[Haskell] Implicit parallel functional programming

Ben Lippmeier Ben.Lippmeier at anu.edu.au
Wed Jan 19 21:13:11 EST 2005

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 

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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