Grand Central Dispatch (simple loop optimization)
Jan,
thanks for sharing this. This is really interesting. We have been
contemplating using GCD for R (mainly pnmath) but at the time OMP was
faster. However, GCD got apparently really good in the meantime:
> system.time(threads(100000,1000,"omp_try"))
user system elapsed
9.671 0.009 2.441
> system.time(threads(100000,1000,"gcd_try"))
user system elapsed
9.592 0.004 2.410
> system.time(threads(100000,1000,"dcg_try"))
user system elapsed
9.784 0.003 9.788
[This is on Harpertown 2.66GHz quad core]
So GCD is surprisingly just a hair faster than OMP (also surprising to
me is that using more threads than cores make OMP faster - the above
is with 16 threads).
On Sep 17, 2009, at 14:24 , Jan de Leeuw wrote:
a) Obviously OpenMP is more portable. Even on a Mac I had to use Apple's gcc in this case (I normally use the GNU gcc-trunk). b) GCD does not require specifying the number of threads -- it determines it at runtime. c) Coding is simpler.
I would not say - OMP takes just one #pragma - no need to change your code whereas GCD requires several special function calls... However, OMP is more limited in the kind of things you can do. Cheers, Simon
d) Since GCD is at a lower OS level than OpenMP, it will probably handle resource allocation better. But my small example, on an otherwise idle Mac Pro (16 cores, 32 GB of RAM), does not really highlight that. e) For more info, and some OpenMP comparisons, see http://www.macresearch.org/cocoa-scientists-xxxi-all-aboard-grand-central http://arstechnica.com/apple/reviews/2009/08/mac-os-x-10-6.ars/12 To quote Syracuse "Write your application as usual, but if there's any part of its operation that can reasonably be expected to take more than a few seconds to complete, then for the love of Zarzycki, get it off the main thread!" On Sep 17, 2009, at 11:03 , Saptarshi Guha wrote:
Nice, how does this compare when using OpenMP? How does it compare when several other core hungry processes are running?( GC is supposed to nicely handle resource allocation, does OpenMP compete with the other processes?). Regards Saptarshi
=== Jan de Leeuw; Distinguished Professor and Chair, UCLA Department of Statistics; Director: UCLA Center for Environmental Statistics (CES); Editor: Journal of Multivariate Analysis, Journal of Statistical Software; US mail: 8125 Math Sciences Bldg, Box 951554, Los Angeles, CA 90095-1554 phone (310)-825-9550; fax (310)-206-5658; email: deleeuw at stat.ucla.edu .mac: jdeleeuw ++++++ aim: deleeuwjan ++++++ skype: j_deleeuw homepages: http://gifi.stat.ucla.edu ++++++ http://www.cuddyvalley.org ------------------------------------------------------------------------------------------------- No matter where you go, there you are. --- Buckaroo Banzai http://gifi.stat.ucla.edu/sounds/nomatter.au
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