[R-SIG-Mac] [R-sig-hpc] Grand Central Dispatch (simple loop optimization)

[Simon Urbanek]

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
>
> _______________________________________________
> R-SIG-Mac mailing list
> R-SIG-Mac at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-mac
>
>