[phobos] Parallelism in Phobos

[Andrei Alexandrescu]

Hello,

Here's a belated answer to your question (hectic times prevented me  
from tending to non-urgent email).

I think a parallel library would be great to have as indeed phobos is  
geared at general concurrency. Such a lib would also expose bugs and  
weaknesses in our model and its implementation.

Andrei

Sent by shouting through my showerhead.

On May 30, 2010, at 12:54 PM, David Simcha <dsimcha at gmail.com> wrote:

> 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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