From: Morton Goldberg <m_goldberg@...>
Date: 2007-09-20T19:23:52+09:00
Subject: Re: thread switching? (p.  144 pickaxe2)

On Sep 20, 2007, at 12:19 AM, 7stud -- wrote:

> On p. 144 in pickaxe2, there are two examples using threads(see  
> below).
> Each thread updates a counter, and the function call that updates the
> counter is synchronized to prevent simultaneous axis to the counter.

Since you are picky enough to complain about the writing in Pickaxe  
2, I feel justified in pointing out that your "axis" should be "access".

> In any case, why doesn't thread switching happen in the example on p.
> 145:

There are two problems with this code.

> playlist = []
> playlist.extend(MonitorMixin)
>
> #Player thread
> Thread.new do
>     record = nil
>     loop do
>         puts "*"
>         playlist.synchronize do

1. The "while playlist.empty?" causes deadlock.

>             sleep 0.1 while playlist.empty?
>             record = playlist.shift
>         end
>         puts record
>     end
> end
>
> #Customer request thread
> Thread.new do
>     loop do
>         puts "-"
>         playlist.synchronize do
>             playlist << "hello"
>         end
>     end
> end

2. The main thread exits immediately killing both child threads.

The following will run for 2 seconds before dying.

<code>
#! /usr/bin/env ruby -w

require "monitor"

playlist = []
playlist.extend(MonitorMixin)
pending = playlist.new_cond

Thread.abort_on_exception = true

Thread.new do
    record = 'none'
    loop do
       puts "*"
       playlist.synchronize do
          pending.wait_while { playlist.empty? }
          record = playlist.shift
       end
       p record
    end
end

Thread.new do
    loop do
       puts "-"
       playlist.synchronize do
          sleep 0.5
          playlist << "hello"
          pending.signal
       end
       p playlist
    end
end

sleep 2
</code>

Regards, Morton