Kshitij Mehta <kvm1...@gmail.com> wrote on 12/08/2009 01:31:52 PM:
> Hello, I am using x10 1.7 and the java backend. I am trying to
parallelize
> nested *for* loops.
>
> This is the original loop:
>
> for(var i:int = 1; i < n1;i++){
> for(var j:int = 1; j < n2; j++){
> for(var k:int = 1; k<n3; k++){
> for(var l:int = 0; l<nc; l++){
> A(i,j,k,l) = B(i,j,k,l) - C(i,j,k,l);
> }
> }
> }
> }
>
>
> I tried to parallelize it as follows:
>
> finish {
> for(shared var i:int = 1; i < n1;i++){
> for(shared var j:int = 1; j < n2; j++){
> async {
> for(shared var k:int = 1; k<n3; k++){
> for(shared var l:int = 0; l<nc; l++){
> A(i,j,k,l) = B(i,j,k,l) - C(i,j,k,l);
> }
> }
> }
> }
> }
> }
>
> I get the error: Local variable "i" is accessed from an inner class, and
> must be declared final.
> And of course, if I declare i as final, I get
> Cannot apply ++ to a final variable
>
> So is there a way to parallelize the loop? Note that I am not using
> distributions, everything occurs at place 0.
The usual idiom for this particular problem is
finish {
for(shared var i:int = 1; i < n1;i++){
for(shared var j:int = 1; j < n2; j++){
val ii = i;
val jj = j;
async {
for(shared var k:int = 1; k<n3; k++){
for(shared var l:int = 0; l<nc; l++){
A(ii,jj,k,l) = B(ii,jj,k,l) - C(ii,jj,k,l);
}
}
}
}
}
}
However, a better way to phrase this in X10 is:
val R1:Region(2) = [1..n1-1,1..n2-1];
finish {
for((i,j) in R1) {
async {
val R2:Region(2) = [1..n3-1,0..nc-1];
for((k,l) in R2) {
A(ii,jj,k,l) = B(ii,jj,k,l) - C(ii,jj,k,l);
}
}
}
}
Hope this helps,
Igor
--
Igor Peshansky (note the spelling change!)
IBM T.J. Watson Research Center
XJ: No More Pain for XML's Gain (http://www.research.ibm.com/xj/)
X10: Parallel Productivity and Performance (http://x10.sf.net/)
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