Question

I have a code snippet like this

 myhash.each_value{|subhash|
  (subhash['key]'.each {|subsubhash|

     statement that modifies the subsubhash and takes about 0.07 s to execute
     })
  }

This loop runs 100+ times and needless to say slows down my application tremendously(about 7 seconds to run this loop).

Any pointers on how to make this faster? I have no control over the really expensive statement. Is there a way I can multi thread within the loop so the statements can be executed in parallel?

Answer 1

threads = []
myhash.each_value{ |subhash|
  threads << Thread.start do
    subhash['key'].each { |subsubhash|
     threads << Thread.start do
       statement that modifies the subsubhash and takes about 0.07 s to execute
     end
    }
  end
}
threads.each { |t| t.join }

Note that MRI 1.8.x doesn’t use real threads, but rather green ones which do not correspond to real OS threads. However, if you use JRuby you might see a performance boost as it supports real threads.