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

[Jan de Leeuw]

****************** threads.c  
*******************************************************

#include <math.h>
#include <dispatch/dispatch.h>

double do_work(double *, int , int);
void gcd_try (int *, int *, double *, double *);
void dgc_try (int *, int *, double *, double *);

void gcd_try (int *npowers, int *nlength, double *data, double  
*results) {
dispatch_queue_t queue = dispatch_get_global_queue 
(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply(*npowers, queue, ^(size_t i) {
     results[i] = do_work(data, *nlength, i);
     });
}

void dcg_try (int *npowers, int *nlength, double *data, double  
*results) {
int i;
for (i = 0; i < *npowers; i++) {
     results[i] = do_work(data, *nlength, i);
     }
}

double do_work(double *data, int nlength, int power) {
int k; double s = 0.0;
for (k = 0; k < nlength; k++) {
     s += pow(data[k],(double) power);
     }
return(s);
}

****************** threads.R  
*******************************************************

dyn.load("threads.so")

threads<-function(nlength,npowers,gcd=FALSE) {
set.seed(12345)
data<-rnorm(nlength)
results<-rep(0,npowers)
if (gcd)
     aList<-.C("gcd_try",npowers=as.integer(npowers),nlength=as.integer 
(nlength),data=as.double(data),results=as.double(results))
else
     aList<-.C("dcg_try",npowers=as.integer(npowers),nlength=as.integer 
(nlength),data=as.double(data),results=as.double(results))
return(aList$results)
}

**************************************************************************************
 > system.time(threads(100000,1000,gcd=FALSE))
    user  system elapsed
   8.967   0.004   8.971
 > system.time(threads(100000,1000,gcd=TRUE))
    user  system elapsed
  10.453   0.008   0.697

 > system.time(threads(1000000,100,gcd=FALSE))
    user  system elapsed
   3.698   0.013   3.712
 > system.time(threads(1000000,100,gcd=TRUE))
    user  system elapsed
   5.253   0.026   0.477

======================================================================
       Jan de Leeuw, 11667 Steinhoff Rd, Frazier Park, CA 93225
       home 661-245-1725 mobile 661-231-5416 work 310-825-9550
       .mac: jdeleeuw +++  aim: deleeuwjan +++ skype: j_deleeuw
======================================================================
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