From: Jason Roelofs <jameskilton@...>
Date: 2009-11-10T06:14:33+09:00
Subject: Re: Understanding Threads...

--001636b2be570ddf3d0477f6ae7c
Content-Type: text/plain; charset=ISO-8859-1

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

--001636b2be570ddf3d0477f6ae7c--