No "last core parallel slowdown" on OS X
marlowsd at gmail.com
Mon Apr 20 10:01:34 EDT 2009
Manuel M T Chakravarty wrote:
> 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.
I suspect some scheduling weirdness in Linux, at least in the kernel we're
using here (2.6.25). Traces appeared to show that one of our threads was
being descheduled for a few ms, and this can be particularly severe in GHC
since our stop-the-world GC needs frequent synchronisations. One advantage
of moving to processor-independent GCs would be that we could degrade more
gracefully if the CPUs are contended, or the OS scheduler just decides to
use a core for something else for a while.
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