8337828: CDS: Trim down minimum GC region alignment

Reviewed-by: iklam, phh

Author: shipilev

src/hotspot/share/cds/archiveHeapWriter.hpp

@@ -112,11 +112,10 @@ class ArchiveHeapWriter : AllStatic {
 public:
   static const intptr_t NOCOOPS_REQUESTED_BASE = 0x10000000;
 
-  // The minimum region size of all collectors that are supported by CDS in
-  // ArchiveHeapLoader::can_map() mode. Currently only G1 is supported. G1's region size
-  // depends on -Xmx, but can never be smaller than 1 * M.
-  // (TODO: Perhaps change to 256K to be compatible with Shenandoah)
-  static constexpr int MIN_GC_REGION_ALIGNMENT = 1 * M;
+  // The minimum region size of all collectors that are supported by CDS.
+  // G1 heap region size can never be smaller than 1M.
+  // Shenandoah heap region size can never be smaller than 256K.
+  static constexpr int MIN_GC_REGION_ALIGNMENT = 256 * K;
 
 private:
   class EmbeddedOopRelocator;