7119027: G1: use atomics to update RS length / predict time of inc CSet

Summary: Make sure that the updates to the RS length and inc CSet predicted time are updated in an MT-safe way.
Reviewed-by: brutisso, iveresov

src/share/vm/gc_implementation/g1/g1CollectorPolicy.cpp

@@ -230,7 +230,9 @@ G1CollectorPolicy::G1CollectorPolicy() :
   _inc_cset_bytes_used_before(0),
   _inc_cset_max_finger(NULL),
   _inc_cset_recorded_rs_lengths(0),
+  _inc_cset_recorded_rs_lengths_diffs(0),
   _inc_cset_predicted_elapsed_time_ms(0.0),
+  _inc_cset_predicted_elapsed_time_ms_diffs(0.0),
 
 #ifdef _MSC_VER // the use of 'this' below gets a warning, make it go away
 #pragma warning( disable:4355 ) // 'this' : used in base member initializer list
@@ -1551,10 +1553,19 @@ void G1CollectorPolicy::record_collection_pause_end(int no_of_gc_threads) {
       }
     }
 
-    // It turns out that, sometimes, _max_rs_lengths can get smaller
-    // than _recorded_rs_lengths which causes rs_length_diff to get
-    // very large and mess up the RSet length predictions. We'll be
-    // defensive until we work out why this happens.
+    // This is defensive. For a while _max_rs_lengths could get
+    // smaller than _recorded_rs_lengths which was causing
+    // rs_length_diff to get very large and mess up the RSet length
+    // predictions. The reason was unsafe concurrent updates to the
+    // _inc_cset_recorded_rs_lengths field which the code below guards
+    // against (see CR 7118202). This bug has now been fixed (see CR
+    // 7119027). However, I'm still worried that
+    // _inc_cset_recorded_rs_lengths might still end up somewhat
+    // inaccurate. The concurrent refinement thread calculates an
+    // RSet's length concurrently with other CR threads updating it
+    // which might cause it to calculate the length incorrectly (if,
+    // say, it's in mid-coarsening). So I'll leave in the defensive
+    // conditional below just in case.
     size_t rs_length_diff = 0;
     if (_max_rs_lengths > _recorded_rs_lengths) {
       rs_length_diff = _max_rs_lengths - _recorded_rs_lengths;
@@ -2436,10 +2447,45 @@ void G1CollectorPolicy::start_incremental_cset_building() {
 
   _inc_cset_max_finger = 0;
   _inc_cset_recorded_rs_lengths = 0;
-  _inc_cset_predicted_elapsed_time_ms = 0;
+  _inc_cset_recorded_rs_lengths_diffs = 0;
+  _inc_cset_predicted_elapsed_time_ms = 0.0;
+  _inc_cset_predicted_elapsed_time_ms_diffs = 0.0;
   _inc_cset_build_state = Active;
 }
 
+void G1CollectorPolicy::finalize_incremental_cset_building() {
+  assert(_inc_cset_build_state == Active, "Precondition");
+  assert(SafepointSynchronize::is_at_safepoint(), "should be at a safepoint");
+
+  // The two "main" fields, _inc_cset_recorded_rs_lengths and
+  // _inc_cset_predicted_elapsed_time_ms, are updated by the thread
+  // that adds a new region to the CSet. Further updates by the
+  // concurrent refinement thread that samples the young RSet lengths
+  // are accumulated in the *_diffs fields. Here we add the diffs to
+  // the "main" fields.
+
+  if (_inc_cset_recorded_rs_lengths_diffs >= 0) {
+    _inc_cset_recorded_rs_lengths += _inc_cset_recorded_rs_lengths_diffs;
+  } else {
+    // This is defensive. The diff should in theory be always positive
+    // as RSets can only grow between GCs. However, given that we
+    // sample their size concurrently with other threads updating them
+    // it's possible that we might get the wrong size back, which
+    // could make the calculations somewhat inaccurate.
+    size_t diffs = (size_t) (-_inc_cset_recorded_rs_lengths_diffs);
+    if (_inc_cset_recorded_rs_lengths >= diffs) {
+      _inc_cset_recorded_rs_lengths -= diffs;
+    } else {
+      _inc_cset_recorded_rs_lengths = 0;
+    }
+  }
+  _inc_cset_predicted_elapsed_time_ms +=
+                                     _inc_cset_predicted_elapsed_time_ms_diffs;
+
+  _inc_cset_recorded_rs_lengths_diffs = 0;
+  _inc_cset_predicted_elapsed_time_ms_diffs = 0.0;
+}
+
 void G1CollectorPolicy::add_to_incremental_cset_info(HeapRegion* hr, size_t rs_length) {
   // This routine is used when:
   // * adding survivor regions to the incremental cset at the end of an
@@ -2455,10 +2501,8 @@ void G1CollectorPolicy::add_to_incremental_cset_info(HeapRegion* hr, size_t rs_l
 
   double region_elapsed_time_ms = predict_region_elapsed_time_ms(hr, true);
   size_t used_bytes = hr->used();
-
   _inc_cset_recorded_rs_lengths += rs_length;
   _inc_cset_predicted_elapsed_time_ms += region_elapsed_time_ms;
-
   _inc_cset_bytes_used_before += used_bytes;
 
   // Cache the values we have added to the aggregated informtion
@@ -2469,37 +2513,33 @@ void G1CollectorPolicy::add_to_incremental_cset_info(HeapRegion* hr, size_t rs_l
   hr->set_predicted_elapsed_time_ms(region_elapsed_time_ms);
 }
 
-void G1CollectorPolicy::remove_from_incremental_cset_info(HeapRegion* hr) {
-  // This routine is currently only called as part of the updating of
-  // existing policy information for regions in the incremental cset that
-  // is performed by the concurrent refine thread(s) as part of young list
-  // RSet sampling. Therefore we should not be at a safepoint.
-
-  assert(!SafepointSynchronize::is_at_safepoint(), "should not be at safepoint");
-  assert(hr->is_young(), "it should be");
+void G1CollectorPolicy::update_incremental_cset_info(HeapRegion* hr,
+                                                     size_t new_rs_length) {
+  // Update the CSet information that is dependent on the new RS length
+  assert(hr->is_young(), "Precondition");
+  assert(!SafepointSynchronize::is_at_safepoint(),
+                                               "should not be at a safepoint");
+
+  // We could have updated _inc_cset_recorded_rs_lengths and
+  // _inc_cset_predicted_elapsed_time_ms directly but we'd need to do
+  // that atomically, as this code is executed by a concurrent
+  // refinement thread, potentially concurrently with a mutator thread
+  // allocating a new region and also updating the same fields. To
+  // avoid the atomic operations we accumulate these updates on two
+  // separate fields (*_diffs) and we'll just add them to the "main"
+  // fields at the start of a GC.
+
+  ssize_t old_rs_length = (ssize_t) hr->recorded_rs_length();
+  ssize_t rs_lengths_diff = (ssize_t) new_rs_length - old_rs_length;
+  _inc_cset_recorded_rs_lengths_diffs += rs_lengths_diff;
 
-  size_t used_bytes = hr->used();
-  size_t old_rs_length = hr->recorded_rs_length();
   double old_elapsed_time_ms = hr->predicted_elapsed_time_ms();
+  double new_region_elapsed_time_ms = predict_region_elapsed_time_ms(hr, true);
+  double elapsed_ms_diff = new_region_elapsed_time_ms - old_elapsed_time_ms;
+  _inc_cset_predicted_elapsed_time_ms_diffs += elapsed_ms_diff;
 
-  // Subtract the old recorded/predicted policy information for
-  // the given heap region from the collection set info.
-  _inc_cset_recorded_rs_lengths -= old_rs_length;
-  _inc_cset_predicted_elapsed_time_ms -= old_elapsed_time_ms;
-
-  _inc_cset_bytes_used_before -= used_bytes;
-
-  // Clear the values cached in the heap region
-  hr->set_recorded_rs_length(0);
-  hr->set_predicted_elapsed_time_ms(0);
-}
-
-void G1CollectorPolicy::update_incremental_cset_info(HeapRegion* hr, size_t new_rs_length) {
-  // Update the collection set information that is dependent on the new RS length
-  assert(hr->is_young(), "Precondition");
-
-  remove_from_incremental_cset_info(hr);
-  add_to_incremental_cset_info(hr, new_rs_length);
+  hr->set_recorded_rs_length(new_rs_length);
+  hr->set_predicted_elapsed_time_ms(new_region_elapsed_time_ms);
 }
 
 void G1CollectorPolicy::add_region_to_incremental_cset_common(HeapRegion* hr) {
@@ -2591,6 +2631,7 @@ void G1CollectorPolicy::choose_collection_set(double target_pause_time_ms) {
   double non_young_start_time_sec = os::elapsedTime();
 
   YoungList* young_list = _g1->young_list();
+  finalize_incremental_cset_building();
 
   guarantee(target_pause_time_ms > 0.0,
             err_msg("target_pause_time_ms = %1.6lf should be positive",

src/share/vm/gc_implementation/g1/g1CollectorPolicy.hpp

@@ -588,16 +588,29 @@ private:
   // Used to record the highest end of heap region in collection set
   HeapWord* _inc_cset_max_finger;
 
-  // The RSet lengths recorded for regions in the collection set
-  // (updated by the periodic sampling of the regions in the
-  // young list/collection set).
+  // The RSet lengths recorded for regions in the CSet. It is updated
+  // by the thread that adds a new region to the CSet. We assume that
+  // only one thread can be allocating a new CSet region (currently,
+  // it does so after taking the Heap_lock) hence no need to
+  // synchronize updates to this field.
   size_t _inc_cset_recorded_rs_lengths;
 
-  // The predicted elapsed time it will take to collect the regions
-  // in the collection set (updated by the periodic sampling of the
-  // regions in the young list/collection set).
+  // A concurrent refinement thread periodcially samples the young
+  // region RSets and needs to update _inc_cset_recorded_rs_lengths as
+  // the RSets grow. Instead of having to syncronize updates to that
+  // field we accumulate them in this field and add it to
+  // _inc_cset_recorded_rs_lengths_diffs at the start of a GC.
+  ssize_t _inc_cset_recorded_rs_lengths_diffs;
+
+  // The predicted elapsed time it will take to collect the regions in
+  // the CSet. This is updated by the thread that adds a new region to
+  // the CSet. See the comment for _inc_cset_recorded_rs_lengths about
+  // MT-safety assumptions.
   double _inc_cset_predicted_elapsed_time_ms;
 
+  // See the comment for _inc_cset_recorded_rs_lengths_diffs.
+  double _inc_cset_predicted_elapsed_time_ms_diffs;
+
   // Stash a pointer to the g1 heap.
   G1CollectedHeap* _g1;
 
@@ -894,6 +907,10 @@ public:
   // Initialize incremental collection set info.
   void start_incremental_cset_building();
 
+  // Perform any final calculations on the incremental CSet fields
+  // before we can use them.
+  void finalize_incremental_cset_building();
+
   void clear_incremental_cset() {
     _inc_cset_head = NULL;
     _inc_cset_tail = NULL;
@@ -902,10 +919,9 @@ public:
   // Stop adding regions to the incremental collection set
   void stop_incremental_cset_building() { _inc_cset_build_state = Inactive; }
 
-  // Add/remove information about hr to the aggregated information
-  // for the incrementally built collection set.
+  // Add information about hr to the aggregated information for the
+  // incrementally built collection set.
   void add_to_incremental_cset_info(HeapRegion* hr, size_t rs_length);
-  void remove_from_incremental_cset_info(HeapRegion* hr);
 
   // Update information about hr in the aggregated information for
   // the incrementally built collection set.