Perl code profiling and mod_perl memory leaks tracking and debugging

[stephanegigandet]

What

• We have not done any profiling of the Perl code of the server, it is very likely that there are performance issues that we could solve, if we find them. Same for memory leaks: our mod_perl process grow very big very fast. There are a number of tools for Perl code profiling and memory usage tracking that we could try to find.
• This bug is to list our experiments and what we find.

Part of

• 8764

[CharlesNepote]

I have no experience in profiling BUT perl benchmark module is really easy to use. It can be a good way to test before/after code changes: https://perldoc.perl.org/Benchmark.html

For a good introduction on profiling and benchmarking, I found: https://www.oreilly.com/library/view/mastering-perl/9780596527242/ch05.html

[stephanegigandet]

I used Devel::MAT and it helped to identify a very big memory leak, details are in https://github.com/openfoodfacts/openfoodfacts-server/issues/2053