--- old/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	2011-12-09 10:54:40.445570695 -0800
+++ new/src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp	2011-12-09 10:54:40.053089419 -0800
@@ -1842,7 +1842,9 @@
   _full_collections_completed(0),
   _in_cset_fast_test(NULL),
   _in_cset_fast_test_base(NULL),
-  _dirty_cards_region_list(NULL) {
+  _dirty_cards_region_list(NULL),
+  _worker_cset_start_region(NULL),
+  _worker_cset_start_region_stamp(NULL) {
   _g1h = this; // To catch bugs.
   if (_process_strong_tasks == NULL || !_process_strong_tasks->valid()) {
     vm_exit_during_initialization("Failed necessary allocation.");
@@ -1863,10 +1865,16 @@
   }
   _rem_set_iterator = iter_arr;
 
+  _worker_cset_start_region = NEW_C_HEAP_ARRAY(HeapRegion*, n_queues);
+  _worker_cset_start_region_stamp = NEW_C_HEAP_ARRAY(unsigned int, n_queues);
+
   for (int i = 0; i < n_queues; i++) {
     RefToScanQueue* q = new RefToScanQueue();
     q->initialize();
     _task_queues->register_queue(i, q);
+    
+    _worker_cset_start_region[i] = NULL;
+    _worker_cset_start_region_stamp[i] = 0;
   }
 
   guarantee(_task_queues != NULL, "task_queues allocation failure.");
@@ -2684,16 +2692,37 @@
 }
 #endif // ASSERT
 
-// We want the parallel threads to start their collection
-// set iteration at different collection set regions to
-// avoid contention.
-// If we have:
-//          n collection set regions
-//          p threads
-// Then thread t will start at region t * floor (n/p)
-
+// Given the id of a worker, obtain or calculate a suitable
+// starting region for iterating over the current collection set.
 HeapRegion* G1CollectedHeap::start_cset_region_for_worker(int worker_i) {
-  HeapRegion* result = g1_policy()->collection_set();
+  assert(_worker_cset_start_region != NULL, "sanity");
+  assert(_worker_cset_start_region_stamp != NULL, "sanity");
+  assert(total_collections() > 0, "should have been updated by now");
+
+  HeapRegion* result = NULL;
+
+  if (_worker_cset_start_region_stamp[worker_i] == total_collections()) {
+    // Cached starting region for current worker was set
+    // during the current pause - so it's valid.
+    // Note: the cached starting heap region may be NULL
+    // (when the collection set is empty).
+    result = _worker_cset_start_region[worker_i];
+    assert(result == NULL || result->in_collection_set(), "sanity");
+    return result;
+  }
+
+  // The cached entry was not valid so let's calculate
+  // a suitable starting heap region for this worker.
+
+  // We want the parallel threads to start their collection
+  // set iteration at different collection set regions to
+  // avoid contention.
+  // If we have:
+  //          n collection set regions
+  //          p threads
+  // Then thread t will start at region t * floor (n/p)
+
+  result = g1_policy()->collection_set();
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     size_t cs_size = g1_policy()->cset_region_length();
     int n_workers = workers()->total_workers();
@@ -2703,6 +2732,14 @@
       result = result->next_in_collection_set();
     }
   }
+
+  // Note: the calculated starting heap region may be NULL
+  // (when the collection set is empty).
+  assert(result == NULL || result->in_collection_set(), "sanity");
+  assert(_worker_cset_start_region_stamp[worker_i] != total_collections(),
+         "should be updated only once per pause");
+  _worker_cset_start_region[worker_i] = result;
+  _worker_cset_start_region_stamp[worker_i] = total_collections();
   return result;
 }
 
@@ -3532,7 +3569,7 @@
       // Please see comment in g1CollectedHeap.hpp and
       // G1CollectedHeap::ref_processing_init() to see how
       // reference processing currently works in G1.
-
+      
       // Enable discovery in the STW reference processor
       ref_processor_stw()->enable_discovery(true /*verify_disabled*/,
                                             true /*verify_no_refs*/);
@@ -5249,7 +5286,7 @@
                         workers()->active_workers() : 1);
 
   assert(active_workers == workers()->active_workers(),
-         "Need to reset active_workers");
+         "Need to reset active_workers");  
   set_par_threads(active_workers);
   G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);