6941378: G1: change default value of G1UseFixedWindowMMUTracker to true

Summary: Rather than changing the default value of the G1UseFixedWindowMMUTracker, the flag and associated guarantee have been removed.
Reviewed-by: jcoomes, tonyp, ysr

hotspot/src/share/vm/gc_implementation/g1/g1MMUTracker.cpp

@@ -81,20 +81,24 @@ void G1MMUTrackerQueue::add_pause(double start, double end, bool gc_thread) {
 
   remove_expired_entries(end);
   if (_no_entries == QueueLength) {
-    // OK, right now when we fill up we bomb out
-    // there are a few ways of dealing with this "gracefully"
+    // OK, we've filled up the queue. There are a few ways
+    // of dealing with this "gracefully"
     //   increase the array size (:-)
     //   remove the oldest entry (this might allow more GC time for
-    //     the time slice than what's allowed)
+    //     the time slice than what's allowed) - this is what we
+    //     currently do
     //   consolidate the two entries with the minimum gap between them
     //     (this might allow less GC time than what's allowed)
-    guarantee(NOT_PRODUCT(ScavengeALot ||) G1UseFixedWindowMMUTracker,
-              "array full, currently we can't recover unless +G1UseFixedWindowMMUTracker");
+
     // In the case where ScavengeALot is true, such overflow is not
     // uncommon; in such cases, we can, without much loss of precision
     // or performance (we are GC'ing most of the time anyway!),
-    // simply overwrite the oldest entry in the tracker: this
-    // is also the behaviour when G1UseFixedWindowMMUTracker is enabled.
+    // simply overwrite the oldest entry in the tracker.
+
+    if (G1PolicyVerbose > 1) {
+      warning("MMU Tracker Queue overflow. Replacing earliest entry.");
+    }
+
     _head_index = trim_index(_head_index + 1);
     assert(_head_index == _tail_index, "Because we have a full circular buffer");
     _tail_index = trim_index(_tail_index + 1);

hotspot/src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -254,9 +254,6 @@
           "If non-0 is the size of the G1 survivor space, "                 \
           "otherwise SurvivorRatio is used to determine the size")          \
                                                                             \
-  product(bool, G1UseFixedWindowMMUTracker, false,                          \
-          "If the MMU tracker's memory is full, forget the oldest entry")   \
-                                                                            \
   product(uintx, G1HeapRegionSize, 0,                                       \
           "Size of the G1 regions.")                                        \
                                                                             \