October 02, 2012
Hi,
I am tempted to start D programming but for me it is crucrial to be able to parallelize for-loops as can be done with openMP for C/C++ (mainly @pragma omp parallel for, @pragma omp critical).
I have already seen the std.parallelism library but I'm unsure whether it can provide me with the same functionality.

Thanks
October 02, 2012
On Tuesday, 2 October 2012 at 19:15:19 UTC, Farmer wrote:
> Hi,
> I am tempted to start D programming but for me it is crucrial to be able to parallelize for-loops as can be done with openMP for C/C++ (mainly @pragma omp parallel for, @pragma omp critical).
> I have already seen the std.parallelism library but I'm unsure whether it can provide me with the same functionality.
>
> Thanks

It can. Here's an example from the docs of parallelising a simple for loop:

auto logs = new double[10_000_000];
foreach(i, ref elem; taskPool.parallel(logs, 100))
{
    elem = log(i + 1.0);
}

This creates a pool of workers that each perform 100 iterations of the loop body in parallel.
October 02, 2012
And is there also a pragma omp critical analogon?

On Tuesday, 2 October 2012 at 20:16:36 UTC, Peter Alexander wrote:
> On Tuesday, 2 October 2012 at 19:15:19 UTC, Farmer wrote:
>> Hi,
>> I am tempted to start D programming but for me it is crucrial to be able to parallelize for-loops as can be done with openMP for C/C++ (mainly @pragma omp parallel for, @pragma omp critical).
>> I have already seen the std.parallelism library but I'm unsure whether it can provide me with the same functionality.
>>
>> Thanks
>
> It can. Here's an example from the docs of parallelising a simple for loop:
>
> auto logs = new double[10_000_000];
> foreach(i, ref elem; taskPool.parallel(logs, 100))
> {
>     elem = log(i + 1.0);
> }
>
> This creates a pool of workers that each perform 100 iterations of the loop body in parallel.


October 02, 2012
On Tuesday, 2 October 2012 at 21:13:33 UTC, Farmer wrote:
> And is there also a pragma omp critical analogon?

For critical sections you could use a low-level mutex. I don't do much parallel stuff in D, so I don't know if this is the preferred way, but it's an option.

http://dlang.org/phobos/core_sync_mutex.html

import std.stdio;
import std.parallelism;
import std.math;
import core.sync.mutex;
void main()
{
	auto logs = new double[1_000_000];
	double x = 0.0;
	Mutex m = new Mutex();
	foreach(i, ref elem; taskPool.parallel(logs, 100))
	{
		elem = log(i + 1.0);
		m.lock();
		x += 1.0;
		m.unlock();
	}
}
October 03, 2012
On Tue, 2012-10-02 at 23:13 +0200, Farmer wrote:
> And is there also a pragma omp critical analogon?

No. The D approach in std.parallelism is to offer explicit parallel constructs that also work on a uniprocessor. The OpenMP approach is to provide meta-data to allow the compiler to parallelize sequential code.

Although OpenMP is high-profile in the C, C++ and Fortran world, it's raison d'être is to be able to use sequential code in a parallel context. Now that C++ has made the jump to using futures and asynchronous function calls as an integral part of the language, I think we will see it drift away from the OpenMP style camp much more towards the TBB style camp. I am not sure if TBB is the right framework, but a drift twowards frameworks that are parallel first and sequential as a special case does seem to be on the cards.

SO rather than working with annotation based systems such as OpenMP, I think D is right to be using parallel map, parallel reduce as functions rather than seeing explicit iterations. Functional languages have always had this. Python, Groovy, Ruby, Clojure brought the ideas to mainstream platforms, as did Scala and C++ (cf. std::foreach). Java 8 will take up this approach. So library based iteration is with us as the tool for abstracting away the details. Obviously we will still have explicit iteration in imperative languages but significantly less than in the past.

To summarize: OpenMP and parallelizing explicit iteration is backward looking, library calls parallelizing using implicit iteration is the future.

-- 
Russel. ============================================================================= Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder@ekiga.net 41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel@winder.org.uk London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder


October 03, 2012
On Wed, 03 Oct 2012 09:08:47 +0100
Russel Winder <russel@winder.org.uk> wrote:
> Now that C++ has made the jump to using futures and asynchronous function calls as an integral part of the language,

Speaking of, do we have futures in D yet? IIRC, way back last time I asked about it there was something that needed taken care of first, though I don't remember what. If we don't have them ATM, is there currently anything in the way of actually creating them?

October 03, 2012
Unless we're using different terminology here, futures are just std.parallelism Tasks.

On Wednesday, 3 October 2012 at 10:17:41 UTC, Nick Sabalausky wrote:
> On Wed, 03 Oct 2012 09:08:47 +0100
> Russel Winder <russel@winder.org.uk> wrote:
>> Now that C++ has made the jump to using futures and
>> asynchronous function calls as an integral part of the language,
>
> Speaking of, do we have futures in D yet? IIRC, way back last time I
> asked about it there was something that needed taken care of first,
> though I don't remember what. If we don't have them ATM, is there
> currently anything in the way of actually creating them?


October 03, 2012
On Wednesday, 3 October 2012 at 14:10:57 UTC, dsimcha wrote:
> Unless we're using different terminology here, futures are just std.parallelism Tasks.

No, std.parallelism.Tasks are not really futures – they offer a constrained [1] future interface, but couple this with the notion that a Task can be executed at some point on a TaskPool chosen by the user. Because of this, I had to implement my own futures for the Thrift async stuff, where I needed a future as a promise [2] by an invoked entity that it kicked off a background activity which will eventually return a value, but which the users can't »start« or choose to »execute it now«, as they can with Tasks.

If TaskPool had an execute() method which took a delegate to execute (or a »Task«, for that matter) and returned a new object which serves as a »handle« with wait()/get()/… methods, _that_ would (likely) be a future.

David


[1] Constrained in the sense that it is only meant for short-/synchronous-running tasks and thus e.g. offer no callback mechanism.

[2] Let's not get into splitting hairs regarding the exact meaning of »Future« vs. »Promise«, especially because C++11 introduced a new interpretation to the mix.
October 03, 2012
Ok, now I vaguely remember seeing stuff about futures in your Thrift code and wondering why it was there.  I'm a little big confused about what you want.  If I understand correctly, std.parallelism can already do it pretty easily, but maybe the docs need to be improved a little to make it obvious how to.

All you have to do is something like this:

auto createFuture() {
    auto myTask = task!someFun();  // Returns a _pointer_ to a Task.
    taskPool.put(myTask);  // Or myTask.executeInNewThread();

    // A task created with task() can outlive the scope it was created in.
    // A scoped task, created with scopedTask(), cannot.  This is safe,
    // since myTask is NOT scoped and is a _pointer_ to a Task.
    return myTask;
}

In this case myTask is already running using the execution resources specified in createFuture().  Does this do what you wanted?  If so, I'll clarify the documentation.  If not, please clarify what you needed and the relevant use cases so that I can fix std.parallelism.

On Wednesday, 3 October 2012 at 15:50:38 UTC, David Nadlinger wrote:
> On Wednesday, 3 October 2012 at 14:10:57 UTC, dsimcha wrote:
>> Unless we're using different terminology here, futures are just std.parallelism Tasks.
>
> No, std.parallelism.Tasks are not really futures – they offer a constrained [1] future interface, but couple this with the notion that a Task can be executed at some point on a TaskPool chosen by the user. Because of this, I had to implement my own futures for the Thrift async stuff, where I needed a future as a promise [2] by an invoked entity that it kicked off a background activity which will eventually return a value, but which the users can't »start« or choose to »execute it now«, as they can with Tasks.
>
> If TaskPool had an execute() method which took a delegate to execute (or a »Task«, for that matter) and returned a new object which serves as a »handle« with wait()/get()/… methods, _that_ would (likely) be a future.
>
> David
>
>
> [1] Constrained in the sense that it is only meant for short-/synchronous-running tasks and thus e.g. offer no callback mechanism.
>
> [2] Let's not get into splitting hairs regarding the exact meaning of »Future« vs. »Promise«, especially because C++11 introduced a new interpretation to the mix.

October 03, 2012
On Wednesday, 3 October 2012 at 19:42:07 UTC, dsimcha wrote:
> If not, please clarify what you needed and the relevant use cases so that I can fix std.parallelism.

In my use case, conflating the notion of a future, i.e. a value that becomes available at some point in the future, with the process which creates that future makes no sense.

For example, let's say you are writing a function which computes a complex database query from its parameters and then submits it to your query manager/connection pool/… for asynchronous execution. You cannot use std.parallelism.Task in this case, because there is no way of expressing the process which retrieves the result as a delegate running inside a TaskPool.

Or, say you want to write an "aggregator", combining the results of several futures together, again offering the same future interface (maybe an array of the original result types) to consumers. Again, there is no computation-bound part to that at all, which would make sense to run on a TaskPool – you are only waiting on the other tasks to finish.

The second problem with std.parallelism.Task is that your only choice is polling (or blocking, for that matter). Yes, callbacks are a hairy thing to do if you can't be sure what thread they are executed on, but not having them severely limits the power of your abstraction, especially if you are dealing with non-CPU-bound tasks (as many of today's "modern" use cases are).

For example, something my mentor asked to implement for Thrift during last year's GSoC was a feature which allows to send a request out to a pool of servers concurrently, returning the first one of the results (apparently, this mechanism is used as a sharding mechanism in some situations – if a server doesn't have the data, it simply ignores the request). How would you implement something like that as a function Task[] -> Task? For what it's worth, Task in C# (which is quite universally praised for its take on the matter) also has a »ContinueWith« method which is really just a completion callback mechanism.

std.parallelism.Task is great for expressing local resource-intensive units of work (and fast!), but I think it is to rigid and specialized for that case to be generally useful.

David
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