No "last core parallel slowdown" on OS X

I'm a huge fan of the recent paper http://ghcmutterings.wordpress.com/2009/03/03/new-paper-runtime-support-for-multicore-haskell/ which put me over the top to get started writing parallel code in Haskell. Parallel code is now integral to my and my Ph.D. students' research. For example, we recently checked an assertion for the roughly 69 billion atomic lattices on six atoms, in a day rather than a week, using perhaps 6 lines of parallel code in otherwise sequential code. When you're anxiously waiting for the answer, a day is a lot better than a week. (The enumeration itself is down to two hours on 7 cores, which astounds me. I see no reason to ever use another language.) 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. I just got an EFi-X "boot processor" (efi-x.com) working on a nearly identical quad core box that I built, and I tested the same computations with OS X. For my test case, there's a mild cost to moving to parallel at all, but... Compared to 2 cores, using 3, 4 cores on a four core Linux box gives speedups of 1.37x, 1.38x Compared to 2 cores, using 3, 4 cores on an equivalent four core box running OS X gives speedups of 1.45x, 1.9x Here 1.5x, 2.0x is ideal, so I'm thrilled. If we can't shame Linux into fixing this, I'm never looking back. How true is this for other parallel languages? Haskell alone is perhaps too fringe to cause a Linux scandal over this, even if it should... The EFi-X boot processor itself is rather expensive ($240 now), and there's sticking to a specific hardware compatibility list, and I needed to update my motherboard BIOS and the EFi-X firmware, but no other fiddling for me. These boxes are just compute servers for me, I would have been ok returning to Linux, but not if it means giving up a core. People worry about compatibility, "I sensed a softness in the surround sound in game X...", but for me the above numbers put all this in perspective. Another way to put this, especially for those who don't have a strong preference for building their own machines, and can't wait for Linux to get its act together: If you're serious about parallel Haskell, buy a Mac Pro.