You should be able to see the resident set size go down (RSS) from time to time as well, but this depends in part on how your OS’s malloc(3) is implemented and how lucky you are in your allocation patterns.

Only VALUEs are stored on the ruby’s heap, not the objects themselves [hand wave over Fixnum]. Arrays and Strings are allocated on the C heap [hand wave over 1.9 behavior for small Arrays and Strings].

Here’s a thought experiment:

Some objects are allocated that cause a new struct heaps_slot to be allocated, then a bunch of objects Strings or Arrays are allocated on the C heap. Memory will look like this:

[old stuff, struct heaps_slot, Array, String, Array]

Then there is a GC pass and one of those items are appended to an object that ends up in an earlier heaps_slot (the object is long lived). Finally the recently allocated objects and their struct heaps_slot are freed. Memory will look like this:

[old stuff, free, free, free, Array]

The free space will still count against you in your processes size. Until the OS swaps out the free pages it may even count against you in RSS.

The problem is that some memory allocation patterns cause fragmentation both in the process address space and the struct heaps_slot address space.