--- old/src/hotspot/share/gc/z/zDirector.cpp	2018-03-05 15:15:01.306448871 +0100
+++ new/src/hotspot/share/gc/z/zDirector.cpp	2018-03-05 15:15:01.131441372 +0100
@@ -162,6 +162,9 @@
   const double time_since_last_gc_threshold = 5 * 60; // 5 minutes
   if (used < used_threshold && time_since_last_gc < time_since_last_gc_threshold) {
     // Don't even consider doing a proactive GC
+    log_debug(gc, director)("Rule: Proactive, UsedUntilEnabled: " SIZE_FORMAT "MB, TimeUntilEnabled: %.3lfs",
+                            (used_threshold - used) / M,
+                            time_since_last_gc_threshold - time_since_last_gc);
     return false;
   }