8244603: G1 incorrectly limiting young gen size when using the reserve can result in repeated full gcs

8238858: G1 Mixed gc young gen sizing might cause the first mixed gc to immediately follow the prepare mixed gc

Reviewed-by: sjohanss, iwalulya

Author: Thomas Schatzl

src/hotspot/share/gc/g1/g1Analytics.cpp

@@ -224,7 +224,11 @@ void G1Analytics::report_rs_length(double rs_length) {
 }
 
 double G1Analytics::predict_alloc_rate_ms() const {
-  return predict_zero_bounded(_alloc_rate_ms_seq);
+  if (enough_samples_available(_alloc_rate_ms_seq)) {
+    return predict_zero_bounded(_alloc_rate_ms_seq);
+  } else {
+    return 0.0;
+  }
 }
 
 double G1Analytics::predict_concurrent_refine_rate_ms() const {