Line By Line Code Profiling in GAP

Line By Line Code Profiling in GAP

The purpose of this article is to show how to use GAP’s new line-by-line profiler / code coverage. Firstly, you need gap version at least 4.8.0, or a build a development version of gap.

Do you just care which lines of code are executed? Then you should switch to the coverage guide (these two guides are very similar!)

Quickstart Guide

We will start with a quick guide to profiling, with some brief comments. We will explain later how to do these things in greater depth!

Let’s start with some code we want to profile. Here I am going to profile the function f given below, and use a group from the atlas package.

a := AtlasGroup("U6(2)", NrMovedPoints, 12474);
b := a^(1,2,3);
f := function() Intersection(a,b); end;

Firstly, we will record a profile of a function f:

# Code between ProfileLineByLine and UnprofileLineByLine is recorded
# to a file output.gz
ProfileLineByLine("output.gz"); f(); UnprofileLineByLine();

You should write this all on a single line in GAP, as profiling records the real time spent executing code, so time spent typing commands will be counted.

This creates a file called output.gz, which stores the result of running f. Now we want to turn that into a nice output. This requires loading the profiling package, like this:

OutputAnnotatedCodeCoverageFiles("output.gz", "outdir");

If loading the profiling package produces errors, make sure you have compiled both the profiling and IO packages.

OutputAnnotatedCodeCoverageFiles reads the previously created output.gz and produces HTML output into the directory outdir.

You must view the result of your profiling in a web-browser outside of GAP. Open index.html from the outdir directory in the web browser of your choice to see what happened.

At the very top is a link to a flame graph. These give a quick overview of this functions took the most time. Functions are stacked, so lower functions call higher functions.

From this graph we can see that f called Intersection, which called a function without a name at line 2942 in This function spent most of it’s time in PartitionBacktrack, and a little time in Stabilizer.

Whenever you generate a profile which contains timing information, a flame graph link will be show on the first page of your generated profile!

FAQ / Problems

  • ProfileLineByLine records wall (also known as clock) time which happens between ProfileLineByLine and the next UnprofileLineByLine. This is why we put starting profiling, our code, and then stopping profile on a single line.

  • If you want to profile how long everything in GAP takes, including the startup, then you can given the command line option --prof <filename> when starting GAP. This is equivalent to GAP calling ProfileLineByLine("<filename>"); before loading any of the standard library (obviously, give your own filename).

  • Giving your output file the gz extension makes GAP automatically compress the file. This is a great idea, because the files can get very big otherwise! Even then, the files can grow quite large very quickly, keep an eye on them.

  • ProfileLineByLine takes an optional second argument which is a record, which can set some configuration options. Here are some of the options.

    • wallTime: Boolean (defaults to true). Sets if time should be measured using wall-clock time (true) or CPU time (false). Measuring CPU-time has a higher overhead, so avoid it if at all possible!

    • resolution: Integer (defaults to 0). By default GAP will record a trace of all executed code. When non-zero, GAP instead samples which piece of code is being executed every resolution nanoseconds. Setting this to a non-zero value improves performance and produces smaller traces, at the cost of accuracy. GAP will still accurately record which statements are executed at least once. This is mainly useful when you wish to consider very ling running code.

Function-Based Profiling

Sometimes you will have code that just runs too long to easily profile line by line. You can profile this in GAP older function-based profiler. You can read more about this profiler in GAP’s documentation, but here is a quick example to get you going!