From: Philip Scott <pscott@...>
Date: 2008-03-07T09:19:48+09:00
Subject: Threading weirdness

Hi guys,

I'd be grateful if someone could take a look at the following two test  
cases I made after noticing some strange behavior with ruby threads. I  
think the
code speaks clearer than I could, but in essence if you create a lot  
of threads from the same block all reading from a single array, things  
go wierd. It is almost as if variables get mixed up between the  
threads. I am fairly sure this is not expected behavior.. If I make  
the threads as part of a class, this doesn't seem to happen.

The first does as you'd expect and never terminates. The second  
invariably stops after a few thousand iterations when my exception  
gets raised.

I hope I am not wasting your time and have overlooked some glaringly  
obvious details about threads, but I am banging my head on the wall  
here :)

Cheers,

Philip

################################ PROGRAM 1: WORKS, NEVER TERMINATES
require 'thread'
require 'digest/md5'

# A little class that takes references to two arrays in its constructor
# then starts a thread which compares random elements from the two and
# throws an exception if they are ever different.
class Tester
   def initialize (arrayA, arrayB)
     @arrayA = arrayA
     @arrayB = arrayB
     Thread.start {mainloop}
   end

   def mainloop
     testThread = Thread.new do
       while true
         i = rand(9)
         if (@arrayA[i] != @arrayB[i])
           raise Exception.new("Threading Fuckup")
         end
       end
     end
   end
end


a = []
b = []

# Fill an array with 10 random strings, and mirror this
# in another identical array.
for i in (0..9)
    a[i] = Digest::MD5.hexdigest(rand(1000).to_s).to_s
    b[i] = a[i]
end

print "Starting threads...\n"
Thread.abort_on_exception = true

# Start 100 threads comparing the elements in the array
for x in (0..100)
   Tester.new(a,b)
end

print "And we're off"

# And spin forever
while true do
end


######################################## PROGRAM 2: DOESNT WORK, TERMINATES
require 'thread'
require 'digest/md5'

a = []
b = []

# Fill two identical arrays full of 10 random strings
for i in (0..9)
    a[i] = Digest::MD5.hexdigest(rand(1000).to_s).to_s
    b[i] = a[i]
end

print "Starting threads\n"
Thread.abort_on_exception = true

# Start a hundred threads reading items from the two arrays and comparing them
for x in (0..100)
     testThread = Thread.new do
         while true
             i = rand(9)
             if (a[i] != b[i])
                 raise Exception.new("Threading Broke")
             end
         end
     end
end

print "And we're off"

# Spin forever (or at least until an Exception gets thrown)
while true do
end

----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.