From: Robert Klemme <shortcutter@...>
Date: 2010-07-01T18:10:58+09:00
Subject: Re: Synchronized Circular Buffer

2010/7/1 Sam Dalton <mail@samdalton.co.nz>:
> I'm relatively new to Ruby, having switched from a PHP and Java
> background, and I'm having some trouble with the Monitor class trying to
> implement a synchronised circular buffer. The other posts here on
> synchronised methods aren't quite relevant enough unfortunately.
>
> My current implementation is the following
>
> require 'monitor'
>
> class SynchronisedBuffer < Monitor
>
>   def initialize(capacity)
>      @capacity = capacity
>      @front = 0
>      @back = 0
>      @elements = Array.new(capacity)
>      @empty_cond = new_cond
>      @full_cond = new_cond
>      super()
>   end
>
>   def get
>       @empty_cond.wait_while {empty?}

The line above must be moved into the synchronized block.

>       element = nil

No need for this.

>       synchronize do
>           element = @elements[@front]
>           @elements[@front] = nil
>           @front = (@front + 1) % @capacity
>           @full_cond.signal
>        end

Just move the next line into the block.

>        return element
>   end
>
>   def put(element)
>       @full_cond.wait_while {full?}

The line above must be moved down 1 line.

>       synchronize do
>           @elements[@back] = element
>           @back = (@back + 1) % @capacity
>           @empty_cond.signal
>        end
>   end
>
>   def full?
>       result = false
>       synchronize do
>           result = (@front == @back and @elements[@front] != nil)
>        end
>        return result

This is sufficient:

def full?
  synchronize do
    (@front == @back and @elements[@front] != nil)
  end
end

>   end
>
>   def empty?

Same as above - saves you some typing. :-)

>       result = false
>       synchronize do
>           result = (@front == @back and @elements[@front] == nil)
>        end
>        return result
>   end
>
> end
>
> This has been adapted from a version I had written in Java. The problem
> is that when I have a thread call 'get' on the buffer, I receive a
> ThreadException saying 'current thread not owner'

You need to be holding the lock when waiting on a condition variable
(same story in Java btw).

> This is being thrown by mon_check_owner, and a bit of poking around
> shows that it fails because mon_owner is set to nil in the condition
> '@mon_owner != Thread.current'.
>
> I am using the buffer for a simple producer/consumer web server, like so
>
> buffer = SynchronisedBuffer.new(10)
> workers = []
>
> for i in (1..10)
>   workers[i] = Worker.new(buffer)
> end
>
> while socket = server.accept
>    buffer.put(socket)
> end
>
> I'm writing this purely to gain a better understanding of some important
> ruby classes like thread, monitor and socket, and am aware that there
> are other ruby web servers I could use straight away :). I'm also trying
> to understand the 'ruby way' of coding, so any comments as to how the
> above code might better be written would be really appreciated.

That's usually a good thing to do.  I do it myself all the time.
Nothing beats one's own experience.

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/