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.
Cheers,
Simon
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