Changes between Initial Version and Version 1 of mpich/collective_communication/cpi

Timestamp:

Jul 24, 2008, 11:55:14 AM (16 years ago)

Author:

wade

Comment:

--

mpich/collective_communication/cpi

@@ +1 @@
+= Source code =
+{{{
+/* Program:
+ *   mpich_example 內建範例，計算 pi 。
+ * History:
+ *   2008-04-11 BETA
+ *   2008-06-19 增加可重複輸入欲計算之精準度
+ *   2008-06-23 加入 MPI_Barrier 以確保每個 node 在接受 n 後才執行
+ * /
+
+#include "mpi.h"
+#include <stdio.h>
+#include <math.h>
+#include <time.h>
+
+double f( double );
+double f( double a )
+{
+    return (4.0 / (1.0 + a*a));
+}
+
+int main( int argc, char *argv[])
+{
+    int done = 0, n, myid, numprocs, i=0;
+    double PI25DT = 3.141592653589793238462643;
+    double mypi, pi, h, sum, x;
+    double startwtime = 0.0, endwtime;
+    int  namelen;
+    char processor_name[MPI_MAX_PROCESSOR_NAME];
+    MPI_Init(&argc,&argv);
+    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
+    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
+    MPI_Get_processor_name(processor_name,&namelen);
+    fprintf(stderr,"Process %d on %s\n",
+            myid, processor_name);
+    n = 0;
+    while (!done)
+    {
+        /* 由 node 0 將使用者輸入的值送給其它的 node  */
+        if (myid == 0)
+        {
+            printf("Enter the number of intervals: (0 quits) ");
+            scanf("%d", &n);
+            startwtime = MPI_Wtime();
+        }
+
+        /* 這非常重要，所有的 node 必需在此同步，才可以收到使用者輸入的 n */
+        MPI_Barrier(MPI_COMM_WORLD);
+
+        /* 將 n 送給其它的 node  */
+        MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
+        if (n == 0)
+            done = 1;
+        else
+        {
+
+            /* 此為計算 pi 的演算法 */
+            h   = 1.0 / (double) n;
+            sum = 0.0;
+            for (i = myid + 1; i <= n; i += numprocs)
+            {
+                x = h * ((double)i - 0.5);
+                sum += f(x);
+            }
+            mypi = h * sum;
+
+            /* 將算完的結果傳給 node 0 加總 */
+            MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
+            if (myid == 0)
+            {
+                printf("pi is approximately %.16f, Error is %.16f\n",
+                       pi, fabs(pi - PI25DT));
+                endwtime = MPI_Wtime();
+                printf("wall clock time = %f\n",
+                       endwtime-startwtime);
+            }
+        }
+    }
+    MPI_Finalize();
+    return 0;
+}
+}}}
+= Result =
+[[Image(demo2-01.png)]]