use geometrical mean for cache time

Signed-off-by: Pavel Boldin <boldin.pavel@gmail.com>

Author: paboldin

meltdown.c

@@ -147,6 +147,18 @@ int usage(void)
 	return 1;
 }
 
+static int mysqrt(long val)
+{
+	int root = val / 2, prevroot = 0, i = 0;
+
+	while (prevroot != root && i < 100) {
+		prevroot = root;
+		root = (val / root + root) / 2;
+	}
+
+	return root;
+}
+
 #define ESTIMATE_CYCLES	1000000
 static void
 set_cache_hit_threshold(void)
@@ -172,7 +184,7 @@ set_cache_hit_threshold(void)
 	cached /= ESTIMATE_CYCLES;
 	uncached /= ESTIMATE_CYCLES;
 
-	CACHE_HIT_THRESHOLD = (cached + uncached) / 2;
+	CACHE_HIT_THRESHOLD = mysqrt(cached * uncached);
 
 	printf("cached = %ld, uncached = %ld, threshold %d\n",
 	       cached, uncached, CACHE_HIT_THRESHOLD);