It's time for another Parallel Haskell Digest! Unfortunately, this may just be our last one, at least within the context of the Parallel GHC project. That said, we may as a community be at the very beginnings of Haskell as the language of choice for your parallel and concurrent needs. Maybe we need to keep something like the Digest going to help our little FP monster through its infancy? Any volunteers in the community? If you're interested in picking up the torch, please give us a shout!

Otherwise, if you can't take on a (perhaps rotating) digest commitment, but still want to help, would you be kind enough to fill out a small survey on the digest? There are just five questions on it, plus a feedback form. Anything you can say will help those of us in the Secret Haskell Propaganda Commitee to fine tune our efforts:

Parallel Haskell Digest Survey

It's been a fantastic year for me, working on the Parallel GHC project, learning about all sorts of neat ideas and technologies (as a basic parallel-naive Haskeller), and trying to reflect them back in a way that hopefully helps the broader community. Thanks to all of you in the parallel Haskell world first for cranking out all this great stuff for us to use, and second for your patience and support. Thanks especially to my follow Well-Typed-ers for all the fun chats, the feedback on drafts, and help getting up to speed.

One last thing before signing off as your Parallel Haskell Digester. While the digest may be coming to an end, there will at least be one encore! It turns out we had so much to say in our last word of the month, that we'll have to put in in a follow-up posting. In the meantime, we'll just leave you with a little teaser…

News

Announce: Haskell Platform 2012.2.0.0 (3 Jun) The new Haskell Platform is out! If you've been waiting for Haskell Platform before moving on GHC 7.4, now's a great time to upgrade. Of particular interest to parallel Haskellers, this latest GHC offers better profiling flags, multicore profiling, vastly improved DPH, event logging [allows ThreadScope spark profiling], and more convenient RTS flags.

Introducing FP Complete (6 Jun) You might have Bartosz Milewski around. If not, have a look at the Downfall of Imperative Programming. Bartosz posted a quick message introducing himself to the community along with the new company FP Complete, which aims to commercialise Haskell. Bartosz believes that “now is the right time for Haskell to become a strong software industry player, especially that functional programming is being widely recognized as the answer to the recent multicore and GPU explosion.” We'll hopefully find out more about FP Complete have in mind as their plans stabilise a bit.

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Word of the month (teaser!)

The word of the month series has given us a chance to survey the arsenal of Haskell parallelism and concurrency constructs:

some low level foundations (sparks and threads),

three ways to do parallelism (parallel arrays, strategies, dataflow),

and some concurrency abstractions (locks, transactions, channels)

The Haskell approach has been to explicitly recognise the vastness of the parallelism/concurrency space, in other words, to provide a multitude of right tools for a multitude of right jobs. Better still, the tools we have are largely interoperable, should we find ourselves with jobs that don't neatly fit into a single category.

The Haskell of 2012 may be in a great place for parallelism and concurrency, but don't think this is the end of the story! What we've seen so far is only a snapshot of the technology as it hurtles through the twenty-tens (How quaint are we, Future Haskeller?). While we can't say what exactly the future will bring, we can look at one of the directions that Haskell might branch into in the coming decade. The series so far has focused on things you might do with a single computer, using parallelism to speed up your software, or using concurrency abstractions to preserve your sanity in the face of non-determinism. But now what if you have more than one computer?

Our final word of the month is actor. Actors are not specific to distributed programming; they are really more of a low level concurrency abstraction on a par with threads. And they certainly aren't new either. The actor model has been around since the early 70s at least, and has been seriously used for distributed programming since the late 80s with Erlang. Can you guess where this word of the month is going? We have a bit more to say about it shortly, so while this is the last Parallel Haskell Digest, watch this space for the final word of the month :-)

Parallel GHC project update

Our work on the distributed-process implementation of Cloud Haskell continues apace. We're almost there, having implemented most of the API described in the original Epstein et al paper, except for node configuration and initialisation. We are very excited to be getting this out of the door soon and into your hands. In fact, we've even submitted a proposal to present this work at the upcoming Haskell Implementors Workshop; so hopefully you'll be able to join Duncan and Edsko in Copenhagen and catch up on the Cloud Haskell news.

As for ThreadScope, we last mentioned that we were working to make use of information from hardware performance counters (specifically, Linux Perf Events). This took a bit more work and trickier GHC patches than we had anticipated, but it does seem to be in order now and we are now in the testing phase for the next release. The next ThreadScope release will also include the use of heap statistics from the (eventual) GHC 7.6 RTS, and some user interface enhancements suggested by our users.

Tutorials

Parallel and Concurrent Haskell Course slides Looks like the recent Summer School was a great time for all! We had expert Haskellers talking parallelism and concurrency, a chateau, good food, and lots of wine. Did you miss out? We can't help with the wine, but if you'd like to get in on some of the parallel action, check out Simon's slides. There are seven lectures in all, and some lab exercises to go with them: Basic pure parallelism The Par Monad Basic concurrency Software Transactional Memory Concurrent network servers Distributed programming GPU programming with Accelerate



Threading and Gtk2Hs “So you're writing a Gtk2Hs application and you need to do some threading.” Daniel Wagner has just the tutorial for you. The post comes in two parts. First Daniel gives it all away with two keys points (and a simple concrete example): Make all Gtk calls from the main thread. If other threads need to affect the interface, use postGUIAsync or postGUISync to send your code to the main thread. Link your program with the threaded runtime system by passing GHC the -threaded option at link time. For readers who want to learn more, Daniel then goes into much more depth: the things that threading hard both in its own right and when working in Gtk2Hs in particular; the perils of using unsafe FFI imports (as opposed to safe ones); a peek into the Gtk2Hs guts showing its interaction with Gtk and glib; and finally, some of the possible pitfalls, wrong things Daniel believed when he started and what he now believes instead.



Blogs and packages

GHC-7.4.2-Eden - Parallel Haskell on multicore and cluster systems (17 Jun) There is more than one way to do parallelism and distributed programming in Haskell. Mischa Dieterle announced a new release of Eden, an extension of Haskell which is “tailored for distributed systems but works equally well on multicore architectures”. These extensions consist of a small number of constructs for working with processes (processes work within disjoint address spaces and do not share any data). Eden provides automatic process handling to reduce the amount of low-level detail needed to implement parallel algorithms, but also allows for the explicit control you may need to get good performance. You can either install the full Eden system including the compiler (GHC with the Eden parallel runtime system), libraries and tools; or just install a thread simulation by using a standard GHC to install the libraries and tools off Hackage.

Forklift - a pattern for performing monadic actions in a worker thread (7 Jun) Apfelmus recently noticed a recurring applied Haskell puzzle: how do you combine two IO-wrapping monads that don't lend themselves to being combined via monad transformers? This arose in the context of Shae Erisson's Summer of Code Project (Web based GHCi) which uses webserver and interpreter packages providing monads of their own. A typical approach would be to run separate threads and have the two communicate using something like an MVar ; but each time reinvent this solution, we end up creating some mini communication protocol specific to the task. Apfelmus suggests a more generic variant of this approach, which he calls the “forklift pattern” because it consists in forking a worker thread to lift arbitrary monadic actions into IO. data ForkLift m = ForkLift requests :: Chan (m ()) carry :: MonadIO m => ForkLift m -> m a -> IO a The worker thread maintains a queue of requests. To run an arbitrary action of type m a , you “carry” it over into IO with a helper function that wraps it up in an MVar sandwich, sticks it on the queue, and reads the MVar to get the result back. See Apfelmus's posting to see this cute trick in action.

The Flavours of MVar (4 Jun) Neil Mitchell finds that that the flexibility of the MVar can leave some room for confusion: both taking from and putting to an MVar one can block, whereas it is likely that you only expect it to block on one of those. In a quick and practical tutorial, Neil whips through three MVar patterns that he tends to use regularly: lock guaranteed single-threaded access to some resource var thread-safe mutable variables that never block on put barrier starts empty, is written to once, then read one or more times. Building on these Neil shows a couple of examples of how one might go one to combine these MVar uses to to get higher level abstractions: an action that can be invoked multiple times but runs at most once, and a queue that collects messages individually and delivers them in bulk. Check his post out, and maybe see why join $ modifyMVar … is becoming one of his favourite idioms.

Being more clever about vectorising nested data parallelism (4 Jun) Manuel Chakravarty tumbles: Our new draft paper on Vectorisation Avoidance introduces a novel program analysis for nested data parallelism that lets us avoid vectorising purely scalar subcomputations. It includes a set of benchmark kernels that suggest that vectorisation avoidance improves runtimes over merely using array stream fusion.

Repa 3: more control over array representations with indexed types (4 Jun) Another paper from the UNSW parallel Haskellers: We have got a new draft paper on Guiding Parallel Array Fusion with Indexed Types. It describes the design and use of the 3rd generation Repa API, which uses type indices to give the programmer control over the various parallel array representations. The result are clearer programs that the compiler can more easily optimise. The implementation of Repa 3 is ready for use on Hackage in the repa package.

Protocol Buffers 2.0.7 (19 May) Chris Kuklewicz is back! In the last digest, some folks in the community were looking for him because they had patches for the protocol buffers package family. Chris has not only resurfaced, but released an update to the packages, making them compile with GHC 7.4.1 and handle missing package names better.

Generics and Protocol Buffers (20 May) Nathan Howell thinks the protocol-buffers package is great: full-featured, well-tested, no complaints about performance. However, maintaining .proto files is “more than just a chore”. The hprotoc tool could help but is trickyp to integrated properly into the build system. Maybe there's another way, one which does not involve separate files or build tools. Have a look at his GitHub Gist for a promising alternative solution using the type-level library.

SafeSemaphore (2 Jun) Chris Kuklewicz has a problem, a solution, and a plea for help. Problem: Control.Concurrent.QSem (and QSemN, SampleVar) are broken (they provide no exception safety). Solution: his SafeSemaphore package, just updated to 0.90, with several safer alternatives. Plea: Would it be possible to replace parts of GHC with SafeSemaphore, so as to unbreak the Haskell Platform? It looks like Chris' plea has been heard, as Simon Marlow has recently suggested importing the STM version for GHC 7.6.1



Paraiso (7 Jun) Kazu Yamamoto announced a couple of new parallel libraries from Japan. The first is Paraiso (by Takayuki Muranushi of Monadius fame), a high-level language for implementing explicit partial-differential equations solvers on supercomputers as well as today's advanced personal computers.

GTALib (7 Jun) Also from the Japan Parallel Haskell workld, is GTALib by Kento Emoto. It provides core functionalities of the GTA programming framework described in the paper Generate, Test, and Aggregate A Calculation-based Framework for Systematic Parallel Programming with MapReduce

Mailing lists

How to write Source for TChan working with LC.take? (20 May) Hiromi Ishii is writing a Data.Conduit Source that supplies its values from a TChan . He has three versions, one using the raw Pipe constructors directly, one using sourceState , and one using yield . The `yield' version does not seem to work as expected. At first this seemed like an unfortunate necessity, but after putting some thought into it, Michael Snoyman proposed some modifications to conduit's await/yield functions, which should allow Hiromi to write things in the intuitive way.

Parallel cooperative multithreading? (22 May) Benjamin Ylvisaker was wondering if it'd be possible to implement something like Observationally Cooperative Multithreading (OCM) in Haskell. The paper discusses Lua, C, and C++ implementations. Ben thinks that Haskell would be an awesome fit such a framework. The premise behind OCM is that cooperative concurrency can be easier than preemptive concurrency, because you can use reason sequentially between invocations of pause/yield/wait. Historically, it has only worked on single processors, because the blocks of code between the p/y/w calls need to be run atomically. Recent research means we know more about how to efficiently run blocks of code atomically, so maybe cooperative concurrency can make a comeback? Ryan Newton thinks the comeback is indeed happening. He points in the Haskell world to monad-par's use of ConT as an example of a a framework in which tasks cooperatively yield control whenever their desired input data is not yet available. Mario Blažević has also thought about cooperative concurrency in Haskell, particularly in context of his monad-coroutine library; however, he found no speedups when he added support for running multiple co-routines in parallel. Ketil Malde is sceptical that the proposed approach would be better than using STM. He wonders if the paper's critique of STM applies to implementations that keep transactional data are segregated by the type system.

How to translate Repa 2 program to efficient Repa 3 code? (26 May) Michael Serra posted a StackOverflow thread asking about the differences between Repa 2 and 3 APIs. He has some simple image convolution tests with which run fast enough in Repa 2 with judicious use of force . But when translating the tests to Repa 3, he can't quite work out how to get the same kind of performance. See the thread on StackOverflow for more details. In short, computeP is the new force .

Is Repa suitable for boxed arrays?... (3 Jun) Stuart Hungerford needs to build a 2D Array of boxed Haskell values. He's attracted to Repa, but couldn't work out from the documentation if it would work with arbitrary values. Moreover, getting the examples to work. Ben Lippmeier replies that it should work (the array type would be something like Array V DIM2 Float ). The documentation is out of date (it's for Repa 2 and Repa 3 is different). Until somebody gets a chance to update the documentation, try the Repa 3 paper, Ben just submitted for Haskell Symposium 2012.

Status and roadmap for Cloud Haskell? (19 May) Ben Lee is very interested in the our work at Well-Typed on distributed-process , the followup implementation of Cloud Haskell. How's progress? As mentioned in the Parallel GHC news above, we're almost there! Edsko de Vries says that so far we've been focusing on two aspects of the new implementation, the design of the transport API, and a robust TCP implementation to sit on top of it. These two parts are nearly done. Meanwhile, we've been laying down some documentation on our GitHub project wiki. If you want to help out, we'd love if you could play with the TCP transport, and try to write some transports of your own.

`anyP' in DPH? (21 May) Rob Stewart is trying to find the anyP function for Data Parallel Haskell. Ben Lippmeier says that DPH is in flux at the moment and that the current user facing API can be found in dph-lifted-vseg which provides an orP function.

Everybody should write everything in Go? (28 May) Ryan Hayes posted a small snippet of Go showing how friendly he found it for writing concurrent programs, “No pthread... not stupid crap... just works!” The program seems to create 4 threads which print out 1 to 100 each. What do Haskellers think? See the comments for some discussion between Haskell people like Simon Marlow, and some folks in the Go community about our respective approaches to the problem.

Proposal: Control.Concurrent.Async (8 June) Deep into writing his book on Parallel Haskell, Simon Marlow proposes a higher-level concurrency API for the base package. The proposed Control.Concurrent.Async would help make sure that exceptions in child threads are dealt with (returned or passed up), and that threads aren't accidentally left running in the background. A few Haskellers commented that they would prefer that base be kept minimal as possible, and have counter-proposed making it a package to be included in the Haskell Platform instead. See the thread for some discussion on the API itself.

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