[Haskell-cafe] Parallel Haskell Digest 5
Eric Y. Kow
eric at well-typed.com
Wed Aug 31 09:39:42 CEST 2011
Parallel Haskell Digest
Eric here, reprising my role as Parallel Haskell Digester. Many thanks
to Nick for good stewardship of the digest and nice new directions for
the future. This month we have Erlang PhDs (and a postdoc), new
partners, two Monad Reader articles in progress and some strategies. As
usual, this digest is made possible by the [Parallel GHC Project][n0].
[Scalable Erlang PostDoc and 2 PhD Studentships][n1] (8 Aug)
What, Erlang?! Yes, you are reading the right digest. If you're
generally into parallelism and concurrency in functional programming
languages, you may be especially interested to know about this
announcement from Phil Trinder.
> RELEASE project - A High-level Paradigm for Reliable Large-scale
> Server Software - is funded by the EU Framework 7 programme for 36
> months from October 2011. Its aim is to scale the radical
> concurrency-oriented programming paradigm to build reliable
> general-purpose software, such as server-based systems, on massively
> parallel machines (100 000 cores).
Word of the Month
Last month, we had `par` and `pseq` as our twin words of the month.
Let's pursue this little train of parallel thought; our next word the
month is *strategy*. Strategies have been around since the 1993
paper [Algorithm + Strategy = Parallelism][asp]; that's before even
we started using monads in Haskell! They've recently been revamped
in Simon Marlow's [Seq no More][seq-nm] paper, and it's this version
of strategies that we'll be exploring here.
Strategies are built on top of the `par` and `pseq` primitives we saw in
the [last digest][phd4]. They provide a nice way to express the often
complicated logic we need to make the best use of parallelism in our
code. Use of strategies can also help to make parallel code easier to
read and maintain because they allow us to more cleanly separate the
core logic from our code that which pertains to our use of parallelism.
Before delving into strategies, let's take a small notational detour
by introducing the `Eval` monad. Suppose we wanted a parallel version
of the `map` function, something that would apply a function to each
item of a list. Using the `par` and `pseq` from the last digest, we
might express this function
parMap :: (a -> b) -> [a] -> [b]
parMap f  = 
parMap f (a:as) = b `par` bs `pseq` (b:bs)
b = f a
bs = parMap f as
If we look carefully at the code we can observe that there is something
inherently sequential in the way we have expressed this parallel
computation: first spark off `f a` then recurse to the tail of the list,
and finally cons.
The `Eval` monad builds off the insight that expressing parallelism is
fundamentally (perhaps counterintuitively) about ordering things.
Monads are well-suited for expressing ordering relationships, and so
they have been been pressed to work for expressing parallel computation
data Eval a
instance Monad Eval
runEval :: Eval a -> a
rpar :: a -> Eval a
rseq :: a -> Eval a
`Eval` is just a strict identity monad, with `rpar` and `rseq` as
counterparts to `par` and `pseq`. We use `Eval` to compose sequences
of parallel computation, which we extract the results of by using the
`runEval` function. If you're not familiar with monads, you can get
away with just treating `Eval` as new notation, or rather, borrowed
notation, the same that we use IO, parser combinator libraries,
QuickCheck and a plethora of other useful monads. It's worth noting
also that despite appearances, we are still in purely functional
territory -- no IO here! -- with the notion of sequencing being
limited to controlling parallelism and evaluation depth.
To make use of `Eval` for our `parMap` function, we could write
a version like the below. It introduces a change of type, from
returning `[b]` to `Eval [b]`. In the general case, we could
just use the `runEval` function to get our result back,
but we are not baking it into `parMap` because we would typically
want to use then function within a greater `Eval` context anyway.
parMap :: (a -> b) -> [a] -> Eval [b]
parMap f  = return 
parMap f (a:as) = do
b <- rpar (f a)
bs <- parMap f as
As before, this function captures the basic idea of its sequential
counterpart `map`: apply function, recurse to tail, cons new head to new
tail. This is a passable parallel map, but there are still two things
which are unfortunate about it. First, we have repeated the
implementation of map, not a big deal for such a small function but a
potential maintenance problem for more complex code. Second, we have
only captured one sort of parallel evaluation firing off sparks for all
the cons cells, but in practice getting parallelism right requires some
often careful tuning to get the right level of granularity and account
for dependencies. For instance, what if instead of doing each cell in
parallel, we wanted to bunch them up into chunks so as to minimise the
coordination overhead? What we need is a way to express ideas about
running code in parallel (such as “break into chunks and run each chunk
simultaneously"), preferably avoid duplicating existing code or
otherwise making it more complicated than it needs to be.
This is where strategies come in. A strategy is just a function that
turns some data into an `Eval` sequence. We've already encountered two
strategies above, `rpar` which sparks off a parallel evaluation, and
`rseq` which evaluates its argument to weak head normal form. Many more
strategies possible particularly when they are custom designed for
specific data types. Also, if we have a strategy, we can imagine
wanting to use it. This we capture with `using`, which applies the
strategy function and runs the resulting sequence. Parallel evaluation
aside, you can almost pretty much think of <code>x `using` s</code> as
being semequivalent to as `id x` but you'll want to watch out for a
couple of caveats described the tutorial [Parallel and Concurrent
Programming in Haskell][smtutorial].
Strategy a = a -> Eval a
using :: a -> Strategy a -> a
x `using` s = runEval (s x)
Now that we've put a name to the idea of building `Eval` sequences,
we can think about trying to generalise our `parMap`. One way would
be to write a higher-order strategy. This `parList` function is
almost identical to `parMap`, except that instead of applying any
function to a list we limit ourselves to just applying some strategy
parList :: Strategy a -> Strategy [a]
parList strat  = return 
parList strat (x:xs) = do
x' <- rpar (x `using` strat)
xs' <- parList xs strat
We can then define `parMap` by parameterising `parList` with `rseq`
to get a parallel list strategy which we then apply to `map f xs`.
(Food for thought: why `parList rseq` instead of `parList r0`?
Reply to the Haskell-Cafe posting if you think you know why!)
parMap :: (a -> b) -> [a] -> Eval [b]
parMap f xs = map f xs `using` parList rseq
We have now achieved two things. First we've improve the modularity
of our code by separating algorithm (`map f xs`) from coordination
(`parList rseq`). Notice how we are now reusing map instead of
reimplementing it, and notice as well how we now have a reusable
`parList` that we can use apply to any situation that involves
a list. Second, by isolating the algorithm from the coordination code
we have given ourselves a lot more flexibility to switch strategies. To
bunch our list up into coarser-grained chunks, for example, we could
just swap out `parList` with `parListChunk`
parMap :: (a -> b) -> [a] -> Eval [b]
parMap f xs = map f xs `using` parListChunk 100 rseq
In this word of the month, we have very lightly touched on the idea
of strategies as a compositional way of expressing parallel
coordination and improving the modularity of our code. To make use of
strategies, we basically need to be aware of two or three things:
1. How to apply a strategy: <code>foo `using` s</code> evaluates the
value `foo` with strategy `s`
2. Useful strategies from the library: the basics are `r0` which does
nothing, `rseq` which evaluates its argument to weak head normal form,
`rdeepseq` which evaluates it all the way down to normal form, and
`rpar` which sparks the value for parallel evaluation. Be sure to check
out [Control.Parallel.Strategies][straddock] for more sophisticated
strategies such as `parListChunk`, which divides the list into chunks
and applies a strategy to each one of the chunks in parallel.
3. (Optionally) How to build strategies, the simplest way being
to use the `dot` function to compose two strategies sequentially.
If you implement your own strategies instead of combining those
from the library, you may want to have a look at the [Seq no
More][seq-nm] for a little safety note.
If you want to know more about strategies, the first place to look is
probably the tutorial [Parallel and Concurrent Programming in
Haskell][smtutorial] (there is a nice example in there, implementing a
parallel K-means algorithm) and followed by the
Control.Parallel.Strategies API. You could also just dive in and give
it a try on some of your own code. It's sometimes just a matter of
grabbing a strategy and `using` it.
Parallel GHC Project Update
The Parallel GHC Project is an MSR-funded project to push forward
the use of parallel Haskell in the real world. The aim is to demonstrate
that parallel Haskell can be employed successfully in industrial projects.
Speaking of industrial projects, our recent search for a new project
partner has been successful! In fact, we will be welcoming two new
partners to the project, the research and development group of Spanish
telecoms company Telefónica, and VETT a UK-based payment processing
company. We are excited to be working with the teams at Telefónica I+D
and VETT. There may be some Cloud Haskell in our future; stay tuned for
more details about the respective partner projects.
Also coming up are a couple of Monad Reader articles featuring work from
the Parallel GHC project.
Kazu Yamamoto has been writing an article for the upcoming Monad Reader
special edition on parallelism and concurrency. He'll be revisiting
Mighttpd (pronounced "Mighty"), a high-performance web server written in
Haskell in late 2009. Since Mighttpd came out the Haskell web
programming landscape has evolved considerably, with the release of GHC
7 and development of several web application frameworks. The new
mighttpd takes advantage of GHC 7's new IO manager as well as the Yesod
Web Application Interface (WAI) and the HTTP engine Warp. Do check out
his article when it comes out for proof that "we can implement a high
performance web server in Haskell, which is comparable to highly tuned
web servers written in C", or if you can't wait, try his [most recent
slides][mighttpd] on the topic.
Bernie Pope and Dmitry Astapov have been working on an article about the
Haskell MPI binding developed in the context of this project. You can
find out more about the binding on its [Hackage][hackage-mpi] and
Blogs, Papers, and Packages
* [Data Parallel Haskell and Repa for GHC 7.2.1][b1] (11 Aug)
Ben Lippmeier has updated the Data Parallel Haskell libraries on Hackage to go
with the newly released GHC 7.2.1. While DPH is still a technology preview,
this new version is significantly more robust than previous ones.
If nested data parallelism isn't what you're after, Ben has also updated
the Repa library for parallel arrays. See [PH Digest 3][phd3] for more
* [thespian: Lightweight Erlang-style Actors for Haskell][b2] (18 Aug)
Alex Constandache update Hackage with this intriguing library.
I wrote to Alex asking about about the relationship with Cloud
Haskell. It turns out Alex had somewhat similar goals in mind
and may be focusing on Cloud Haskell instead.
* [dbus-core 0.9][b3] (23 Jul)
John Millikin announced "the first significant release to dbus-core in
a while" with significant improvements to both the API and
implementation. In particular, dbus-client has been merged into
dbus-core. See the announcement for more details then a quick code
sample showing the new simple API in action. For the curious,
D-Bus is an inter-process communication mechanism used in desktop
environments like Gnome.
Mailing list discussions
* [How to ensure code executes in the context of a specific OS thread?][m1] (4 Jul)
Jason Dagit has found the correct handling of GUI events on OS X
requires checking for events and responding to them from the main
thread. He wanted to know if such thing was even possible on the
threaded RTS, particularly so that he could use the library from GHCi.
David Pollak has a [bit of code][hs-ipad] that provides a
runOnMain function using an FFI call to a bit objective C code. Simon
Marlow pointed out that for compiled code, the main thread is a bound
thread, bound to the main thread of the process. Simon is specifically
talking about the threaded RTS here as this is already trivially true
of the non-thread one. The question still remains open for GHCi.
* [Cloud Haskell][m2] (22 Jul)
Tom Murphy was curious to see anybody was using [Cloud
Haskell][cloud-hs] yet. It looks like we have a couple people who are
*thinking* of using it: Tim Cowlishaw might be using it for his
masters thesis project (A Haskell EDSL for agent-based simulation).
Julian Porter, who we mentioned in the last digest for his Monad
Reader article, is planning to make his MapReduce monad framework work
on the distribute cluster. Now how about those of us who have
actually given it try?
As for the Parallel GHC project, it's worth mentioning that our new
partners are planning to use Cloud Haskell in their projects. We'll
be sure to report back to the community about the results.
* [A language that runs on the JVM or .NET...][m4] (31 Jul)
KC was hoping for a version of Haskell "that runs on the JVM or .NET
has the advantage of Oracle & Microsoft making those layers more
parallelizable". Austin Seipp replied that it basically boils down to
it being [a lot of work][why-no-jvm]. Chris Smith also cautions that
while there may be many good reasons to easy to want a Haskell.Net or
a Jaskell, Haskell parallelism and concurrency may not be one of them.
Parallel and concurrent Haskell code relies generates a large volume
of lightweight threads. Simply using their JVM or CLR counterparts as
they are would not do.
Are you interested in seeing a serious and long-term effort towards
Haskell on the JVM? See the [FAQ][why-no-jvm] and get in touch!
* [Error in the asynchronous exception operational semantics][m6] (9 Aug)
Edward Z. Yang noticed at the end of [Asynchronous Exceptions as an
Effect][async1] (Harrison et. al) that the authors have found an
error in the operational semantics described in [Asynchronous
Exceptions in Haskell][async2]. Anybody know what it is?
Meanwhile, David Barbour took the opportunity to say
> All you need to know about asynchronous exceptions is: *don't use them!*
> They're too difficult to reason about, in terms of failure modes and such.
> Use TVars or MVars instead.
* [Haskell Actors, Linda, publish / subscribe models?][m7] (13 Aug)
Dmitri O. Kondratiev needs to "build a framework to coordinate task
producers / consumers distributed in the same and different address
spaces. He needs to scale a data processing application somewhat
Hadoop-like way yet in more flexible manner, without Hadoop-specific
distributed FS constraints." At this point, Ryan Newton suggested
"Cloud Haskell", which was greeted by with joy and great enthusiasm.
"Finally Erlang actor model of communication comes to Haskell!"
Keep us posted, Dmitri.
* [More generic parfoldr than this... ][m8](23 Jul)
Prabhat Totoo tried to implement a parallel foldr function by breaking
the input listed chunks, mapping foldr over each chunk, and running
foldr over the list of results. This solution only works if the input
and output of the folded function have the same type. Prabhat wanted
to know how he could go about generalising his parallel fold. Also
while doing some timings, he noticed that the regular presumably
sequential fold improved with multiple cores. What's up with that?
Conal Elliott make the point that Prabhat's current
parallelise-by-reassociating solution is only correct for an
associative operation, which is forcibly of type (a -> a -> a). Have
a look at the the rest of the thread for an exploration of parallel
fold by Kevin Hammond, Conal Elliot and Sebastien Fischer.
Stack Overflow and Reddit
* [Are 'par' and 'pseq' good for data parallelism?][s1]
* [Difference between MVar and a TVar?][s2]
* [How to sort a list [MVar a] using a values?][s3]
* [Understanding BlockedIndefinitelyOnMVar in Concurrent code][s4]
* [Poor performance / lockup with STM][s5]
* [If one of Haskell's goals is concurrency, then why is it based on the λ-calculus and not on a process calculus?][s6]
Help and Feedback
If you'd like to make an announcement in the next Haskell Parallel
Digest, then get in touch with me, Eric Kow, at
<parallel at well-typed.com>. Please feel free to leave any comments and
Eric Kow <http://erickow.com>
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