Concurrency architecture for D2

[Andrei Alexandrescu]

Walter Bright wrote:
> dsimcha wrote:
>> Ok, one thing that leads to a lot of misunderstanding and confusion is 
>> that
>> there's (as far as I can tell) absolutely no documentation of what shared
>> does/will do or what its semantics are/will be, other than a few 
>> newsgroup posts
>> from years ago,which were tentative, and the "migrating to shared" 
>> article, which
>> is mostly about thread-local storage.  Also, as far as I can tell, 
>> shared is not
>> fully implemented yet, because core.thread doesn't require a shared 
>> delegate.  Is
>> this correct?
> 
> Yes.
> 
>> Here are some questions, and this is just what I've been able to think 
>> of off the
>> top of my head in a few minutes:
>>
>> What guarantees is shared supposed to provide?
> 
> Shared means that multiple threads can access the data. The guarantee is 
> that if it is not shared, and not immutable, that only the current 
> thread can see it.
> 
>> What does shared have to do with synchronization?
> 
> Only shared data can be synchronized. It makes no sense to synchronize 
> thread local data.
> 
>> What does shared have to do with memory barriers?
> 
> Reading/writing shared data emits memory barriers to ensure sequential 
> consistency (not implemented).
> 
>> What are the semantics of casting FROM unshared TO shared?
> 
> You'd better be sure there are no other unshared references to that same 
> data.
> 
>> What are the semantics of casting FROM shared TO unshared?
> 
> You'd better be sure there are no other shared references to that same 
> data.
> 
>> Why doesn't shared appear anywhere in core.thread?
> 
> Because nobody has spent the effort to do it.

Thanks Walter for doing this. These questions and their answers could 
start an FAQ page on D's website.

Andrei