[Haskell] Implicit parallel functional programming

John Hughes rjmh at cs.chalmers.se
Thu Jan 20 04:29:10 EST 2005

Lennart Augustsson and Thomas Johnsson got some encouraging results 
fifteen years ago with their nu-G-machine. They compiled Lazy ML for a 
shared memory multiprocessor, and benchmarked against the sequential LML 
compiler, the precursor of hbc and at that time the best compiler for a 
lazy functional language. They observed speed-ups of at least 20% even 
with only two processors, and with four the speed-ups were in the range 
2-3.3x. The idea was to use "sparks" to start parallel computations, 
which were ignored, and thus cheap, unless a processor was actually 
available. OK, then benchmarks were tiny (n-queens), and the LML 
compiler had higher overheads for ordinary computation than GHC, but 
still, it's an encouraging result for exploiting multi-core machines.

Here's the reference:

> author = {Lennart Augustsson and Thomas Johnsson},
> title = {Parallel graph reduction with the (v , G)-machine},
> booktitle = {FPCA '89: Proceedings of the fourth international conference on Functional programming languages and computer architecture},
> year = {1989},
> isbn = {0-89791-328-0},
> pages = {202--213},
> location = {Imperial College, London, United Kingdom},
> doi = {http://doi.acm.org/10.1145/99370.99386},
> publisher = {ACM Press},
> }

>> I thought the "lazy functional languages are great for implicit 
>> parallelism" thing died out some time ago - at least as far as 
>> running the programs on conventional hardware is concerned.
>> Designing an algorithm that breaks apart a "sequential" lazy program 
>> into parallel chunks of the appropriate size is **HARD** (with double 
>> asterixes). The time and space behavior of a lazy program is complex 
>> enough for the _programmer_ to reason about, let alone an automated 
>> analysis - which has no knowledge of what the program is actually 
>> trying to do.
>> I think a more likely approach lies in the direction of the so called 
>> "parallel strategies". If you haven't already, I would strongly 
>> suggest reading: Algorithm + Strategy = Parallelism, 1998, PW 
>> Trinder, et al.
>> You can get this paper from Simon Peyton Jones's homepage.
>> Also, at the end of Hans Wolfgang-Loidl's thesis he develops a 
>> granularity analysis for a Haskell subset - one of the first steps in 
>> any kind of implicit parallelism. It's a pretty good effort, but at 
>> the end of it all it still relies on a pre-existing table of 
>> information about recursive functions. I think that these kind of 
>> analyses tend suffer from uncomputability problems more than most.
>> If you've still got your heart set on implicit parallelism, then 
>> there's a (very slow) simulator you might want to poke around with. I 
>> wrote it based around Clem Baker-Finch's "Abstract machine for 
>> parallel lazy evaluation", which supports fully speculative implicit 
>> parallelism.
>> There's a link to it on my homepage at 
>> http://cs.anu.edu.au/people/Ben.Lippmeier
>> Keean Schupke wrote:
>>> I have to say I disagree... I feel Haskell is highly suited to 
>>> implicit parallel execution... The key to "implicit" parallelisation 
>>> is that it is implicit - not explicit, so the programmer should feel 
>>> like they are programming a sequential language. If we can assume 
>>> little memory access penalties for threads running on other CPUs 
>>> (shared cache model), it seems to be a matter of putting locks on 
>>> the right structures, and allowing any worker-thread to take the 
>>> next function ready to run from the scheduler.
>>>    Keean.
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