thing to do. I tried not to repeat code fragments, but I'm sure I did some stuff in really awkward ways at times. Please send suggestions for improvements to: jar@netscape.com. I won't promise any support. ...but I'd appreciate the feedback.

Introduction to the profilers

A :dfn:`profiler` is a program that describes the run time performance of a program, providing a variety of statistics. This documentation describes the profiler functionality provided in the modules :mod:`profile` and :mod:`pstats`. This profiler provides :dfn:`deterministic profiling` of any Python programs. It also provides a series of report generation tools to allow users to rapidly examine the results of a profile operation.

The Python standard library provides three different profilers:

1. :mod:`profile`, a pure Python module, described in the sequel. Copyright © 1994, by InfoSeek Corporation.
2. :mod:`cProfile`, a module written in C, with a reasonable overhead that makes it suitable for profiling long-running programs. Based on :mod:`lsprof`, contributed by Brett Rosen and Ted Czotter.
3. :mod:`hotshot`, a C module focusing on minimizing the overhead while profiling, at the expense of long data post-processing times.

The :mod:`profile` and :mod:`cProfile` modules export the same interface, so they are mostly interchangeables; :mod:`cProfile` has a much lower overhead but is not so far as well-tested and might not be available on all systems. :mod:`cProfile` is really a compatibility layer on top of the internal :mod:`_lsprof` module. The :mod:`hotshot` module is reserved to specialized usages.

Instant User's Manual

This section is provided for users that "don't want to read the manual." It provides a very brief overview, and allows a user to rapidly perform profiling on an existing application.

To profile an application with a main entry point of :func:`foo`, you would add the following to your module:

import cProfile
cProfile.run('foo()')

(Use :mod:`profile` instead of :mod:`cProfile` if the latter is not available on your system.)

The above action would cause :func:`foo` to be run, and a series of informative lines (the profile) to be printed. The above approach is most useful when working with the interpreter. If you would like to save the results of a profile into a file for later examination, you can supply a file name as the second argument to the :func:`run` function:

import cProfile
cProfile.run('foo()', 'fooprof')

The file :file:`cProfile.py` can also be invoked as a script to profile another script. For example:

python -m cProfile myscript.py

:file:`cProfile.py` accepts two optional arguments on the command line:

cProfile.py [-o output_file] [-s sort_order]

:option:`-s` only applies to standard output (:option:`-o` is not supplied). Look in the :class:`Stats` documentation for valid sort values.

When you wish to review the profile, you should use the methods in the :mod:`pstats` module. Typically you would load the statistics data as follows:

import pstats
p = pstats.Stats('fooprof')

The class :class:`Stats` (the above code just created an instance of this class) has a variety of methods for manipulating and printing the data that was just read into p. When you ran :func:`cProfile.run` above, what was printed was the result of three method calls:

p.strip_dirs().sort_stats(-1).print_stats()

The first method removed the extraneous path from all the module names. The second method sorted all the entries according to the standard module/line/name string that is printed. The third method printed out all the statistics. You might try the following sort calls:

p.sort_stats('name')
p.print_stats()

The first call will actually sort the list by function name, and the second call will print out the statistics. The following are some interesting calls to experiment with:

p.sort_stats('cumulative').print_stats(10)

This sorts the profile by cumulative time in a function, and then only prints the ten most significant lines. If you want to understand what algorithms are taking time, the above line is what you would use.

If you were looking to see what functions were looping a lot, and taking a lot of time, you would do:

p.sort_stats('time').print_stats(10)

to sort according to time spent within each function, and then print the statistics for the top ten functions.

You might also try:

p.sort_stats('file').print_stats('__init__')

This will sort all the statistics by file name, and then print out statistics for only the class init methods (since they are spelled with __init__ in them). As one final example, you could try:

p.sort_stats('time', 'cum').print_stats(.5, 'init')

This line sorts statistics with a primary key of time, and a secondary key of cumulative time, and then prints out some of the statistics. To be specific, the list is first culled down to 50% (re: .5) of its original size, then only lines containing init are maintained, and that sub-sub-list is printed.

If you wondered what functions called the above functions, you could now (p is still sorted according to the last criteria) do:

p.print_callers(.5, 'init')

and you would get a list of callers for each of the listed functions.

If you want more functionality, you're going to have to read the manual, or guess what the following functions do:

p.print_callees()
p.add('fooprof')

Invoked as a script, the :mod:`pstats` module is a statistics browser for reading and examining profile dumps. It has a simple line-oriented interface (implemented using :mod:`cmd`) and interactive help.

What Is Deterministic Profiling?

:dfn:`Deterministic profiling` is meant to reflect the fact that all function call, function return, and exception events are monitored, and precise timings are made for the intervals between these events (during which time the user's code is executing). In contrast, :dfn:`statistical profiling` (which is not done by this module) randomly samples the effective instruction pointer, and deduces where time is being spent. The latter technique traditionally involves less overhead (as the code does not need to be instrumented), but provides only relative indications of where time is being spent.

In Python, since there is an interpreter active during execution, the presence of instrumented code is not required to do deterministic profiling. Python automatically provides a :dfn:`hook` (optional callback) for each event. In addition, the interpreted nature of Python tends to add so much overhead to execution, that deterministic profiling tends to only add small processing overhead in typical applications. The result is that deterministic profiling is not that expensive, yet provides extensive run time statistics about the execution of a Python program.

Call count statistics can be used to identify bugs in code (surprising counts), and to identify possible inline-expansion points (high call counts). Internal time statistics can be used to identify "hot loops" that should be carefully optimized. Cumulative time statistics should be used to identify high level errors in the selection of algorithms. Note that the unusual handling of cumulative times in this profiler allows statistics for recursive implementations of algorithms to be directly compared to iterative implementations.

Reference Manual -- :mod:`profile` and :mod:`cProfile`

The primary entry point for the profiler is the global function :func:`profile.run` (resp. :func:`cProfile.run`). It is typically used to create any profile information. The reports are formatted and printed using methods of the class :class:`pstats.Stats`. The following is a description of all of these standard entry points and functions. For a more in-depth view of some of the code, consider reading the later section on Profiler Extensions, which includes discussion of how to derive "better" profilers from the classes presented, or reading the source code for these modules.

Analysis of the profiler data is done using the :class:`Stats` class.

The :class:`Stats` Class

:class:`Stats` objects have the following methods:

Limitations

One 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 than the 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 (less than one clock tick), but it can accumulate and become very significant.

The problem is more important with :mod:`profile` than with the lower-overhead :mod:`cProfile`. For this reason, :mod:`profile` provides a means of calibrating itself for a given platform so that this error can be probabilistically (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.

Calibration

The profiler of the :mod:`profile` module subtracts a constant from each event handling time to compensate for the overhead of calling the time function, and socking away the results. By default, the constant is 0. The following procedure can be used to obtain a better constant for a given platform (see discussion in section Limitations above).

import profile
pr = profile.Profile()
for i in range(5):
    print pr.calibrate(10000)

The method executes the number of Python calls given by the argument, directly and again under the profiler, measuring the time for both. It then computes the hidden overhead per profiler event, and returns that as a float. For example, on an 800 MHz Pentium running Windows 2000, and using Python's time.clock() as the timer, the magical number is about 12.5e-6.

The object of this exercise is to get a fairly consistent result. If your computer is very fast, or your timer function has poor resolution, you might have to pass 100000, or even 1000000, to get consistent results.

When you have a consistent answer, there are three ways you can use it: [2]

import profile

# 1. Apply computed bias to all Profile instances created hereafter.
profile.Profile.bias = your_computed_bias

# 2. Apply computed bias to a specific Profile instance.
pr = profile.Profile()
pr.bias = your_computed_bias

# 3. Specify computed bias in instance constructor.
pr = profile.Profile(bias=your_computed_bias)

If you have a choice, you are better off choosing a smaller constant, and then your results will "less often" show up as negative in profile statistics.

Extensions --- Deriving Better Profilers

The :class:`Profile` class of both modules, :mod:`profile` and :mod:`cProfile`, were written so that derived classes could be developed to extend the profiler. The details are not described here, as doing this successfully requires an expert understanding of how the :class:`Profile` class works internally. Study the source code of the module carefully if you want to pursue this.

If all you want to do is change how current time is determined (for example, to force use of wall-clock time or elapsed process time), pass the timing function you want to the :class:`Profile` class constructor:

pr = profile.Profile(your_time_func)

The resulting profiler will then call :func:`your_time_func`.

:class:`profile.Profile`

:func:`your_time_func` should return a single number, or a list of numbers whose sum is the current time (like what :func:`os.times` returns). If the function returns a single time number, or the list of returned numbers has length 2, then you will get an especially fast version of the dispatch routine.

Be warned that you should calibrate the profiler class for the timer function that you choose. For most machines, a timer that returns a lone integer value will provide the best results in terms of low overhead during profiling. (:func:`os.times` is pretty bad, as it returns a tuple of floating point values). If you want to substitute a better timer in the cleanest fashion, derive a class and hardwire a replacement dispatch method that best handles your timer call, along with the appropriate calibration constant.

:class:`cProfile.Profile`

:func:`your_time_func` should return a single number. If it returns plain integers, you can also invoke the class constructor with a second argument specifying the real duration of one unit of time. For example, if :func:`your_integer_time_func` returns times measured in thousands of seconds, you would constuct the :class:`Profile` instance as follows:

pr = profile.Profile(your_integer_time_func, 0.001)

As the :mod:`cProfile.Profile` class cannot be calibrated, custom timer functions should be used with care and should be as fast as possible. For the best results with a custom timer, it might be necessary to hard-code it in the C source of the internal :mod:`_lsprof` module.

Footnotes

[1]	Updated and converted to LaTeX by Guido van Rossum. Further updated by Armin Rigo to integrate the documentation for the new :mod:`cProfile` module of Python 2.5.

[2]	Prior to Python 2.2, it was necessary to edit the profiler source code to embed the bias as a literal number. You still can, but that method is no longer described, because no longer needed.