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alternative separate thread implementation of shared variables


The standard shared variable scheme used by Perl, is based on tie-ing the variable to some very special dark magic. This dark magic ensures that shared variables, which are copied just as any other variable when a thread is started, update values in all of the threads where they exist as soon as the value of a shared variable is changed.

The Thread::Tie module is a proof-of-concept implementation of another approach to shared variables. Instead of having shared variables exist in all the threads from which they are accessible, shared variable exist as "normal", unshared variables in a separate thread. Only a tied object exists in each thread from which the shared variable is accessible.

Through the use of a client-server model, any thread can fetch and/or update variables living in that thread. This client-server functionality is hidden under the hood of tie(). So you could say that one dark magic (the current shared variables implementation) is replaced by another dark magic.

The Thread::Tie approach has the following advantages:

 * Memory usage - This implementation circumvents the memory leak that
   currently (threads::shared version 0.90) plagues any shared array or shared
   hash access.
 * Tieing shared variables - Because the current implementation uses tie-ing,
   you can not tie a shared variable. The same applies for this implementation
   you might say. However, it is possible to specify a non-standard tie
   implementation for use within the thread. So with this implementation you
   can tie() a shared variable. So you could tie a shared hash to a DBM file à
   la dbmopen() with this module.

Of course there are disadvantages to this approach:

 * Pure Perl implementation - This module is currently a pure Perl
   implementation. This is ok for a proof of concept, but may need re-
   implementation in pure XS or in Inline::C for production use.

 * Tradeoff between cpu and memory - This implementation currently uses (much)
   more cpu than the standard shared variables implementation. Whether this
   would still be true when re-implemented in XS or Inline::C, remains to be

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