DMD 1.029 and 2.013 releases

[Sean Kelly]

== Quote from Russell Lewis (webmaster at villagersonline.com)'s article
> Sean Kelly wrote:
> > 1) The cost of acquiring or committing a lock is generally roughly equivalent to
> >      a memory synchronization, and sometimes less than that (futexes, etc).  So
> >      it's not insignificant, but also not as bad as people seem to think.  I suspect
> >      that locked operations are often subject to premature optimization.
> What exactly do you mean by "memory synchronization?"  Just a write
> barrier instruction, or something else?
> If what you mean is a write barrier, then what you said isn't
> necessarily true, especially as we head toward more and more cores, and
> thus more and more caches.  Locks are almost always atomic
> read/modify/write operations, and those can cause terrible cache
> bouncing problems.  If you have N cores (each with its own cache) race
> for the same lock (even if they are trying to get shared locks), you can
> have up to N^2 bounces of the cache line around.

Yeah I meant an atomic RMW, or at least a load barrier for the acquire.  Releasing
a mutex can often be done using a plain old store though, since write ops are
typically ordered anyway and moving loads up into the mutex doesn't break
anything.  My point, however, was simply that mutexes aren't terribly slower than
atomic operations, since a mutex acquire/release is really little more than an atomic
operation itself, at least in the simple case.


Sean