Monday, 23 April 2012

Ruby Mutex Reentrancy

This morning I was making some Ruby code of mine thread-safe which is always fun. (I'm serious btw. I frikking love multithreaded programming!) In doing so I came across something that I found a bit surprising.

Consider the following snippet: Think it will work? Let's try...

<internal:prelude>:8:in `lock': deadlock; recursive locking (ThreadError)
 from :8:in `synchronize'
 from reentrancy.rb:5:in `block in 
' from :10:in `synchronize' from reentrancy.rb:4:in `
Mutex is not reentrant. Wow. Ok. Let's try something else...

Let's change that Mutux into a Monitor and try again. Alrighty, let's put on fresh underwear and give it a whirl...

Monitor is reentrant.
Ah, the world makes sense again. If I had to code my own reentrancy I would've cried and hated Ruby a little bit. My love and faith in Ruby remains, yay!

Is There A Cost?

Nothing is free. Is there a performance penalty? Time for some benchmarks.

Here is a little benchmarking script that acquires and releases both a mutex and monitor 1 million times each: Benchmarking results:

                 user     system      total        real
Mutex        0.400000   0.000000   0.400000 (  0.406259)
Monitor      0.870000   0.010000   0.880000 (  0.864888)
Ouch, monitor takes over the double the time that mutex does. That's the trade-off.

What About JRuby

I'm curious, let's try JRuby too. We'll change bm to bmbm and fire it up.

Rehearsal ---------------------------------------------
Mutex       0.571000   0.000000   0.571000 (  0.539000)
Monitor     2.012000   0.000000   2.012000 (  2.012000)
------------------------------------ total: 2.583000sec

                user     system      total        real
Mutex       0.321000   0.000000   0.321000 (  0.321000)
Monitor     1.696000   0.000000   1.696000 (  1.696000)
Wow, Monitor is 5.3x slower when using JRuby!!! Hmmm, I suspect JIT just need more time to warmup. Here's a new benchmarking script with a big warmup: And the results:
> jruby --1.9 --fast reentrancy-benchmark-jruby.rb
Warmup #1/20
Warmup #20/20
                user     system      total        real
Mutex       0.357000   0.000000   0.357000 (  0.357000)
Monitor     0.768000   0.000000   0.768000 (  0.768000)
Ok, that's on-par with the MRI results. Mutex is fast off-the-bat with JRuby where as Monitor will be a lot slower at first then decrease to a little over double the speed of mutex.


Mutex: No reentrancy. Fast, less than half the speed of Monitor.
Monitor: Reentrancy. Slow, little over twice as slow as Mutex.


  1. Thanks for your post!
    It really helped me to understand!

  2. Thanks for taking the time to benchmark these!