From: Rob Biedenharn <Rob@...>
Date: 2008-06-19T07:15:48+09:00
Subject: Re: Threads and synchronized access to an array

On Jun 18, 2008, at 6:11 PM, Rob Biedenharn wrote:
> On Jun 18, 2008, at 5:23 PM, Bob Bobrov wrote:
>> Hi everyone!
>> A n00b question - I have a following ruby code:
>> ===============================================
>>

snip

>> 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:
>>

snip

> 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


If you remove the sleep and initialize with @arr=(1..10000).to_a you  
see the kind of result you probably want where the @mutex.synchronize  
block is really controlling the access.

-Rob

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