[Haskell-beginners] parallelizing a function for generating prime numbers

[Mateusz Kowalczyk]

On 05/16/2014 10:13 PM, Norbert Melzer wrote:
> I had some fears, that there will be answers like this ;)
> 
> The problem with improving the generation itself is, that I don't
> understand the faster implementations that I found (namely the
> implementation of `Data.Numbers.Primes` in the `primes`-package and some
> other Wheel-Sieves).
> 
> And for the Project-Euler-Problems I only use code that I have created
> myself or at least I have a small idea how it works if it is from a package…
> 

Euler is a mathematics (mostly number theory) exercise, not a
programming one, so it's understandable that in some cases the problems
and some of their solutions are not suited for parallelism. It's not of
much benefit to try and parallelise an inherently linear solution.

> 2014-05-16 18:33 GMT+02:00 Benjamin Edwards <edwards.benj at gmail.com>:
> 
>> Given the linear dependencies in prime number generation, shy of using a
>> probabilistic sieving method, I'm not sure that it's possible to hope for
>> any kind of parallel number generation. All you are going to do is for
>> yourself to eat the cost of synchronisation for no gain.
>>
>> Ben
>>
>> On Fri May 16 2014 at 16:53:38, Norbert Melzer <timmelzer at gmail.com>
>> wrote:
>>
>>> Hi there!
>>>
>>> I am trying to enhence the speed of my Project Euler solutions…
>>>
>>> My original function is this:
>>>
>>> ```haskell
>>> problem10' ::  Integer
>>> problem10' = sum $ takeWhile (<=2000000) primes
>>>   where
>>>     primes                  = filter isPrime possiblePrimes
>>>     isPrime n               = n == head (primeFactors n)
>>>      possiblePrimes          = (2:3:concat [ [6*pp-1, 6*pp+1] | pp <-
>>> [1..] ])
>>>     primeFactors m          = pf 2 m
>>>     pf n m | n*n > m        = [m]
>>>            | n*n       == m  = [n,n]
>>>            | m `mod` n == 0  = n:pf n (m `div` n)
>>>            | otherwise      = pf (n+1) m
>>> ```
>>>
>>> Even if the generation of primes is relatively slow and could be much
>>> better, I want to focus on parallelization, so I tried the following:
>>>
>>> ```haskell
>>> parFilter :: (a -> Bool) -> [a] -> [a]
>>> parFilter _ [] = []
>>> parFilter f (x:xs) =
>>>   let px = f x
>>>       pxs = parFilter f xs
>>>   in par px $ par pxs $ case px of True -> x : pxs
>>>                                    False -> pxs
>>>
>>> problem10' ::  Integer
>>> problem10' = sum $ takeWhile (<=2000000) primes
>>>   where
>>>     primes                  = parFilter isPrime possiblePrimes
>>>     isPrime n               = n == head (primeFactors n)
>>>     possiblePrimes          = (2:3:concat [ [6*pp-1, 6*pp+1] | pp <-
>>> [1..] ])
>>>     primeFactors m          = pf 2 m
>>>     pf n m | n*n > m        = [m]
>>>            | n*n       == m  = [n,n]
>>>            | m `mod` n == 0  = n:pf n (m `div` n)
>>>            | otherwise      = pf (n+1) m
>>> ```
>>>
>>> This approach was about half as slow as the first solution (~15 seconds
>>> old, ~30 the new one!).
>>>
>>> Trying to use `Control.Parallel.Strategies.evalList` for `possiblePrimes`
>>> resulted in a huge waste of memory, since it forced to generate an endless
>>> list, and does not stop…
>>>
>>> Trying the same for `primeFactors` did not gain any speed, but was not
>>> much slower at least, but I did not expect much, since I look at its head
>>> only…
>>>
>>> Only thing I could imagine to parallelize any further would be the
>>> takeWhile, but then I don't get how I should do it…
>>>
>>> Any ideas how to do it?
>>>
>>> TIA
>>> Norbert
>>>
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>>>
>>
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-- 
Mateusz K.