std.concurrency and efficient returns

[dsimcha]

== Quote from Jonathan M Davis (jmdavisprog at gmail.com)'s article
> Okay. From what I can tell, it seems to be a recurring pattern with threads that
> it's useful to spawn a thread, have it do some work, and then have it return the
> result and terminate. The appropriate way to do that seems to spawn the thread
> with the data that needs to be passed and then using send to send what would
> normally be the return value before the function (and therefore the spawned
> thread) terminates. I see 2 problems with this, both stemming from immutability.

I think the bottom line is that D's threading model is designed to put safety and
simplicity over performance and flexibility.  Given the amount of bugs that are
apparently generated when using threading for concurrency in large-scale software
written by hordes of programmers, this may be a reasonable tradeoff.

Within the message-passing model, one thing that would help a lot is a Unique type
that can be implicitly and destructively converted to immutable or shared.  In D
as it stands right now, immutable is basically useless in all but the simplest
cases because it's just too hard to build complex immutable data structures,
especially if you want to avoid unnecessary copying or having to rely on casts and
manually checked assumptions in at least small areas of the program.  In theory,
immutable solves tons of problems, but in practice it solves very few.  While I
don't understand shared that well, I guess a Unique type would help in creating
shared data, too.

There are two reasons for using multithreading:  Parallelism (using multiple cores
to increase throughput) and concurrency (making things appear to be happening
simultaneously to decrease latency; this makes sense even on a single-core
machine).  One may come as a side effect of the other, but usually only one is the
goal.  It sounds like you're looking for parallelism.  When using threading for
parallelism as opposed to concurrency, this tradeoff of simplicity and safety in
exchange for flexibility and performance doesn't work so well because:

1.  When using threading for parallelism instead of concurrency, it's reasonable
to do some unsafe stuff to get better performance, since performance is the whole
point anyhow.

2.  Unlike the concurrency case, the parallelism case usually occurs only in small
hotspots of a program, or in small scientific computing programs.  In these cases
it's not that hard for the programmer to manually track what's shared, etc.

3.  In my experience at least, parallelism often requires finer grained
communication between threads than concurrency.  For example, an OS timeslice is
about 15 milliseconds, meaning that on single core machines threads being used for
concurrency simply can't communicate more often than that.  I've written useful
parallel code that scaled to at least 4 cores and required communication between
threads several times per millisecond.  It could have been written more
efficiently w.r.t. communication between threads, but it would have required a lot
more memory allocations and been less efficient in other respects.

While I completely agree that message passing should be D's **flagship** threading
model because it's been proven to work well in a lot of cases, I'm not sure if it
should be the **only** one well-supported out of the box because it's just too
inflexible when you want pull-out-all-stops parallelism.  As Robert Jacques
mentioned, I've been working on a parallelism library.  The code is at:

http://dsource.org/projects/scrapple/browser/trunk/parallelFuture/parallelFuture.d

The docs are at:

http://cis.jhu.edu/~dsimcha/parallelFuture.html

I've been thinking lately about how to integrate this into the new threading
model, as it's currently completely unsafe, doesn't use shared at all, and was
written before the new threading model was implemented.  (core.thread still takes
an unshared delegate).  I think before we can solve the problems you've brought
up, we need to clarify how non-message passing based multithreading (i.e. using
shared) is going to work in D, as right now it is completely unclear at least to me.