src/share/vm/gc/shared/genCollectedHeap.cpp

@@ -1254,25 +1254,25 @@
     _time = MIN2(_time, gen->time_of_last_gc(_now));
   }
 };
 
 jlong GenCollectedHeap::millis_since_last_gc() {
-  // javaTimeNanos() is guaranteed to be monotonically non-decreasing
-  // provided the underlying platform provides such a time source
-  // (and it is bug free). So we still have to guard against getting
-  // back a time later than 'now'.
+  // We need a monotonically non-decreasing time in ms but
+  // os::javaTimeMillis() does not guarantee monotonicity.
   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
   GenTimeOfLastGCClosure tolgc_cl(now);
   // iterate over generations getting the oldest
   // time that a generation was collected
   generation_iterate(&tolgc_cl, false);
 
+  // javaTimeNanos() is guaranteed to be monotonically non-decreasing
+  // provided the underlying platform provides such a time source
+  // (and it is bug free). So we still have to guard against getting
+  // back a time later than 'now'.
   jlong retVal = now - tolgc_cl.time();
   if (retVal < 0) {
-    log_warning(gc)("Detected clock going backwards. "
-      "Milliseconds since last GC would be " JLONG_FORMAT
-      ". returning zero instead.", retVal);
+    NOT_PRODUCT(log_warning(gc)("time warp: " JLONG_FORMAT, retVal);)
     return 0;
   }
   return retVal;
 }