waste can become memory bound

scripts/waste.c

@@ -1,4 +1,5 @@
 // gcc -O2 waste.c -lpthread -owaste
+// gcc -O2 waste.c -lpthread -owaste -DMEM
 
 #define _GNU_SOURCE
 #include <stdio.h>
@@ -19,24 +20,28 @@ void waste(int pid) {
   int ret = sched_setaffinity(0, sizeof(cpu_set_t), &my_set);
   printf("set affinity to %d: %d\n", pid, ret);
 
-  float32x4_t *tmp = (float32x4_t *)malloc(0x1000008*sizeof(float32x4_t));
+  // 256 MB
+  float32x4_t *tmp = (float32x4_t *)malloc(0x1000001*sizeof(float32x4_t));
+
+  // comment out the memset for CPU only and not RAM
+  // otherwise we need this to avoid the zero page
+#ifdef MEM
+  memset(tmp, 0xaa, 0x1000001*sizeof(float32x4_t));
+#endif
+
   float32x4_t out;
 
-  uint64_t i = 0;
   double sec = seconds_since_boot();
-  while(1) {
-    int j;
-    for (j = 0; j < 0x1000000; j++) {
-      out = vmlaq_f32(out, tmp[j], tmp[j+1]);
+  while (1) {
+    for (int i = 0; i < 8; i++) {
+      for (int j = 0; j < 0x1000000; j++) {
+        out = vmlaq_f32(out, tmp[j], tmp[j+1]);
+      }
     }
-    if (i == 0x8) {
-      double nsec = seconds_since_boot();
-      ttime[pid] = nsec-sec;
-      oout[pid] = out[0] + out[1] + out[2] + out[3];
-      i = 0;
-      sec = nsec;
-    }
-    i++;
+    double nsec = seconds_since_boot();
+    ttime[pid] = nsec-sec;
+    oout[pid] = out[0] + out[1] + out[2] + out[3];
+    sec = nsec;
   }
 }
 
@@ -50,10 +55,12 @@ int main() {
     pthread_create(&waster[i], NULL, waste, (void*)i);
   }
   while (1) {
+    double avg = 0.0;
     for (int i = 0 ; i < CORES; i++) {
+      avg += ttime[i];
       printf("%.2f ", ttime[i]);
     }
-    printf("\n");
+    printf("-- %.2f\n", avg/CORES);
     sleep(1);
   }
 }