From: Markus <markus@...>
Date: 2004-10-16T01:27:56+09:00
Subject: Re: Again some problem with my multithreaded teadrinker app

I'd suggested this before (as the solution to your first problem) but it
seemed to have gotten missed--the nesting of your looping/locking needs
to be swapped.  In the present case, reversing both of the "
mutex.synchronize"/"while true" pairs (and their associated "}"/"end"
pairs) should fix the problem.

-- Markus

P.S. If the reason isn't clear, ask yourself when/under what condition
(in the code as posted) did it ever leave the synchronize block?  If it
still isn't clear after pondering that, post again & I'll give you a
more detailed answer.


On Fri, 2004-10-15 at 05:39, Tassilo Horn wrote:
> Hi,
> 
> I still have some problems with my teadrinker app. After I implemented
> the things SER told me in
> 
>     Message-ID: <1097253853.581059.173460@z14g2000cwz.googlegroups.com>
> 
> the program works very good for 3 drinkers.
> 
> Now I wanted it to support more than 3 drinker Threads.
> 
> Here's the code:
> 
> <----- CODE ----->
> #! /usr/bin/env ruby
> 
> require 'thread'
> 
> # For synchronisation
> mutex = Mutex.new
> drinker_cv = ConditionVariable.new
> waiter_cv = ConditionVariable.new
> 
> # items on the table:
> # 0: cup
> # 1: water
> # 2: tea
> table = []
> items = %w{ Cup Water Tea }
> 
> # min. 3 drinkers.
> if ARGV[0].to_i >= 3
>   drinkers_count = ARGV[0].to_i
> else
>   drinkers_count = 3
> end
> 
> drinkers_count.times do |i|
>   case i%3
>   when 0
>     # this drinker has water and tea
>     needs = [1,2]
>   when 1
>     # this drinker has a cup and tea
>     needs = [0,2]
>   when 2
>     # this drinker has a cup and water
>     needs = [0,1]
>   end
>   Thread.new {
>     puts "Starting Drinker#{i}"
>     mutex.synchronize {
>       while true
>         drinker_cv.wait(mutex)
>         # The things on the table are the things the waiter needs.
>         if needs.sort == table.sort
>           puts "Drinker#{i}: #{items[table[0]]} and 
>                #{items[table[1]]} on table. I'll cook my tea!"
>           # The Drinker took the items from the table. Now it's empty
>           # till the waiter puts new items on it.
>           table = []
>           sleep 1                                               # (1) #
>           puts "Drinker#{i}: Drinking my tea. I call the waiter again."
>           # Wake up the waiter Thread
>           waiter_cv.signal
>           sleep 1                                               # (2) #
>           puts "Drinker#{i}: Now I'll read my newspaper."
>         end
>       end
>     }
>   }
> end
> 
> waiter = Thread.new {
>   mutex.synchronize {
>     while true
>       puts "WAITER: Looking at the table!"
>       if table.length == 0
>         puts "WAITER: Oh, nothing on the table!"
>         #sleep 2
>         while table[0] == table[1] do
>           table = [rand(3), rand(3)]
>         end
>         $stdout << "WAITER: Putting " << table[0] 
>                 << " and " << table[1] << " on the table.\n"
>       else
>         puts "WAITER: Why did you call me???"
>       end
>       # Call waiting tea drinkers.
>       drinker_cv.broadcast                                      # (3) #
>       puts "WAITER: Waiting for new appointments..."
>       waiter_cv.wait(mutex)
>     end
>   }
> }
> 
> waiter.join
> <----- END_OF_CODE ----->
> 
> If I start it with let's say 20 drinkers (./TeaRoom.rb 20) it starts all
> threads, but only the drinkers 0..2 cook their tea. If the table
> contains the items 1 and 2 always drinker0 will cook his tea although
> drinker3, drinker6, drinker9 etc need the same items.
> 
> Why is it the way it is and how do I get it the way I want which means
> that the probability of using drinkerX is the same as using drinkerX+-3?
> Why does drinker_cv.broadcast (line marked with '# (3) #') always wake
> up the same threads?
> 
> One interesting this is that if I remove the lines '# (2) #' and '# (2)
> #' (the sleep() method calls) more drinkers get their turn. But still
> there are drinkers who never drink a tea and others who extremely often
> have to drink. 
> 
> Can anybody help me before any of my tearoom customers die with thirst
> or because of too much tea?
> 
> Thanks,
> Tassilo