From: "ara.t.howard" <ara.t.howard@...>
Date: 2008-08-22T23:26:52+09:00
Subject: Re: Newly created thread blocks all others in Ruby 1.8.7 patchlevel 72 on Cygwin


On Aug 19, 2008, at 9:22 AM, Daniel Merk wrote:

>    require 'thread'
>
>    count1 = count2 = 0
>    puts "starting thread a"
>    a = Thread.new do
>          loop { count1 += 1 }
>        end
>    a.priority = -1
>
>    puts "starting thread b"
>    b = Thread.new do
>          loop { count2 += 1 }
>        end
>    b.priority = -2
>    sleep 1   #=> 1
>    Thread.critical = 1
>
>    puts "count1: #{count1}"
>    puts "count2: #{count2}"
>
>
> This code never starts thread b. It seems that thread a blocks all
> other threads once it runs (though its priority is lowered), even the
> main thread.
> If I put a sleep command into the thread a loop, thread b starts, but
> the problem remains, since then thread b blocks all others from
> running.

that code looks completely non-deterministic to me and i've had  
similar issues many times in the past.  i think the fundamental error  
is mistaking the creation of a thread for scheduling it.  i've used  
the following pattern many times to force my code to know that a  
thread has started processing before continuing:

cfp:~ > ruby186 a.rb
starting thread a
starting thread b
count1: 4580083
count2: 44732



cfp:~ > ruby187 a.rb
starting thread a
starting thread b
count1: 4394267
count2: 37032



cfp:~ > cat a.rb

require 'thread'

q = Queue.new
count1 = count2 = 0

a =
   Thread.new do
     puts "starting thread a"
     q.push Thread.current
     loop { count1 += 1 }
   end

a = q.pop # wait for the thread to start
a.priority = -1


b =
   Thread.new do
     puts "starting thread b"
     q.push Thread.current
     loop { count2 += 1 }
   end

b = q.pop # wait for the thread to start
b.priority = -2

sleep 1   #=> 1
Thread.critical = 1

puts "count1: #{count1}"
puts "count2: #{count2}"


i'm sorry that i cannot test this on cygwin so i'm not *sure* it's the  
issue.  but to me it simply looks like non-deterministic code.  in  
fact, even the code above has a race condition, i only force the  
program to ensure each thread calls puts - there is nothing to make  
sure that stdout has been flushed or that the increments have run in  
the loop.  to do that i think you'd need something like this


cfp:~ > cat a.rb
require 'thread'

q = Queue.new
count1 = count2 = 0

a =
   Thread.new do
     STDERR.puts "starting thread a"
     loop do
       count1 += 1
       q.push Thread.current if count1 == 1
     end
   end

a = q.pop # wait for the thread to start
a.priority = -1


b =
   Thread.new do
     puts "starting thread b"
     q.push Thread.current
     loop do
       count2 += 1
       q.push Thread.current if count2 == 1
     end
   end

b = q.pop # wait for the thread to start
b.priority = -2

Thread.critical = 1

puts "count1: #{count1}"
puts "count2: #{count2}"



cfp:~ > ruby186 a.rb
starting thread a
starting thread b
count1: 19297
count2: 28490



cfp:~ > ruby187 a.rb
starting thread a
starting thread b
count1: 17435
count2: 24603


i'd be curious how that fairs on cygwin.  notice that the sleep is not  
required with the queues.

regards.

a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama