Question

I wrote a persistent network service in Perl that runs on Linux.

Unfortunately, as it runs, its Resident Stack Size (RSS) just grows, and grows, and grows, slowly but surely.

This is despite diligent efforts on my part to expunge all unneeded hash keys and delete all references to objects that would otherwise cause reference counts to remain in place and obstruct garbage collection.

Are there any good tools for profiling the memory usage associated with various native data primitives, blessed hash reference objects, etc. within a Perl program? What do you use for tracking down memory leaks?

I do not habitually spend time in the Perl debugger or any of the various interactive profilers, so a warm, gentle, non-esoteric response would be appreciated. 🙂

Answer 1

You could have a circular reference in one of your objects. When the garbage collector comes along to deallocate this object, the circular reference means that everything referred to by that reference will never get freed. You can check for circular references with Devel::Cycle and Test::Memory::Cycle. One thing to try (although it might get expensive in production code, so I’d disable it when a debug flag is not set) is checking for circular references inside the destructor for all your objects:

# make this be the parent class for all objects you want to check;
# or alternatively, stuff this into the UNIVERSAL class's destructor
package My::Parent;
use strict;
use warnings;
use Devel::Cycle;   # exports find_cycle() by default

sub DESTROY
{
    my $this = shift;

    # callback will be called for every cycle found
    find_cycle($this, sub {
            my $path = shift;
            foreach (@$path)
            {
                my ($type,$index,$ref,$value) = @$_;
                print STDERR "Circular reference found while destroying object of type " .
                    ref($this) . "! reftype: $type\n";
                # print other diagnostics if needed; see docs for find_cycle()
            }
        });

    # perhaps add code to weaken any circular references found,
    # so that destructor can Do The Right Thing
}