[Haskell-cafe] Parallel Karatsuba - A Weird speed up value greater than 4 on an Intel Quadcore CPU!

[Eugene Kirpichov]

If the cache was infinitely faster, then doubling it would give an infinite speedup for an algorithm whose working set was exactly one core's cache size.



24.12.2011, в 19:58, Burak Ekici <ekcburak at hotmail.com> написал(а):

> 
> First of all, thanks a lot for your quick answer!
> However, the question is what are the approximate limits 
> of this super-linear speedup? I mean, is it acceptable, if
> parallelization happens even 100 time faster?
> 
> How can I calculate the limits of this speedup via the 
> cache size of my processor?
> 
> Cheers,
> Burak.
> 
> CC: haskell-cafe at haskell.org
> From: ekirpichov at gmail.com
> Subject: Re: [Haskell-cafe] Parallel Karatsuba - A Weird speed up value greater than 4 on an Intel Quadcore CPU!
> Date: Sat, 24 Dec 2011 19:53:26 +0400
> To: ekcburak at hotmail.com
> 
> Superlinear speedup can occur due to the increased cache size.
> 
> 
> 
> 24.12.2011, в 19:49, Burak Ekici <ekcburak at hotmail.com> написал(а):
> 
> Dear List,
> 
> I am trying to parallelize Karatsuba multiplication with Haskell's
> second generation strategies. Although, I am running the code on an
> Intel quad-core  CPU, I abnormally have a speedup much greater 
> than 4, around 10, which means a weird parallelization or something 
> occurs.
> 
> I would be appreciated, if anyone make some comments on the issue 
> explaining the possible reasons why this weird incident occurs?
> 
> Here is the basic parallel portion of the code:
> 
> karatsuba :: Int -> [Bool] -> [Bool] -> [Bool]
> karatsuba _ [] _ = []
> karatsuba _ _ [] = []
> karatsuba currentDepth xs ys 
>  | (l < 32 || currentDepth >= limit) = mul xs ys
>  | otherwise = (x `add` (replicate l False ++ (z `add` (replicate l False ++ y)))) `Main.using` strategy  
>   where 
>    l = (min (length xs) (length ys)) `div` 2
>    (xs0, xs1) = splitAt l xs
>    (ys0, ys1) = splitAt l ys
>    x  = (normalize (karatsuba (currentDepth+1) xs0 ys0))
>    y  = (normalize (karatsuba (currentDepth+1) xs1 ys1)) 
>    z  = ((karatsuba (currentDepth+1) (add xs0 xs1) (add ys0 ys1)) `sub` (normalize (karatsuba (currentDepth+1) xs0 ys0)) `sub` (normalize (karatsuba (currentDepth+1) xs1 ys1)))
>    strategy res = do (Main.rpar) (x)
>                      (Main.rpar) (y)
>                      (Main.rpar) (z)
>                      Main.rdeepseq res
> 
> Many thanks in advance and kind regards.
> 
> Saluti,
> Burak.
> 
> 
> 
> 
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