No "last core parallel slowdown" on OS X

Manuel M T Chakravarty chak at
Sun Apr 19 05:58:46 EDT 2009

Dave Bayer:
> In that paper, they routinely benchmark N-1 cores on an N core Linux  
> box, because of a noticeable falloff using the last core, which can  
> do more harm than good. I had confirmed this on my four core Linux  
> box, but was puzzled that my two core MacBook showed no such  
> falloff. Hey, two cores isn't representative of many cores, cache  
> issues yada yada, so I waited.
> Compared to 2 cores, using 3, 4 cores on an equivalent four core box  
> running OS X gives speedups of
> 	1.45x, 1.9x

As another data point, in our work on Data Parallel Haskell, we ran  
benchmarks on an 8-core Xserve (OS X) and an 8-core Sun T2 (Solaris).   
On both machines, we had no problem using all 8 cores.


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