http://news.squeak.org/2007/10/26/wait-for-it-the-little-book-of-semaphores/


I like it too!  :) 

Ron Teitelbaum
Squeak News Team Leader

> -----Original Message-----
> From: Peter William Lount
> 
> Rob Withers wrote:
> > Peter,
> >
> > I also want to thank you for this link:
> > http://www.greenteapress.com/semaphores/downey05semaphores.pdf
> > I started to read it after David's comment about it and it is
> > entertaining and I am learning lots.
> >
> > I also plan on using it in phase 3 of my multi-threaded vm project.
> >
> > Phase 1, my current phase, is to get all msg sends to be interceptable
> > by the SqueakElib promise framework.  This includes things that have
> > been macro transformed by the Compiler, like #ifTrue:, #ifNil:,
> > #whileTrue:, and so on. It also includes bytecodeMethods like #class
> > and #==.
> >
> > Phase 2 is to allow all primitives and bytecode methods to have a
> > promise as an argument.  Here, my plan is to stop short the primitive
> > call and send the excapsulated primitive call to the promise(s) as
> > part of a whenMoreResolved call.  When the promise resolves, the
> > primitive call will be made.  QoS can be satisfied by joining the
> > promise with a timer, such that if the promise does not resolve in xxx
> > milliseconds, it will become broken and the primitive call will "fail".
> >
> > Phase 3 is to make the Interpreter multithreaded, while protecting
> > ObjectMemory with Semaphores.  I have a quad-core chip and so I want 4
> > Interpreter threads (Vats).  Only one of them can be inside of
> > ObjectMemory at a time and that could be for purposes of allocation,
> > mutation, or GC. It's possible that a simple mutex semaphore would
> > suffice, initially.   In this model, references to objects in other
> > Vats will be ThreadRefs (a form of a FarRef) and msgs will be
> > serialized to the other Vat (reassigned the VatID in the same shared
> > ObjectMemory, or copied to a different but co-located ObjectMemory).
> >
> > I don't think having a single ObjectMemory will scale to 10's of
> > "processors", but will probably also need to be multithreaded with one
> > per Vat.  It's good from the standpoint of no shared memory.  One
> > challenge then is what if refs from 2 Vats are involved in the same
> > primitive call.  Well, memory reads don't have to be protected, unless
> > memory can be relocated, that is.  One thing at a time, I tell myself.
> >
> > I have 0 experience in this area (Interpreter+ObjectMemory), but I
> > thought it would be fun.  Your link will help tremendously.
> >
> > Cheers,
> > Rob
> >
> >
> Hi Rob,
> 
> Yeah, it's an awesome little book that cuts right to the chase. I
> particularly like that they show some solutions that won't work as
> concurrency is quite difficult and sometimes you think it's correct when
> it isn't. It's good to learn about those pitfalls.
> 
> You're plan sounds excellent. Thank you for taking up the task of making
> Squeak VM multi-threaded with native threads!
> 
> If you need anything...
> 
> All the best,
> 
> Peter
>