How To Create A Memory Leak in Ruby

There are a few articles out there about how to find memory leaks.

But how about creating one?

I think it will be an interesting exercise so you know what a memory leak looks like in Ruby.

Let’s see some examples.

A Simple Leak

We can create a memory leak by simply adding new objects to an array.

Like this:

a = []
b = {}

loop {
  sleep(1)

  10_000.times { a << "abc" }

  puts GC.stat(b)[:heap_live_slots]
}

This creates 10k strings every second & it prints an object count:

The count keeps going up because the GC can't collect these strings, they are being referenced by the containing array (a).

If a goes out of scope that will allow the GC to collect all these "abc" strings. You can test this with the example above by setting a to nil, then running GC.start.

You can find a live example here, just click run to see the results.

C Extension Leak

When you create an object from Ruby the GC keeps track of how much memory it is using, but when using a C extension Ruby has no control over what happens.

If you create a C extension like this:

#include <ruby.h>
#include "extconf.h"

void *ptr;

void Init_extension()
{
  allocate_memory();
}

void allocate_memory() {
  for(int i = 0; i < 10000; i++) {
   ptr = malloc(1000);
  }
}

The allocate_memory() function will leak memory because it's using malloc & it doesn't call free to release that memory.

As you can see here:

`ps -o rss -p #{$$}`.lines.last
# "49036"

require './extension'

`ps -o rss -p #{$$}`.lines.last
# "89512"

This kind of leak won't show up on any heap dump or on GC.stat, but you will see memory usage grow.

Summary

Now you know what a memory leak looks like, hopefully that will help you find one faster if you ever have this issue. Btw Ruby 2.4.1 has a known memory leak, so you may want to upgrade if you are using this specific version.

Do you have any questions, feedback or an interesting memory leak debugging story? Leave a comment below 🙂

Thanks for reading!