8329223: Parallel: Parallel GC resizes heap even if -Xms = -Xmx

Backport-of: aca1e8365bf0f64bf18caf798bbca1d25b3c4117

Author: Ben Taylor

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -276,6 +276,9 @@ void GenArguments::initialize_size_info() {
   // and maximum heap size since no explicit flags exist
   // for setting the old generation maximum.
   MaxOldSize = MAX2(MaxHeapSize - max_young_size, GenAlignment);
+  MinOldSize = MIN3(MaxOldSize,
+                    InitialHeapSize - initial_young_size,
+                    MinHeapSize - MinNewSize);
 
   size_t initial_old_size = OldSize;
 
@@ -287,9 +290,8 @@ void GenArguments::initialize_size_info() {
     // with the overall heap size).  In either case make
     // the minimum, maximum and initial sizes consistent
     // with the young sizes and the overall heap sizes.
-    MinOldSize = GenAlignment;
     initial_old_size = clamp(InitialHeapSize - initial_young_size, MinOldSize, MaxOldSize);
-    // MaxOldSize has already been made consistent above.
+    // MaxOldSize and MinOldSize have already been made consistent above.
   } else {
     // OldSize has been explicitly set on the command line. Use it
     // for the initial size but make sure the minimum allow a young
@@ -304,9 +306,10 @@ void GenArguments::initialize_size_info() {
                             ", -XX:OldSize flag is being ignored",
                             MaxHeapSize);
       initial_old_size = MaxOldSize;
+    } else if (initial_old_size < MinOldSize) {
+      log_warning(gc, ergo)("Inconsistency between initial old size and minimum old size");
+      MinOldSize = initial_old_size;
     }
-
-    MinOldSize = MIN2(initial_old_size, MinHeapSize - MinNewSize);
   }
 
   // The initial generation sizes should match the initial heap size,