From: Brian Candler <b.candler@...>
Date: 2011-01-02T05:08:59+09:00
Subject: Re: Newbie: Thread variables in Ruby Threads

It's not consistent. In my case, running your program I got

8, 7, 6, 5, 4, 3, 2, 1, 0, 9, count = 10

The thing is: you create 10 threads in quick succession. They all sleep 
for 10 seconds. Suddenly they all start waking up, storing the current 
value of count and incrementing it.

You can't predict what order they'll all wake up in. If you change 
10.times to 100.times this might be clearer.

Alternatively, try changing sleep(1) to sleep(0.1*i) and it will be a 
lot more predictable (although not entirely guaranteed).

However, the problems are worse than that. You might not even end up 
with count=10 at the end! This is because the operation

  count += 1

is not atomic. It is really count = count + 1, which translates as
1. read the current value of count into a register
2. add 1 to the value in the register
3. write the register back to count

Let's say count=5 at some point in time, and two threads are ready to 
run.

The first does step 1, reading value 5, and then is pre-empted by the 
second thread. The second thread reads value 5, and writes value 6. The 
first thread then wakes up and carries on. It also calculates 6 and 
writes this to count.

So you need to protect the read-update-write operation with a mutex.

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