Ruby Memory Pitfalls

Ruby has an automatic memory management. In most cases this is good; sometimes it becomes sad.

Ruby memory management is both elegant and cumbersome. It stores objects (named RVALUE s) in so-called heaps of size of approx 16KB. On a low level, RVALUE is a c -struct, containing a union of different standard ruby object representations.

So, heaps store RVALUE objects, which size is not more than 40 bytes. For such objects as String , Array , Hash etc. this means that small objects can fit in the heap, but as soon as they reach a threshold, an extra memory outside of the Ruby heaps will be allocated.

This extra memory is flexible; is will be freed as soon as an object became GC’ed. But the heaps themselves are not released to OS anymore.

Let’s take a look at the simple example:

  def report
    puts 'Memory ' + `ps ax -o pid,rss | grep -E "^[[:space:]]*#{$$}"`
            .strip.split.map(&:to_i)[1].to_s + 'KB'
  end

  report
  big_var = " " * 10_000_000
  report
  big_var = nil
  report
  ObjectSpace.garbage_collect
  sleep 1
  report

  # ⇒ Memory 11788KB
  # ⇒ Memory 65188KB
  # ⇒ Memory 65188KB
  # ⇒ Memory 11788KB

Here we allocate the huge amount of memory, use it somehow and then release back to OS. Everything seems to be fine. Let’s now slightly change the source code:

-  big_var = " " * 10_000_000
+  big_var = 1_000_000.times.map(&:to_s)

That was a humdrum modification, wasn’t it? But wait:

  # ⇒ Memory 11788KB
  # ⇒ Memory 65188KB
  # ⇒ Memory 65188KB
  # ⇒ Memory 57448KB

WTF? The memory is not released to OS anymore. That’s because each element of the array we introduced suits the RVALUE size and is stored in the ruby heap .

In most cases this is OK. There are more empty slots in ruby heap now; code re-run will not eat any additional memory; GC[:heap_used] value is decreased as expected every time we dispose big_var and a lot of empty heaps, ready for operation are returned back to Ruby. To Ruby that said, not to OS.

So, be careful with creating a lot of temporary variables suiting the 40 bytes:

  big_var = " " * 10_000_000
  big_var.gsub(/\s/) { |c| '-' }

results in growth of memory guzzled by Ruby as well. And this memory will not be returned back to OS during the whole long run:

  # ⇒ Memory 10156KB
  # ⇒ Memory 13788KB
  # ⇒ Memory 13788KB
  # ⇒ Memory 12808KB

Not so crucial, but noteworthy enough.