Why does Ruby release memory only sometimes?

Question

Depending on how I generate a string Ruby will release the memory to the OS or it won\'t. The first test code will take up about 235MB

size = 2**22
string =          


        

Answer 1

Ruby MRI does not release memory back to the OS.

Here's what I see with Ruby MRI 2.2 on OSX 10.10, using typical ps -o rss:

• Allocating the big string by using * uses ~220MB.

• Allocating the big string by using map uses ~340MB.

On my system, the GC.start doesn't do anything to the RSS. In other words, I see the RAM usage stay the same.

Notably, the map is using a lot of RAM:

• (0...size).map{ '' } uses ~300MB.

When I loop your examples, something interesting emerges:

• Allocating the big string by using * continues to use the same RAM, i.e. RSS doesn't change much.

• Allocating the big string by using map grows by ~40M per loop.

• Doing just (0...size).map{ '' } grows by ~40M per loop.

This shows me that Ruby map may have a RAM-related issue. It's not exactly a problem, because Ruby isn't raising NoMemoryException, but does seem to be a non-optimal use of RAM.