8337269: G1ConfidencePercent interpreted inconsistently

Reviewed-by: kbarrett, iwalulya

src/hotspot/share/gc/g1/g1Policy.cpp

@@ -53,7 +53,7 @@
 #include "gc/shared/gcTraceTime.inline.hpp"
 
 G1Policy::G1Policy(STWGCTimer* gc_timer) :
-  _predictor(G1ConfidencePercent / 100.0),
+  _predictor((100 - G1ConfidencePercent) / 100.0),
   _analytics(new G1Analytics(&_predictor)),
   _remset_tracker(),
   _mmu_tracker(new G1MMUTracker(GCPauseIntervalMillis / 1000.0, MaxGCPauseMillis / 1000.0)),

src/hotspot/share/gc/g1/g1Predictions.hpp

@@ -29,8 +29,9 @@
 
 // Utility class containing various helper methods for prediction.
 class G1Predictions {
- private:
-  double _sigma;
+private:
+  // Scale factor indicating to which degree stddev should be taking into account in predictions.
+  double _stddev_scale;
 
   // This function is used to estimate the stddev of sample sets. There is some
   // special consideration of small sample sets: the actual stddev for them is
@@ -46,16 +47,14 @@ class G1Predictions {
     }
     return estimate;
   }
- public:
-  G1Predictions(double sigma) : _sigma(sigma) {
-    assert(sigma >= 0.0, "Confidence must be larger than or equal to zero");
+
+public:
+  G1Predictions(double stddev_scale) : _stddev_scale(stddev_scale) {
+    assert(stddev_scale >= 0.0, "must be");
   }
 
-  // Confidence factor.
-  double sigma() const { return _sigma; }
-
   double predict(TruncatedSeq const* seq) const {
-    return seq->davg() + _sigma * stddev_estimate(seq);
+    return seq->davg() + _stddev_scale * stddev_estimate(seq);
   }
 
   double predict_in_unit_interval(TruncatedSeq const* seq) const {

src/hotspot/share/gc/g1/g1_globals.hpp

@@ -111,7 +111,8 @@
           range(1, max_intx)                                                \
                                                                             \
   product(uint, G1ConfidencePercent, 50,                                    \
-          "Confidence level for MMU/pause predictions")                     \
+          "Confidence level for MMU/pause predictions. A higher value "     \
+          "means that G1 will use less safety margin for its predictions.") \
           range(1, 100)                                                     \
                                                                             \
   product(uintx, G1SummarizeRSetStatsPeriod, 0, DIAGNOSTIC,                 \