6679764: enable parallel compaction by default

Reviewed-by: phh, jmasa

src/share/vm/runtime/arguments.cpp

@@ -1400,10 +1400,11 @@ void Arguments::set_ergonomics_flags() {
 
 void Arguments::set_parallel_gc_flags() {
   assert(UseParallelGC || UseParallelOldGC, "Error");
-  // If parallel old was requested, automatically enable parallel scavenge.
-  if (UseParallelOldGC && !UseParallelGC && FLAG_IS_DEFAULT(UseParallelGC)) {
-    FLAG_SET_DEFAULT(UseParallelGC, true);
+  // Enable ParallelOld unless it was explicitly disabled (cmd line or rc file).
+  if (FLAG_IS_DEFAULT(UseParallelOldGC)) {
+    FLAG_SET_DEFAULT(UseParallelOldGC, true);
   }
+  FLAG_SET_DEFAULT(UseParallelGC, true);
 
   // If no heap maximum was requested explicitly, use some reasonable fraction
   // of the physical memory, up to a maximum of 1GB.