From: Rick DeNatale <rick.denatale@...>
Date: 2006-10-06T02:46:20+09:00
Subject: Re: Associating data with a function

On 10/5/06, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:

> Watch out for threads, then. If your program can have multiple threads
> running these calculations, a mutex will keep them from fighting over
> the expensive objects.

This got me thinking about the idea of thread local instance variables.

Thread#[] and Thread#[]= give you 'global' thread locals, here's an
UNTESTED 'SKETCH' of a class which would let you do something like:

class Foo

   def meth1
        @tliv1 = ThreadLocalInstanceVariable.new {ExpensiveVariable1.new}
        @tiiv2 = ThreadLocalInstanceVariable.new {ExpensiveVariable2.new}
        .. do something with those expensive variables
   end
...
end

rick@frodo:/public/rubyscripts$ cat thread_local.rb
require 'delegator'
class ThreadLocalInstanceVariable < Delegator

  def initialize(&init_block)
    super
    @init_block = init_block
  end

  def __get_obj__
    # thread local instance variable proxies are kept in a hash
    # which is itself a thread local variable
    iv_hash = Thread.current[:tl_iv_hash] || = Hash.new
    local_iv = iv_hash[self.object_id]
    unless local_iv
      # store using object_id to avoid direct reference
      my_id = self.object_id
      # clean out when local iv gets GC'd, e.g. when the instance holding
      # it dies
     ObjectSpace.define_finalizer(self, lambda {iv_hash.delete(my_id)})
      local_iv = iv_hash[self.object_id] = init_block.call
    end
    local_iv
  end

end



-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/