8008546: Wrong G1ConfidencePercent results in GUARANTEE(VARIANCE() > -1.0) FAILED

Reviewed-by: brutisso, johnc
Contributed-by: vladimir.kempik@oracle.com

src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp

@@ -267,7 +267,15 @@ G1CollectorPolicy::G1CollectorPolicy() :
   double max_gc_time = (double) MaxGCPauseMillis / 1000.0;
   double time_slice  = (double) GCPauseIntervalMillis / 1000.0;
   _mmu_tracker = new G1MMUTrackerQueue(time_slice, max_gc_time);
-  _sigma = (double) G1ConfidencePercent / 100.0;
+
+  uintx confidence_perc = G1ConfidencePercent;
+  // Put an artificial ceiling on this so that it's not set to a silly value.
+  if (confidence_perc > 100) {
+    confidence_perc = 100;
+    warning("G1ConfidencePercent is set to a value that is too large, "
+            "it's been updated to %u", confidence_perc);
+  }
+  _sigma = (double) confidence_perc / 100.0;
 
   // start conservatively (around 50ms is about right)
   _concurrent_mark_remark_times_ms->add(0.05);

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -32,7 +32,7 @@
 
 #define G1_FLAGS(develop, develop_pd, product, product_pd, diagnostic, experimental, notproduct, manageable, product_rw) \
                                                                             \
-  product(intx, G1ConfidencePercent, 50,                                    \
+  product(uintx, G1ConfidencePercent, 50,                                   \
           "Confidence level for MMU/pause predictions")                     \
                                                                             \
   develop(intx, G1MarkingOverheadPercent, 0,                                \