I have done some test, measures here:

pure calculus :
    http://regisaubarede.posterous.com/threading-and-smp

many disk read (grep -r ):
  http://regisaubarede.posterous.com/threading-and-smp

So with jruby/iron ruby, optimum is one thread by core, for pure 
calculus
When many I/O, that depend on system, type of io. anyway, jruby and 
ironruby are the best for multi core environment (no giant lock).

for net processing, its seem that event machine + multi process is the 
must.

Regards,

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