- scheduler: in some cases the system may have a too-low estimate

of the performance of an app version on a host.
    It will then stop using that app version,
    so the estimate never has a chance to converge to its correct value.
    Fix: multiply by a random factor (mean 1, stddev .1)
    when comparing the FLOPS estimates of app versions.

svn path=/trunk/boinc/; revision=22837

checkin_notes

@@ -8682,3 +8682,18 @@ David  8 Dec 2010
 
 	api/
 		ttffont.cpp,h
+
+David  8 Dec 2010
+    - scheduler: in some cases the system may have a too-low estimate
+        of the performance of an app version on a host.
+        It will then stop using that app version,
+        so the estimate never has a chance to converge to its correct value.
+        Fix: multiply by a random factor (mean 1, stddev .1)
+        when comparing the FLOPS estimates of app versions.
+
+    client/
+        sim_util.cpp
+    lib/
+        util.cpp,h
+    sched/
+        sched_version.cpp

client/sim_util.cpp

@@ -124,30 +124,9 @@ int ACTIVE_TASK::init(RESULT* rp) {
 
 //////////////// OTHER
 
-// http://www.cs.wm.edu/~va/software/park/rvgs.c
 double NORMAL_DIST::sample() {
     if (!std_dev) return mean;
-
-  const double p0 = 0.322232431088;     const double q0 = 0.099348462606;
-  const double p1 = 1.0;                const double q1 = 0.588581570495;
-  const double p2 = 0.342242088547;     const double q2 = 0.531103462366;
-  const double p3 = 0.204231210245e-1;  const double q3 = 0.103537752850;
-  const double p4 = 0.453642210148e-4;  const double q4 = 0.385607006340e-2;
-  double u, t, p, q, z;
-
-  u   = drand();
-  if (u < 0.5)
-    t = sqrt(-2.0 * log(u));
-  else
-    t = sqrt(-2.0 * log(1.0 - u));
-  p   = p0 + t * (p1 + t * (p2 + t * (p3 + t * p4)));
-  q   = q0 + t * (q1 + t * (q2 + t * (q3 + t * q4)));
-  if (u < 0.5)
-    z = (p / q) - t;
-  else
-    z = t - (p / q);
-  return (mean + std_dev * z);
-
+    return (mean + std_dev * rand_normal());
 }
 
 inline double exponential(double mean) {

lib/util.cpp

@@ -551,3 +551,22 @@ bool boinc_is_finite(double x) {
 #endif
 }
 
+#define PI2 (2*3.1415926)
+
+// generate normal random numbers using Box-Muller.
+// this generates 2 at a time, so cache the other one
+//
+double rand_normal() {
+    static bool cached;
+    static double cached_value;
+    if (cached) {
+        cached = false;
+        return cached_value;
+    }
+    double u1 = drand();
+    double u2 = drand();
+    double z = sqrt(-2*log(u1));
+    cached_value = z*sin(PI2*u2);
+    cached = true;
+    return z*cos(PI2*u2);
+}

lib/util.h

@@ -42,6 +42,7 @@ extern void push_unique(std::string, std::vector<std::string>&);
 static inline double drand() {
     return (double)rand()/(double)RAND_MAX;
 }
+extern double rand_normal();
 
 #ifdef _WIN32
 #include <windows.h>

sched/sched_version.cpp

@@ -659,9 +659,11 @@ BEST_APP_VERSION* get_app_version(
 
             estimate_flops(host_usage, av);
 
-            // pick the fastest version
+            // pick the fastest version.
+            // Throw in a random factor in case the estimates are off.
             //
-            if (host_usage.projected_flops > bavp->host_usage.projected_flops) {
+            double r = 1 + .1*rand_normal();
+            if (r*host_usage.projected_flops > bavp->host_usage.projected_flops) {
                 bavp->host_usage = host_usage;
                 bavp->avp = &av;
                 bavp->reliable = app_version_is_reliable(av.id);