Stefano Zacchiroli said:

To balance this, according to my first read of Ruby’s threading capability, it’s my impression that not only at most one thread can execute Ruby code at a time (limitation shared by OCaml, due to the non-distributed nature of mark and sweep garbage collectors), but also a thread blocked on a syscall will block all other threads to run.

Dumb Ruby threads (but I still hope I’m wrong …)

This is not totally true:

• Yes, Ruby threads are user-level: you won’t get speedup from dual-cores or dual-processors system. In python, this presentation claims (slide 39) that python threads are mapped to native threads, but that a giant lock always prevents the execution of more than one thread at a time (so library writers don’t have to write thread-safe code).
• The ruby interpreter tries very hard to map blocking syscalls to their non-blocking counterparts (for example, all I/Os are passed to a select() call). This gets really dirty when you mix I/O and other syscalls. For example, see the strace output the following Ruby code:

  require 'thread'
  th1 = Thread::new do
    pid = fork { sleep 2 }
    Process::waitpid(pid)
    puts "Finished."
  end
  th2 = Thread::new do
    f = STDIN.read
    puts "Finished2."
  end
  th1.join
  

  select(1, [0], [], [], {0, 349})        = 0 (Timeout)
  gettimeofday({1168847486, 478832}, NULL) = 0
  select(1, [0], [], [], {0, 0})          = 0 (Timeout)
  waitpid(25249, 0xbf926810, WNOHANG)     = 0
  select(1, [0], [], [], {0, 0})          = 0 (Timeout)
  gettimeofday({1168847486, 479013}, NULL) = 0
  gettimeofday({1168847486, 479053}, NULL) = 0
  select(1, [0], [], [], {0, 59959})      = 0 (Timeout)
  gettimeofday({1168847486, 538971}, NULL) = 0
  select(1, [0], [], [], {0, 41})         = 0 (Timeout)
  gettimeofday({1168847486, 543017}, NULL) = 0
  select(1, [0], [], [], {0, 0})          = 0 (Timeout)
  waitpid(25249, 0xbf926810, WNOHANG)     = 0
  select(1, [0], [], [], {0, 0})          = 0 (Timeout)
  gettimeofday({1168847486, 543200}, NULL) = 0
  gettimeofday({1168847486, 543240}, NULL) = 0
  select(1, [0], [], [], {0, 59959})      = 0 (Timeout)
  gettimeofday({1168847486, 602842}, NULL) = 0
  select(1, [0], [], [], {0, 357})        = 0 (Timeout)
  gettimeofday({1168847486, 606842}, NULL) = 0
  select(1, [0], [], [], {0, 0})          = 0 (Timeout)