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    <p>The problem appears to be that <code>parVector</code> doesn't force evaluation of the elements of the vector. Each element remains a thunk and nothing is sparked until the vector is consumed (by being printed), which is too late for the sparks to do work. You can force evaluation of each element by composing the <code>parVector</code> strategy with <code>rdeepseq</code>.</p> <pre><code>import qualified Data.Vector as V import qualified Data.Vector.Unboxed as U import Data.Vector.Strategies import Control.DeepSeq import Control.Parallel.Strategies main = do let res = genVVec 200 `using` (rdeepseq `dot` parVector 20) print res genUVec :: Int -&gt; U.Vector Int genUVec n = U.map (ack 2) $ U.enumFromN n 75 genVVec :: Int -&gt; V.Vector (U.Vector Int) genVVec n = V.map genUVec $ V.enumFromN 0 n ack :: Int -&gt; Int -&gt; Int ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1)) instance (NFData a, U.Unbox a) =&gt; NFData (U.Vector a) where rnf vec = seq vec () instance (NFData a) =&gt; NFData (V.Vector a) where rnf = rnf . V.toList </code></pre> <p>I also changed your <code>NFData (U.Vector a)</code> instance. Since a <code>U.Vector</code> is unboxed, evaluation to WHNF is sufficient, and forcing each element via the list conversion is wasteful. In fact the default definition for <code>rnf</code> works fine if you like.</p> <p>With these two changes, I get the following</p> <pre><code>SPARKS: 200 (200 converted, 0 pruned) </code></pre> <p>and the runtime has been reduced by nearly 50%. I also adjusted the vector chunk size to 20, but the result is very similar to a chunk size of 2.</p>
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    1. USJohn L
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    1. COExcellent! It's odd, because `parVector` already calls `rdeepseq` on each element of the outer vector, but it doesn't seem to have any effect without the strategy composition `dot` you use. Also, I've emailed the maintainer of vector-strategies as I believe the chunk size is currently being ignored incorrectly.
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      1. USJKnight
    2. CO@JKnight: I agree the chunk size is ignored. Or more properly, it controls how far `parVector` recurses but each element of the vector is evaluated in parallel regardless. No matter what the chunk size is, I always see 200 sparks.
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      1. USJohn L
    3. CO@John L, yes, 200 sparks. That was me being rather dumb. I'm going to upload a new version soonish (but not immediately for fear of discovering something else that needs changes 15 minutes later).
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      1. USThomas M. DuBuisson
 

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