[phobos] std.parallelism: Request for review/comment

[David Simcha]

  On 8/31/2010 3:07 PM, Sean Kelly wrote:
> A few general comments:
>
> I'm wondering whether the use of a TaskPool should always be explicit.  Perhaps a default pool should be considered to exist, and calling parallel() instead of pool.parallel() will use that?  It's rare for apps to have multiple distinct pools anyway, so this may be an easy way to help obscure a tiny bit more of the implementation.

Ok, I've done the following:

1.  There is now a default pool that is initialized lazily with the 
default number of threads.  The function that retrieves it is called 
taskPool().

2.  Just to save typing and make the API a little more elegant, I added 
a free function parallel() as a synonym for taskPool.parallel().

3.  I don't want to make map() and reduce() free functions that forward 
to taskPool.map() and taskPool.reduce() because these would collide with 
std.algorithm which I expect would also be imported in most modules 
using std.parallelism.  This would force use of qualified names and not 
actually be any less verbose than just typing taskPool.map().  I could 
make parallelMap() and parallelReduce() functions, but these are not any 
less verbose.

4.  For tasks, I added a little syntactic sugar:

// Submit to default pool.
auto myTask = task!myFun(args).submit();

// Submit to explicitly instantiated pool.
auto pool = new TaskPool();
auto myTask = task!myFun(args).submitTo(pool);

Unfortunately, core.cpuid is still kind of buggy, so on a lot of newer 
Intel machines the default pool will have the wrong number of threads 
until this gets fixed.  If anyone can think of a decent API, I can make 
the number of threads in the default pool configurable.