From: Judson Lester <nyarly@...>
Date: 2009-11-10T06:24:42+09:00
Subject: Re: Understanding Threads...

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I completely agree with Jason's diagnosis.  I'd like to make two
observations, though.

First, you can avoid the mutex entirely by using thread-local variables:

  threads << Thread.new { Thread.current[:result] = method1(string) }

  results = threads.inject({}) do |results, thread|
    thread.join
    results.merge(thread[:result])
  end

Second, and (possibly) more controversially, just because you can eval
doesn't mean you should.  To my eye, this looks nicer:

  threads = [
   Thread.new { Thread.current[:result] = method1(string) },
   Thread.new { Thread.current[:result] = method2(string) },
   Thread.new { Thread.current[:result] = method3(string) },
   Thread.new { Thread.current[:result] = method4(string) }
  ]

And exception handling, etc, will be ever so much clearer.

Judson

On Mon, Nov 9, 2009 at 1:14 PM, Jason Roelofs <jameskilton@gmail.com> wrote:

> On Mon, Nov 9, 2009 at 3:45 PM, Matt White <mattw922@gmail.com> wrote:
>
> > I am writing an app that retrieves multiple web pages in one method
> > call. Threading has improved performance drastically for me, but I
> > need some help understanding how exactly the call to join is going to
> > affect my program.
> >
> > Here's some code:
> >
> > def method(string)
> >  result = {}
> >  mutex = Mutex.new
> >  threads = []
> >
> >  %w{methodname1 methodname2 methodname3 methodname4}.each do |method|
> >    threads << Thread.new(method) do |m|
> >      r = eval("#{m}(string)")     # each method call makes an HTTP
> > request
> >      mutex.synchronize { result.merge!(r) }
> >    end
> >  end
> >  threads.each { |t| t.join }
> >  result
> > end
> >
> > Seems like the call to join on each thread is necessary to keep the
> > script from getting ahead of itself, but if I exclude that line, it
> > doesn't seem to hurt my results and the program runs a lot faster.
> > Also, sometimes I get deadlocked somehow if I do use the call to join
> > and I'm not certain as to why. Can someone help shed some light on the
> > situation? Do I need to call join? Any idea why I'm deadlocking?
> > Thanks!
> >
> >
> Thread#join simply says "Wait here until this thread has finished
> executing".
>
> So what you're doing is waiting for all threads to finish before execution
> continues, aka blocking main thread execution. Without the #join, the
> values
> in results will be nonderministic. Any perceived deadlocking is probably
> whatever is in your eval call not timing out. You'll have to watch out
> carefully for that.
>
> Jason
>

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