From: Carlos Ortega <caof2005@...>
Date: 2008-10-06T12:40:08+09:00
Subject: Concurent (using threads) slower than sequential -doubt

Hi Folks.
While starting to study the benefits of using threads in Ruby, I tried
to solve the following problem:

I have 3 text files ( numbers0.txt, numbers1.txt, c:\numbers2.txt ),
each file contains a very large list of numbers.
I attempt to read and compute each file by using a different thread.
Finally I tried to sum all subtotals to provide the final result.

Here is the code.
===================

require 'thread'
m_threads = []

print "INITIAL TIME  := ", initial_time = Time.now, "\n"
3.times do |i|
  m_threads[i] = Thread.new do
    total_per_thread = 0
    case i
       when 0 then path = "C:\\numbers0.txt"
       when 1 then path = "C:\\numbers1.txt"
       when 2 then path = "C:\\numbers2.txt"
    end
    File.open( path, "r"  ) do |m_file|
      while line = m_file.gets
          total_per_thread = line.to_i + total_per_thread
      end
      Thread.current[:INDEX] = total_per_thread
    end
  end
end

result = 0
m_threads.each{ |t| t.join; result = t[:INDEX] + result; }

print "FINAL TIME   := ", final_time = Time.now, "\n"
print "TOTAL TIME  := ", total_time = final_time-initial_time, "\n"
print "Total                 := ", result, "\n"

=======================================
Output (CONCURRENT - Using Threads):

INITIAL TIME  := Sun Oct 05 22:07:26 -0500 2008

FINAL TIME    := Sun Oct 05 22:07:38 -0500 2008
TOTAL TIME  := 11.485
Total                 := 1150000000
========================================

I verified and each thread made the job, result is OK too.
I also solved the same problem by using a sequential program with no
threads at all
Here is the code:

print "INITIAL Time := ", initial_time = Time.now, "\n"

paths = [ "C:\\numbers0.txt", "C:\\numbers1.txt", "C:\\numbers2.txt" ]
result = 0
for m_path in paths
  File.open( m_path, "r+"  ) do |m_file|
    while line = m_file.gets
      result = line.to_i + result
    end
  end
end

print "FINAL time     := ", final_time = Time.now, "\n"
print "TOTAL time    := ", total_time = final_time - initial_time, "\n"
print "Total                 := ", result, "\n"

=======================================
Output: (SECUENCIAL- NO Threads)

INITIAL TIME := Sun Oct 05 22:34:47 -0500 2008
FINAL TIME    := Sun Oct 05 22:34:57 -0500 2008
TOTAL TIME   := 10.656
Total                  := 1150000000

=======================================
As you see, the thread based program run slower.
I thought that by using threads it will be faster, but it didn't....Why
is it slower?

Any help will be very appreciated
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