[phobos] Parallelism in Phobos

[David Simcha]

I have a few questions/comments about the possible inclusion of a library
for parallelism in Phobos:

1.  What is the status of std.concurrency?  It's in the source tree, but
it's not in the documentation or the changelogs.  It appears to have been
checked in quietly ~3 months ago, and I just noticed now.

2.  From reading the description of std.concurrency in TDPL it seemed more
geared toward concurrency (i.e. making stuff appear to be happening
simultaneously, useful for things like GUIs and servers) rather than
parallelism (i.e. the use of multiple CPU cores to increase throughput,
useful for things like scientific computing and video encoding).  It seems
fairly difficult (though I haven't tried yet) to write code that's designed
for pull-out-all-stops maximal performance on a multicore machine,
especially since immutability is somewhat of a straight jacket.  I find
implicit sharing and the use of small synchronized blocks or atomic ops to
be very useful in writing parallel programs.

3.  Most code where parallelism, as opposed to concurrency, is the goal (at
least most that I write) is parallelized in one or two small, performance
critical sections, and the rest is written serially.  Therefore, it's easy
to reason about things and safety isn't as important as the case of
concurrency-oriented multithreading over large sections of code.

4.  I've been eating my own dogfood for awhile on my ParallelFuture
library.  (http://cis.jhu.edu/~dsimcha/parallelFuture.html;
http://dsource.org/projects/scrapple/browser/trunk/parallelFuture/parallelFuture.d)
It's geared toward throughput-oriented parallelism on multicore machines,
not concurrency for GUIs, servers, etc. and is higher level than
std.concurrency.  Is there any interest in including something like this in
Phobos?  If so, would we try to make it fit into the explicit-sharing-only
model, or treat it as an alternative method of multithreading geared towards
pull-out-all-stops parallelism on multicore computers?

One last note:  Walter claimed a while back on the NG that Parallelfuture
doesn't compile.  I use it regularly and it compiles for me.  Walter, can
you please point out what the issue was?
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