Continuing to catch on with older email...
David, the intent of shared is to prevent sharing of everything that
isn't shared. I didn't get to review your parallelism library yet, but I
think it's likely your library uses things that shouldn't actually work
:o). If it does, we should work towards making the type system better to
accept your code without inadvertent sharing.
Andrei
On 07/31/2010 11:35 PM, David Simcha wrote:
I've started thinking about how to make ParallelFuture jive with D's new
threading model, since it was designed before shared and std.concurrency
were implemented and is basically designed around default sharing.
(core.thread takes a non-shared delegate, and allows you to completely
bypass the shared system, and from what I remember of newsgroup
discussions, this isn't going to change.)
I've re-read the concurrency chapter in TDPL and I'm still trying to
understand what the model actually is for shared data. For example, the
following compiles and, IIUC shouldn't:
shared real foo;
void main() {
foo++;
}
I guess my high-level question that I'm *still* not quite getting is
"What is shared besides a piece of syntactic salt to make it harder to
inadvertently share data across threads?"
Secondly, my parallel foreach loop implementation relies on sharing the
current stack frame and anything reachable from it across threads. For
example:
void main() {
auto pool = new TaskPool;
uint[] nums = fillNums();
uint modBy = getSomeOtherNum();
foreach(num; pool.parallel(nums)) {
if(isPrime(num % modBy)) {
writeln("Found prime number: ", num % modBy);
}
}
}
Allowing stuff like this is personally useful to me, but if the idea is
that we have no implicit sharing across threads, then I don't see how
something like this can be implemented. When you call a parallel
foreach loop like this, **everything** on the current stack frame is
**transitively** shared. Doing anything else would require a complete
redesign of the library. Is calling pool.parallel enough of an explicit
asking for "here be dragons" that the delegate should simply be cast to
shared? If not, does anyone see any other reasonable way to do parallel
foreach?
On 7/31/2010 7:31 AM, Andrei Alexandrescu wrote:
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 <[email protected]
<mailto:[email protected]>> 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://cis.jhu.edu/%7Edsimcha/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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