[Haskell-cafe] A parallelization problem

wren ng thornton wren at freegeek.org
Mon Apr 25 01:06:39 CEST 2011

Hello all,

I have some loopy code I'd like to make parallel, but I'm not sure the 
best way to go about it. Everywhere else in this program I'm using STM, 
but that seems pretty heavy-handed for this task, and it looks like the 
perfect place for `par` and friends.

So the 10,000ft view of the code is something like:

     foldl' step (0,state0) xs

     step (n,stateN) x = (n+1, k stateN)
         k = case lookup n stateN of
             Nothing        -> error "oh noes"
             Just substateN -> insert (n+1) $
                 foldl' maybeInsert empty (views substateN x)

         maybeInsert state (k,v)
             | p (k,v)   = state
             | otherwise = insert k v state

That is, for each x that comes in we add a bunch of things to a map 
datastructure. The thing that makes the inner loop parallelizable is 
that we happen to know that none of the new keys will (a) conflict with 
old keys (as indicated by the fact that we're nesting maps), (b) 
conflict with the other new keys, nor (c) depend on the other new 
key/value pairs. Thus, we know that we can reassociate the order of 
insertions without changing the semantics.

So the question is, how do I do it? One issue not apparent in this 
high-level look at the code is that I'm actually using ST for doing the 
inner loop efficiently, which is why I'm not just using fromList.

So far I'm using an IntMap for storing the state. The STM approach would 
be to store the IntMap in a TMVar and fire off threads to compute and 
insert each one of the views. But even as lightweight as Haskell's 
threads are, I worry that the overhead would swamp the benefits.

(Of course, it may be viable to push the ST down far enough to use 
(fromList . catMaybes). Then we could just construct the list and call 
par on all the elements. That wouldn't do much to parallelize the 
insertions themselves, but that shouldn't matter too much I hope. 
Perhaps I've just answered my own question; any other ideas?)

Live well,

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