8345323: Parallel GC does not handle UseLargePages and UseNUMA gracefully

Reviewed-by: sjohanss, tschatzl

src/hotspot/share/gc/shared/genArguments.cpp

@@ -37,7 +37,11 @@ size_t MinNewSize = 0;
 size_t MinOldSize = 0;
 size_t MaxOldSize = 0;
 
-size_t OldSize = 0;
+// If InitialHeapSize or MinHeapSize is not set on cmdline, this variable,
+// together with NewSize, is used to derive them.
+// Using the same value when it was a configurable flag to avoid breakage.
+// See more in JDK-8346005
+size_t OldSize = ScaleForWordSize(4*M);
 
 size_t GenAlignment = 0;