[mpich-discuss] Bug fix for dims_create

[Ian Hutchinson]

The file src/mpi/topo/dims_create.c contains the code that determines the 
result of

MPI_DIMS_CREATE

It contains a bug which causes it to produce improper distributions of the 
processes among dimensions that do not satisfy the objective of being "as close 
to each other as possible". For example, if called in 3-dimensions,
with 16 nodes, the topology returned is 4, 4, 1. It ought to be 4, 2, 2.

This bug is caused by some longstanding cobbled-together code that is called 
when all the factors of the nnodes are 2 (which is not an unusual case).

I attach (and include below) a patch to correct this bug. It would be great if 
it could find its way into the distribution.

Thanks
 	Ian Hutchinson
 	http://www.psfc.mit.edu/people/hutch/

=========================================================================

--- dims_create.c.dist	2012-12-08 13:46:46.000000000 -0500
+++ dims_create.c	2012-12-08 13:48:02.000000000 -0500
@@ -317,28 +317,22 @@
  	    int cnt    = factors[0].cnt; /* Numver of factors left */
  	    int cnteach = ( cnt + dims_needed - 1 ) / dims_needed;
  	    int factor_each;
- -	    factor_each = factor;
-	    for (i=1; i<cnteach; i++) factor_each *= factor;

-	    for (i=0; i<ndims; i++) {
-		if (dims[i] == 0) {
-		    if (cnt > cnteach) {
-			dims[i] = factor_each;
-			cnt -= cnteach;
-		    }
-		    else if (cnt > 0) {
-			factor_each = factor;
-			for (j=1; j<cnt; j++) -			    factor_each 
*= factor;
-			dims[i] = factor_each;
-			cnt = 0;
-		    }
-		    else {
-			dims[i] = 1;
-		    }
+	    for (i=0;i<ndims;i++){ +		if(dims[i]==0)dims[i]=-1;
+	    }
+	    i=0;
+	    while(cnt > 0){
+		if(dims[i] < 0){
+		    dims[i]=dims[i]*factor;
+		    cnt--;
  		}
+		if(++i >= ndims)i=0;
+	    }
+	    for (i=0;i<ndims;i++){
+		if(dims[i] < 0)dims[i]=-dims[i];
  	    }
+
  	}
  	else {
  	    /* Here is the general case.  */
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