On Wednesday, 3 October 2012 at 21:02:07 UTC, David Nadlinger wrote:
> 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.

So the "process which creates the future" is a Task that executes in a different thread than the caller?  And an alternative way that a value might become available in the future is e.g. if it's being retrieved from some slow I/O process like a database or network?

>
> 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.

Ok, I'm confused here.  Why can't the process that retrieves the result be expressed as a delegate running in a TaskPool or a new thread?

>
> 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.

Maybe I'm just being naive since I don't understand the use cases, but why couldn't you just create an array of Task objects?

>
> 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).

I'm a little confused about how the callbacks would be used here.  Is the idea that some callback would be called when the task is finished?  Would it be called in the worker thread or the thread that submitted the task to the pool?  Can you provide a use case?

>
> 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).

"First one of the results" == the result produced by the the first server to return anything?

> 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.

I'll look into ContinueWith and see if it's implementable in std.parallelism without breaking anything.

>
> 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.

Right.  I wrote std.parallelism with resource-intensive units of work in mind because that's the use case I was familiar with.  It was designed first and foremost to make using SMP parallelism _simple_.  In hindsight I might have erred to much on the side of making simple things simple vs. complicated things possible or over-specialized it and avoided solving the an important, more general problem.  I'll try to understand your use cases and see if they can be addressed without making simple things more complicated.

I think the best way you could help me understand what I've overlooked in std.parallelism's design is to give a quick n' dirty example of how an API that does what you want would be used.  Even more generally, any _concise, concrete_ use cases, even toy use cases, would be a huge help.

On Wednesday, 3 October 2012 at 23:02:25 UTC, dsimcha wrote:
> So the "process which creates the future" is a Task that executes in a different thread than the caller?  And an alternative way that a value might become available in the future is e.g. if it's being retrieved from some slow I/O process like a database or network?

Yes.


>> 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.
>
> Ok, I'm confused here.  Why can't the process that retrieves the result be expressed as a delegate running in a TaskPool or a new thread?

Because you already have a system in place for managing these tasks, which is separate from std.parallelism. A reason for this could be that you are using a third-party library like libevent. Another could be that the type of workload requires additional problem knowledge of the scheduler so that different tasks don't tread on each others's toes (for example communicating with some servers via a pool of sockets, where you can handle several concurrent requests to different servers, but can't have two task read/write to the same socket at the same time, because you'd just send garbage).

Really, this issue is just about extensibility and/or flexibility. The design of std.parallelism.Task assumes that all values which "becomes available at some point in the future" are the product of a process for which a TaskPool is a suitable scheduler. C++ has std::future separate from std::promise, C# has Task vs. TaskCompletionSource, etc.


>> 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).
>
> I'm a little confused about how the callbacks would be used here.
>  Is the idea that some callback would be called when the task is finished?  Would it be called in the worker thread or the thread that submitted the task to the pool?  Can you provide a use case?

Maybe using the word "callback" was a bit misleading, but it callback would be invoked on the worker thread (or by whoever invokes the hypothetical Future.complete(<result>) method).

Probably most trivial use case would be to set a condition variable in it in order to implement a waitAny(Task[]) method, which waits until the first of a set of tasks is completed. Ever wanted to wait on multiple condition variables? Or used select() with multiple sockets? This is what I mean.

For more advanced/application-level use cases, just look at any use of ContinueWith in C#. std::future::then() is also proposed for C++, see e.g. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3327.pdf.

I didn't really read the the N3327 paper in detail, but from a brief look it seems to be a nice summary of what you might want to do with tasks/asynchronous results – I think you could find it an interesting read.

David

Ok, I think I see where you're coming from here.  I've replied to some points below just to make sure and discuss possible solutions.

On Thursday, 4 October 2012 at 16:07:35 UTC, David Nadlinger wrote:
> On Wednesday, 3 October 2012 at 23:02:25 UTC, dsimcha wrote:

> Because you already have a system in place for managing these tasks, which is separate from std.parallelism. A reason for this could be that you are using a third-party library like libevent. Another could be that the type of workload requires additional problem knowledge of the scheduler so that different tasks don't tread on each others's toes (for example communicating with some servers via a pool of sockets, where you can handle several concurrent requests to different servers, but can't have two task read/write to the same socket at the same time, because you'd just send garbage).
>
> Really, this issue is just about extensibility and/or flexibility. The design of std.parallelism.Task assumes that all values which "becomes available at some point in the future" are the product of a process for which a TaskPool is a suitable scheduler. C++ has std::future separate from std::promise, C# has Task vs. TaskCompletionSource, etc.

I'll look into these when I have more time, but I guess what it boils down to is the need to separate the **abstraction** of something that returns a value later (I'll call that **abstraction** futures) from the **implementation** provided by std.parallelism (I'll call this **implementation** tasks), which was designed only with CPU-bound tasks and multicore in mind.

On the other hand, I like std.parallelism's simplicity for handling its charter of CPU-bound problems and multicore parallelism.  Perhaps the solution is to define another Phobos module that models the **abstraction** of futures and provide an adapter of some kind to make std.parallelism tasks, which are a much lower-level concept, fit this model.  I don't think the **general abstraction** of a future should be defined in std.parallelism, though.  std.parallelism includes parallelism-oriented things besides tasks, e.g. parallel map, reduce, foreach.  Including a more abstract model of values that become available later would make its charter too unfocused.

>
> Maybe using the word "callback" was a bit misleading, but it callback would be invoked on the worker thread (or by whoever invokes the hypothetical Future.complete(<result>) method).
>
> Probably most trivial use case would be to set a condition variable in it in order to implement a waitAny(Task[]) method, which waits until the first of a set of tasks is completed. Ever wanted to wait on multiple condition variables? Or used select() with multiple sockets? This is what I mean.

Well, implementing something like ContinueWith or Future.complete for std.parallelism tasks would be trivial, and I see how waitAny could easily be implemented in terms of this.  I'm not sure I want to define an API for this in std.parallelism, though, until we have something like a std.future and the **abstraction** of a future is better-defined.

>
> For more advanced/application-level use cases, just look at any use of ContinueWith in C#. std::future::then() is also proposed for C++, see e.g. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3327.pdf.
>
> I didn't really read the the N3327 paper in detail, but from a brief look it seems to be a nice summary of what you might want to do with tasks/asynchronous results – I think you could find it an interesting read.

I don't have time to look at these right now, but I'll definitely look at them sometime soon.  Thanks for the info.

On Thursday, 4 October 2012 at 16:07:35 UTC, David Nadlinger wrote:
> For more advanced/application-level use cases, just look at any use of ContinueWith in C#. std::future::then() is also proposed for C++, see e.g. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3327.pdf.
>
> I didn't really read the the N3327 paper in detail, but from a brief look it seems to be a nice summary of what you might want to do with tasks/asynchronous results – I think you could find it an interesting read.
>
> David

Thanks for posting this.  It was an incredibly useful read for me!  Given that the code I write is generally compute-intensive, not I/O intensive, I'd never given much thought to the value of futures in I/O intensive code before this discussion.  I stand by what I said before:  Someone (not me because I'm not intimately familiar with the use cases; you might be qualified) should write a std.future module for Phobos that properly models the **abstraction** of a future.  It's only tangentially relevant to std.parallelism's charter, which includes both a special case of futures that's useful to SMP parallelism and other parallel computing constructs.  Then, we should define an adapter that allows std.parallelism Tasks to be modeled more abstractly as futures when necessary, once we've nailed down what the future **abstraction** should look like.

On Thursday, 4 October 2012 at 18:34:29 UTC, dsimcha wrote:

>
> I don't have time to look at these right now, but I'll definitely look at them sometime soon.  Thanks for the info.

You will finds this interesting too, a code snippet from Daniel
Keep.

http://www.dsource.org/projects/scrapple/browser/trunk/future/future.d

On Friday, 5 October 2012 at 10:26:57 UTC, Pragma Tix wrote:
> You will finds this interesting too, a code snippet from Daniel
> Keep.
>
> http://www.dsource.org/projects/scrapple/browser/trunk/future/future.d

The code only allows you to do something equivalent to »auto t = std.parallelism.task!dg(); t.executeInNewThread()«. So no, I don't think David, being the author of std.parallelism, would find it interesting… ;)

David