No "last core parallel slowdown" on OS X

Daniel Peebles pumpkingod at
Sat Apr 18 16:46:44 EDT 2009

That looks great! I wonder what about Mac OS leads to such good performance...

Now if only we could get a nice x86_64-producing GHC for Mac OS too, I
could use all my RAM and the extra registers my Mac Pro gives me :)

On Sat, Apr 18, 2009 at 2:39 PM, Dave Bayer <bayer at> wrote:
> I'm a huge fan of the recent paper
> 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" ( 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.
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