From: Rob Biedenharn <Rob@...>
Date: 2008-06-19T07:11:02+09:00
Subject: Re: Threads and synchronized access to an array


On Jun 18, 2008, at 5:23 PM, Bob Bobrov wrote:

> Hi everyone!
> A n00b question - I have a following ruby code:
> ===============================================
> @arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]
>
> def iterate(thread_number)
>  @arr.each do |line|
>    puts "#{line} - I am thread number #{thread_number}"
>  end
> end
>
> number_of_threads = 4
>
> threads = []
> number_of_threads.times do |t|
>  threads[t] = Thread.new do
>    iterate(t)
>  end
> end
>
> threads.each {|t| t.join}
> ================================================
>
>
>
> The output for this code is:
> ================================================
> 1 - I am thread number 0
> 2 - I am thread number 0
> 3 - I am thread number 0
> 4 - I am thread number 0
> 5 - I am thread number 0
> 6 - I am thread number 0
> 7 - I am thread number 0
> 8 - I am thread number 0
> 1 - I am thread number 1
> 2 - I am thread number 1
> 3 - I am thread number 1
> 4 - I am thread number 1
> 5 - I am thread number 1
> 6 - I am thread number 1
> 7 - I am thread number 1
> 8 - I am thread number 1
> 1 - I am thread number 2
> 2 - I am thread number 2
> 3 - I am thread number 2
> 4 - I am thread number 2
> 5 - I am thread number 2
> 6 - I am thread number 2
> 7 - I am thread number 2
> 8 - I am thread number 2
> 1 - I am thread number 3
> 2 - I am thread number 3
> 3 - I am thread number 3
> 4 - I am thread number 3
> 5 - I am thread number 3
> 6 - I am thread number 3
> 7 - I am thread number 3
> 8 - I am thread number 3
> ================================================
>
>
> What should be changed in this code to make the output looks like  
> this?
> ================================================
> 1 - I am thread number #{thread_number}
> 2 - I am thread number #{thread_number}
> 3 - I am thread number #{thread_number}
> 4 - I am thread number #{thread_number}
> 5 - I am thread number #{thread_number}
> 6 - I am thread number #{thread_number}
> 7 - I am thread number #{thread_number}
> 8 - I am thread number #{thread_number}
> ================================================
>
> I want to iterate the array values only one time by some number of
> threads, how should I control the access to array values? Each thread
> must be prohobited from using the array values, which have been  
> already
> used by other threads.
>
> I was trying to change the code like this:
> @mutex = Mutex.new
> @arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]
>
> @mutex.synchronize {
>  def iterate(thread_number)
>    @arr.each do |line|
>      puts "#{line} - I am thread number #{thread_number}"
>    end
>  end
> }
>
> number_of_threads = 4
>
> threads = []
> number_of_threads.times do |t|
>  threads[t] = Thread.new do
>    iterate(t)
>  end
> end
>
> threads.each {|t| t.join}
>
> and this:
> @mutex = Mutex.new
> @arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]
>
>
> def iterate(thread_number)
>  @arr.each do |line|
>    puts "#{line} - I am thread number #{thread_number}"
>  end
> end
>
>
> number_of_threads = 4
>
> threads = []
> number_of_threads.times do |t|
>  threads[t] = Thread.new do
>    @mutex.synchronize {
>      iterate(t)
>    }
>  end
> end
>
> threads.each {|t| t.join}

Perhaps:

require 'thread'

@mutex = Mutex.new
@arr = [ 1, 2, 3, 4, 5, 6, 7, 8 ]

def iterate(thread_number)
   while @arr.length > 0 do
     sleep 0.01
     @mutex.synchronize {
       if line = @arr.shift
         puts "#{line} - I am thread number #{thread_number}"
       end
     }
   end
end

number_of_threads = 4

threads = []
number_of_threads.times do |t|
  threads[t] = Thread.new do
     iterate(t)
  end
end

threads.each {|t| t.join}

__END__

without the sleep, the first thread started grabs them all before the  
next one gets a chance.  You never did anything to alter the @arr that  
all threads saw.

I haven't played with threads for a long time, but unless you change  
something about @arr, every iterate() will operate once for each  
element.  You run into trouble with the Array#each if you modify the  
contents of the array inside the block.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com