From: Robert Klemme <shortcutter@...>
Date: 2008-10-06T23:39:26+09:00
Subject: Re: Concurent (using threads) slower than sequential -doubt

2008/10/6 Carlos Ortega <caof2005@yahoo.com>:
> Thank all of you (Matz, Charles and Robert)
>
> Just one more doubt.....
>
>  Since the threads I created really resides as an array that holds
> threads object I tried to access each one by using [ ] notation:
>
> for t in m_threads
>  print t[:INDEX], "\n"
> end
>
> The interpreter does not throw any error, but results always indicate:
> nil
> nil
> nil
>
> I tried to verify if they are still running:
>
> Thread.list.each{|t| p t}
>
> Results were:
> #<Thread:0x29c5fc0 run>
> #<Thread:0x29c6100 run>
> #<Thread:0x29c6240 run>
> #<Thread:0x294c74c run>
>
> So indeed they are running... the doubt is...why I can't access the
> content of the array?
> In fact in the statement
>     m_threads.each{ |t| t.join; result = t[:INDEX] + result; }
>
> I just can compute result variable only after executing  t.join..... if
> I take out the t.join statement the interpreter throws an error:
>
> PbaThreads.rb:10 : undefined method `+' for nil:NilClass (NoMethodError)
>
> Could you clarify this, please.

Well, this is obvious: you cannot access the result before it's there.
 Since you are setting this as the last statement in the thread you
need to wait (i.e. join) until the thread finishes.

Btw, you can use Thread#value for this.  Here's a variant:


require 'benchmark'

files = (1..3).map {|i| "C:\\numbers#{i}.txt"}

Benchmark.bmbm 10 do |b|
  b.report "threaded" do
    threads = files.map do |file|
      Thread.new file do |f|
        File.open f do |io|
          io.inject(0) {|sum, l| sum + l.to_i}
        end
      end
    end

    puts threads.inject(0) {|sum, th| sum + th.value}
  end

  b.report "sequential" do
    puts files.inject(0) {|s, f|
      File.open f do |io|
        io.inject(s) {|sum, l| sum + l.to_i}
      end
    }
  end
end


Kind regards

robert

-- 
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