From: Robert Klemme <shortcutter@...>
Date: 2010-11-17T17:57:06+09:00
Subject: Re: Thread problem

On Wed, Nov 17, 2010 at 9:16 AM, Justin Collins <justincollins@ucla.edu> wrote:
> On 11/16/2010 11:50 PM, Jaeyong Lee wrote:
>>
>> Hi all,
>>
>> I learning ruby thread and faced to thread problem.
>>
>> Here's my test code that sum 1 to 1000.
>>
>> --
>> threads = []
>>
>> sum = 0
>>
>> 1.upto(1000) { |external|
>>   threads<<  Thread.new(external) { |local|
>>     sleep(rand(0.01))
>>     sum = sum + local
>>   }
>> }
>>
>> threads.each() { |thread| thread.join() }
>>
>> puts sum
>> --
>>
>> I expected '500500' but output is smaller than. (ex: 499864)
>>
>> Please let me know what problem is.
>>
>>
>
> http://en.wikipedia.org/wiki/Race_condition#Computing
>
> You are updating a shared variable "simultaneously" in separate threads,
> which is a very bad idea.

Absolutely correct.  In this particular case using a shared resource
is not necessary since the state of that shared resource is only
accessed when all threads have terminated.  One could do this as well:

threads = []

1.upto(1000) { |external|
  threads << Thread.new(external) { |local|
   sleep(rand(0.01))
   local
 }
}

sum = 0

threads.each() { |thread| sum += thread.value()}

puts sum

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/