--- a/hotspot/src/share/vm/memory/sharedHeap.hpp Tue Nov 22 04:47:10 2011 -0500
+++ b/hotspot/src/share/vm/memory/sharedHeap.hpp Tue Aug 09 10:16:01 2011 -0700
@@ -49,6 +49,62 @@
class CollectorPolicy;
class KlassHandle;
+// Note on use of FlexibleWorkGang's for GC.
+// There are three places where task completion is determined.
+// In
+// 1) ParallelTaskTerminator::offer_termination() where _n_threads
+// must be set to the correct value so that count of workers that
+// have offered termination will exactly match the number
+// working on the task. Tasks such as those derived from GCTask
+// use ParallelTaskTerminator's. Tasks that want load balancing
+// by work stealing use this method to gauge completion.
+// 2) SubTasksDone has a variable _n_threads that is used in
+// all_tasks_completed() to determine completion. all_tasks_complete()
+// counts the number of tasks that have been done and then reset
+// the SubTasksDone so that it can be used again. When the number of
+// tasks is set to the number of GC workers, then _n_threads must
+// be set to the number of active GC workers. G1CollectedHeap,
+// HRInto_G1RemSet, GenCollectedHeap and SharedHeap have SubTasksDone.
+// This seems too many.
+// 3) SequentialSubTasksDone has an _n_threads that is used in
+// a way similar to SubTasksDone and has the same dependency on the
+// number of active GC workers. CompactibleFreeListSpace and Space
+// have SequentialSubTasksDone's.
+// Example of using SubTasksDone and SequentialSubTasksDone
+// G1CollectedHeap::g1_process_strong_roots() calls
+// process_strong_roots(false, // no scoping; this is parallel code
+// collecting_perm_gen, so,
+// &buf_scan_non_heap_roots,
+// &eager_scan_code_roots,
+// &buf_scan_perm);
+// which delegates to SharedHeap::process_strong_roots() and uses
+// SubTasksDone* _process_strong_tasks to claim tasks.
+// process_strong_roots() calls
+// rem_set()->younger_refs_iterate(perm_gen(), perm_blk);
+// to scan the card table and which eventually calls down into
+// CardTableModRefBS::par_non_clean_card_iterate_work(). This method
+// uses SequentialSubTasksDone* _pst to claim tasks.
+// Both SubTasksDone and SequentialSubTasksDone call their method
+// all_tasks_completed() to count the number of GC workers that have
+// finished their work. That logic is "when all the workers are
+// finished the tasks are finished".
+//
+// The pattern that appears in the code is to set _n_threads
+// to a value > 1 before a task that you would like executed in parallel
+// and then to set it to 0 after that task has completed. A value of
+// 0 is a "special" value in set_n_threads() which translates to
+// setting _n_threads to 1.
+//
+// Some code uses _n_terminiation to decide if work should be done in
+// parallel. The notorious possibly_parallel_oops_do() in threads.cpp
+// is an example of such code. Look for variable "is_par" for other
+// examples.
+//
+// The active_workers is not reset to 0 after a parallel phase. It's
+// value may be used in later phases and in one instance at least
+// (the parallel remark) it has to be used (the parallel remark depends
+// on the partitioning done in the previous parallel scavenge).
+
class SharedHeap : public CollectedHeap {
friend class VMStructs;
@@ -84,11 +140,6 @@
// If we're doing parallel GC, use this gang of threads.
FlexibleWorkGang* _workers;
- // Number of parallel threads currently working on GC tasks.
- // O indicates use sequential code; 1 means use parallel code even with
- // only one thread, for performance testing purposes.
- int _n_par_threads;
-
// Full initialization is done in a concrete subtype's "initialize"
// function.
SharedHeap(CollectorPolicy* policy_);
@@ -107,6 +158,7 @@
CollectorPolicy *collector_policy() const { return _collector_policy; }
void set_barrier_set(BarrierSet* bs);
+ SubTasksDone* process_strong_tasks() { return _process_strong_tasks; }
// Does operations required after initialization has been done.
virtual void post_initialize();
@@ -198,13 +250,6 @@
FlexibleWorkGang* workers() const { return _workers; }
- // Sets the number of parallel threads that will be doing tasks
- // (such as process strong roots) subsequently.
- virtual void set_par_threads(int t);
-
- // Number of threads currently working on GC tasks.
- int n_par_threads() { return _n_par_threads; }
-
// Invoke the "do_oop" method the closure "roots" on all root locations.
// If "collecting_perm_gen" is false, then roots that may only contain
// references to permGen objects are not scanned; instead, in that case,
@@ -240,6 +285,13 @@
virtual void gc_prologue(bool full) = 0;
virtual void gc_epilogue(bool full) = 0;
+ // Sets the number of parallel threads that will be doing tasks
+ // (such as process strong roots) subsequently.
+ virtual void set_par_threads(int t);
+
+ int n_termination();
+ void set_n_termination(int t);
+
//
// New methods from CollectedHeap
//