diff -r 10c6e9066819 -r e7fdc9d9c376 src/hotspot/share/runtime/synchronizer.cpp
--- a/src/hotspot/share/runtime/synchronizer.cpp	Tue Nov 27 17:53:17 2018 -0500
+++ b/src/hotspot/share/runtime/synchronizer.cpp	Tue Nov 27 18:35:16 2018 -0500
@@ -45,6 +45,7 @@
 #include "runtime/stubRoutines.hpp"
 #include "runtime/synchronizer.hpp"
 #include "runtime/thread.inline.hpp"
+#include "runtime/timer.hpp"
 #include "runtime/vframe.hpp"
 #include "runtime/vmThread.hpp"
 #include "utilities/align.hpp"
@@ -1618,9 +1619,10 @@
 }
 
 void ObjectSynchronizer::prepare_deflate_idle_monitors(DeflateMonitorCounters* counters) {
-  counters->nInuse = 0;          // currently associated with objects
-  counters->nInCirculation = 0;  // extant
-  counters->nScavenged = 0;      // reclaimed
+  counters->nInuse = 0;            // currently associated with objects
+  counters->nInCirculation = 0;    // extant
+  counters->nScavenged = 0;        // reclaimed
+  counters->perThreadTimes = 0.0;  // per-thread scavenge times
 }
 
 void ObjectSynchronizer::deflate_idle_monitors(DeflateMonitorCounters* counters) {
@@ -1660,6 +1662,14 @@
 }
 
 void ObjectSynchronizer::finish_deflate_idle_monitors(DeflateMonitorCounters* counters) {
+  if (log_is_enabled(Info, safepoint, cleanup)) {
+    // Report the cumulative time for deflating each thread's idle
+    // monitors. Note: if the work is split among more than one
+    // worker thread, then the reported time will likely be more
+    // than a beginning to end measurement of the phase.
+    log_info(safepoint, cleanup)("deflating per-thread idle monitors, %3.7f secs", counters->perThreadTimes);
+  }
+
   gMonitorFreeCount += counters->nScavenged;
 
   // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
@@ -1680,9 +1690,16 @@
 
   ObjectMonitor * freeHeadp = NULL;  // Local SLL of scavenged monitors
   ObjectMonitor * freeTailp = NULL;
+  elapsedTimer timer;
+
+  if (log_is_enabled(Info, safepoint, cleanup)) {
+    timer.start();
+  }
 
   int deflated_count = deflate_monitor_list(thread->omInUseList_addr(), &freeHeadp, &freeTailp);
 
+  timer.stop();
+
   Thread::muxAcquire(&gListLock, "scavenge - return");
 
   // Adjust counters
@@ -1690,6 +1707,8 @@
   thread->omInUseCount -= deflated_count;
   counters->nScavenged += deflated_count;
   counters->nInuse += thread->omInUseCount;
+  // For now, we only care about cumulative per-thread deflation time.
+  counters->perThreadTimes += timer.seconds();
 
   // Move the scavenged monitors back to the global free list.
   if (freeHeadp != NULL) {