8349906: G1: Improve initial survivor rate for newly used young regions

Reviewed-by: kbarrett, iwalulya

src/hotspot/share/gc/g1/g1SurvRateGroup.cpp

@@ -68,13 +68,26 @@ void G1SurvRateGroup::stop_adding_regions() {
     _accum_surv_rate_pred = REALLOC_C_HEAP_ARRAY(double, _accum_surv_rate_pred, _num_added_regions, mtGC);
     _surv_rate_predictors = REALLOC_C_HEAP_ARRAY(TruncatedSeq*, _surv_rate_predictors, _num_added_regions, mtGC);
 
+    // Assume that the prediction for the newly added regions is the same as the
+    // ones at the (current) end of the array. Particularly predictions at the end
+    // of this array fairly seldom get updated, so having a better initial value
+    // that is at least somewhat related to the actual application is preferable.
+    double new_pred = _stats_arrays_length > 1
+                    ? _accum_surv_rate_pred[_stats_arrays_length - 1] - _accum_surv_rate_pred[_stats_arrays_length - 2]
+                    : InitialSurvivorRate;
+
     for (size_t i = _stats_arrays_length; i < _num_added_regions; ++i) {
       // Initialize predictors and accumulated survivor rate predictions.
       _surv_rate_predictors[i] = new TruncatedSeq(10);
-      _surv_rate_predictors[i]->add(InitialSurvivorRate);
-      _accum_surv_rate_pred[i] = ((i == 0) ? 0.0 : _accum_surv_rate_pred[i-1]) + InitialSurvivorRate;
+      if (i == 0) {
+        _surv_rate_predictors[i]->add(InitialSurvivorRate);
+        _accum_surv_rate_pred[i] = 0.0;
+      } else {
+        _surv_rate_predictors[i]->add(_surv_rate_predictors[i-1]->last());
+        _accum_surv_rate_pred[i] = _accum_surv_rate_pred[i-1] + new_pred;
+      }
     }
-    _last_pred = InitialSurvivorRate;
+    _last_pred = new_pred;
 
     _stats_arrays_length = _num_added_regions;
   }