8236176: Parallel GC SplitInfo comment should be updated for shadow r…

…egions

Reviewed-by: tschatzl, ayang

src/hotspot/share/gc/parallel/psParallelCompact.cpp

@@ -1792,7 +1792,7 @@ bool PSParallelCompact::invoke_no_policy(bool maximum_heap_compaction) {
     ref_processor()->start_discovery(maximum_heap_compaction);
 
     marking_start.update();
-    marking_phase(vmthread_cm, maximum_heap_compaction, &_gc_tracer);
+    marking_phase(vmthread_cm, &_gc_tracer);
 
     bool max_on_system_gc = UseMaximumCompactionOnSystemGC
       && GCCause::is_user_requested_gc(gc_cause);
@@ -2077,7 +2077,6 @@ class ParallelCompactRefProcProxyTask : public RefProcProxyTask {
 };
 
 void PSParallelCompact::marking_phase(ParCompactionManager* cm,
-                                      bool maximum_heap_compaction,
                                       ParallelOldTracer *gc_tracer) {
   // Recursively traverse all live objects and mark them
   GCTraceTime(Info, gc, phases) tm("Marking Phase", &_gc_timer);
@@ -2282,7 +2281,6 @@ void PSParallelCompact::prepare_region_draining_tasks(uint parallel_gc_threads)
   FillableRegionLogger region_logger;
   for (unsigned int id = to_space_id; id + 1 > old_space_id; --id) {
     SpaceInfo* const space_info = _space_info + id;
-    MutableSpace* const space = space_info->space();
     HeapWord* const new_top = space_info->new_top();
 
     const size_t beg_region = sd.addr_to_region_idx(space_info->dense_prefix());
@@ -3092,12 +3090,6 @@ bool PSParallelCompact::steal_unavailable_region(ParCompactionManager* cm, size_
 // the shadow region by copying live objects from source regions of the unavailable one. Once
 // the unavailable region becomes available, the data in the shadow region will be copied back.
 // Shadow regions are empty regions in the to-space and regions between top and end of other spaces.
-//
-// For more details, please refer to §4.2 of the VEE'19 paper:
-// Haoyu Li, Mingyu Wu, Binyu Zang, and Haibo Chen. 2019. ScissorGC: scalable and efficient
-// compaction for Java full garbage collection. In Proceedings of the 15th ACM SIGPLAN/SIGOPS
-// International Conference on Virtual Execution Environments (VEE 2019). ACM, New York, NY, USA,
-// 108-121. DOI: https://doi.org/10.1145/3313808.3313820
 void PSParallelCompact::initialize_shadow_regions(uint parallel_gc_threads)
 {
   const ParallelCompactData& sd = PSParallelCompact::summary_data();

src/hotspot/share/gc/parallel/psParallelCompact.hpp

@@ -950,7 +950,6 @@ inline void ParMarkBitMapClosure::decrement_words_remaining(size_t words) {
 // has been updated.  KKK likely resides in a region to the left of the region
 // containing AAA.  These AAA's have there references updated at the end in a
 // clean up phase.  See the method PSParallelCompact::update_deferred_objects().
-// An alternate strategy is being investigated for this deferral of updating.
 //
 // Compaction is done on a region basis.  A region that is ready to be filled is
 // put on a ready list and GC threads take region off the list and fill them.  A
@@ -961,6 +960,25 @@ inline void ParMarkBitMapClosure::decrement_words_remaining(size_t words) {
 // regions and regions compacting into themselves.  There is always at least 1
 // region that can be put on the ready list.  The regions are atomically added
 // and removed from the ready list.
+//
+// During compaction, there is a natural task dependency among regions because
+// destination regions may also be source regions themselves.  Consequently, the
+// destination regions are not available for processing until all live objects
+// within them are evacuated to their destinations.  These dependencies lead to
+// limited thread utilization as threads spin waiting on regions to be ready.
+// Shadow regions are utilized to address these region dependencies.  The basic
+// idea is that, if a region is unavailable because it still contains live
+// objects and thus cannot serve as a destination momentarily, the GC thread
+// may allocate a shadow region as a substitute destination and directly copy
+// live objects into this shadow region.  Live objects in the shadow region will
+// be copied into the target destination region when it becomes available.
+//
+// For more details on shadow regions, please refer to §4.2 of the VEE'19 paper:
+// Haoyu Li, Mingyu Wu, Binyu Zang, and Haibo Chen.  2019.  ScissorGC: scalable
+// and efficient compaction for Java full garbage collection.  In Proceedings of
+// the 15th ACM SIGPLAN/SIGOPS International Conference on Virtual Execution
+// Environments (VEE 2019).  ACM, New York, NY, USA, 108-121.  DOI:
+// https://doi.org/10.1145/3313808.3313820
 
 class TaskQueue;
 
@@ -1045,7 +1063,6 @@ class PSParallelCompact : AllStatic {
 
   // Mark live objects
   static void marking_phase(ParCompactionManager* cm,
-                            bool maximum_heap_compaction,
                             ParallelOldTracer *gc_tracer);
 
   // Compute the dense prefix for the designated space.  This is an experimental
@@ -1114,7 +1131,6 @@ class PSParallelCompact : AllStatic {
   DEBUG_ONLY(static void write_block_fill_histogram();)
 
   // Move objects to new locations.
-  static void compact_perm(ParCompactionManager* cm);
   static void compact();
 
   // Add available regions to the stack and draining tasks to the task queue.