How to force Ruby release memory to OS

Question

As the title, I have a ruby program deal with a large term of data. The program took all memory and there is a calling of system command hostname in it, and an error happened

Cannot allocate memory - hostname

I've tried GC.start and it doesn't work.

So how can I force ruby to release the memory unused?

OK, this is a test code from other people, and the error at last shows that big_var has been recycled. However the memory is still not released.

require "weakref"
def report
  puts "#{param}:\t\t Memory " + `ps ax -o pid,rss | grep -E "^[[:space:]]*#{$$}"`
      .strip.split.map(&:to_i)[1].to_s + 'KB'
end
big_var = ""
#big_var = WeakRef.new(big_var)

report
big_var = 1_000_000.times.map(&:to_s)
report

big_var = WeakRef.new(big_var)

GC.start

sleep 1
report

p big_var.length

#Memory 7508KB
#Memory 61516KB
#Memory 53700KB
#test.rb:20:in `<main>': Invalid Reference - probably recycled (WeakRef::RefError)

OK, I tried things blow and I do not understand why the GC.stat[:heap_used] is still that large after I did $big_var=nil and GC.start

puts GC.stat[:heap_used]
$big_var = []
  5000000.times { |i|
    $big_var << i.to_s
  }
puts GC.stat[:heap_used]
$big_var = nil
puts GC.stat[:heap_used]
GC.start
puts GC.stat[:heap_used]

#70
#12286
#12286
#9847    

In addition, I use Ruby 2.1 and CentOS 6.4

Answer 1

I can see the garbage-collection is done by WeakRef class as documented. here is my try to prove this :

require 'objspace'
require "weakref"

big_var = ""
puts "memory size: #{ObjectSpace.memsize_of big_var}"

big_var = 1_000_000.times.map(&:to_s)
puts "memory size: #{ObjectSpace.memsize_of big_var}"

big_var = WeakRef.new(big_var)
GC.start

puts "memory size: #{ObjectSpace.memsize_of big_var}"

output

[shreyas@arup_ruby (master)]$ ruby a.rb
memory size: 40
memory size: 11636312
memory size: 40
[shreyas@arup_ruby (master)]$

Look at the method : memsize_of.