[Haskell-cafe] Performance of concurrent array access

[Andreas Voellmy]

One more observation... I tried a third variation in which the test program
still uses a single shared IOArray but each thread writes to different
indices in the array. In this case I get good scaling with performance
similar to the use of IOUArray. In detail, I made the following two changes
to give each thread a disjoint set of indices to write to:

bunchOfKeys threadNum = take numElems $ zip (cycle $ indices numThreads
threadNum) $ drop threadNum cyclicChars

and

indices :: Int -> Int -> [Int]
indices numThreads threadNum =
  let numixs      = arraySize `div` numThreads
      startIx     = numixs * threadNum
      allIndices  = [0..highestIndex]
  in take numixs $ drop startIx allIndices


--Andreas

On Tue, Aug 23, 2011 at 5:07 PM, Andreas Voellmy
<andreas.voellmy at gmail.com>wrote:

> Thanks for the suggestions. I tried to add strictness in the following
> ways:
>
> (1) Changing "insertDAT a j c" to "insertDAT a j $! c"
> (2) Changing "insertDAT a j c" to "deepseq c (insertDAT a j c)"
>
> I also used Int instead of Int32 throughout and changed the DAT data type
> to a newtype definition. These changes improved the performance slightly,
> but still, the multithreaded runs perform significantly worse than the
> single-threaded runs, by about the same amount (i.e. 0.5 seconds more for
> the 2 core run than for the 1 core run).
>
> I used ghc 7.0.3 for the performance measurements I gave in my message.
> I've also tried under 7.2.1, and I get basically the same behavior there.
>
> --Andreas
>
>
> On Tue, Aug 23, 2011 at 4:38 PM, Johan Tibell <johan.tibell at gmail.com>wrote:
>
>> On Tue, Aug 23, 2011 at 10:04 PM, Andreas Voellmy
>> <andreas.voellmy at gmail.com> wrote:
>> > data DAT = DAT (IOArray Int32 Char)
>>
>> Try to make this a newtype instead. The data type adds a level of
>> indirection.
>>
>> >   do let p j c = insertDAT a j c >> lookupDAT a j >>= \v -> v `pseq`
>> return
>> > ()
>>
>> You most likely want (insertDAT a j $! c) to make sure that the
>> element is force, to avoid thunks building up in the array.
>>
>> > -- Parameters
>> > arraySize :: Int32
>>
>> Int might work better than Int32. While they should behave the same on
>> 32-bit machines Int might have a few more rewrite rules that makes it
>> optimize better.
>>
>> -- Johan
>>
>
>
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