From: Robert Klemme <shortcutter@...>
Date: 2010-02-21T22:50:05+09:00
Subject: Re: Threads vs Processes

On 02/21/2010 01:50 AM, Tim Ferrell wrote:
> I have just switched back to Windows from the Mac/Linux world and my
> first non-web Ruby project is a program to manage a bunch of independent
> backup tasks ... the spec calls for allowing many tasks to be queued but
> to only allow a certain number to run at a time. I don't foresee the
> tasks needing to share any data with the parent process or each other
> but that might be an option to keep open...
> 
> My first thought is to do something along the lines of a thread pool and
> have each task as a thread... but then I thought of processes... I am
> admittedly a but fuzzy on the distinction between processes and threads
> in Ruby, especially on Windows... can someone shine some light on this
> for me?

Processes are fairly independent.  Threads share the same memory space 
and if the process exits they are all gone.  In Ruby 1.8 there was only 
one OS level thread that did the work for all Ruby threads so you could 
not make good use of multiple cores that way.  OTOH, if your threads 
just control external programs that you execute (e.g. via "system" or 
"IO.popen") then the single thread might be sufficient.  In 1.9 things 
have been improved but still there are some limitations to the 
concurrency of multiple threads.  Using JRuby with real threads is also 
an option.

> Would one approach be easier to manage in this type of scenario? Are
> there any performance or portability issues I should be aware of?

Performance wise and from a robustness point of view multiple processes 
are probably better.  AFAIK the windows version of Ruby does not have 
support for "fork" (unless you are using cygwin) so there you might 
rather want to use threads.

Using processes is fairly easy - you can try it out with something like 
this:

#! /usr/bin/env ruby19

def log msg
   printf "pid %5d %-10s %s\n", $$, Time.now, msg
end

tasks = (1..10).map { 2 + rand(5) }

limit = 2
processes = []

log "starting"

tasks.each do |t|
   if processes.size == limit
     processes.delete Process.wait
   end

   processes << fork do
     log "start #{t}"
     sleep t
     log "end   #{t}"
   end
end

log "all started"
Process.waitall
log "done"


Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
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