Memory Management in D: Request for Comment

[dsimcha]

== Quote from BCS (none at anon.com)'s article
> Hello dsimcha,
> > 3.  This one is an order of magnitude less likely than the other two
> > to actually get implemented, at least by me, but how about
> > thread-local allocators so you can call malloc() without taking a
> > lock?  I vaguely remember Sean saying he was working on that a while
> > back, but I never heard anything about it again.  It's probably best
> > to wait for shared to be implemented for this so that unshared objects
> > can also be collected w/o stopping the world, but we should start at
> > least discussing this now.
> I think there are general malloc functions that are lock free. On the same
> note, I've been toying with an idea for how to do memory heap structure that
> would be totally lock free (think CAS) but has some downsides (like it won't
> handle a non-continues memory space and expanding the memory could get very
> expensive. Both of these could be non issues in some cases; short lived job
> specific heaps, memory mapped (shared?) data(base) files, resource limited
> systems, etc.
> I've been meaning to get around to implementing it but never had time. I'd
> be willing to collaborate or, if even that doesn't get me moving, just hand
> over the whole project in exchange for a "coauthor" line in the comments.


But x86 atomic ops, while not as slow as locks, are pretty slow.  Some benchmarks
I did a while back indicated that, on Windows, atomic increment is only about
twice as fast as an uncontested synchronized { var++; }.  I think using
thread-local storage and having each thread-local heap interact with the global
heap only occasionally, for very large transactions, is a much better bet than
using atomic CAS.

I'm also a little bit confused about what you're suggesting, specifically about
the non-continuous memory space.  Does this mean that things would have to be
freed LIFO?  If so, what use cases would you be covering that TempAlloc doesn't
already cover?  If not, what does it mean?