src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

rev 2891 : 7120038: G1: ParallelGCThreads==0 is broken
Summary: Running G1 with ParallelGCThreads==0 results in various crashes and asserts. Most of these are caused by unguarded references to the worker threads array or an incorrect number of active workers.
Reviewed-by:

@@ -3721,12 +3721,13 @@
         }
 
         double end_time_sec = os::elapsedTime();
         double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
         g1_policy()->record_pause_time_ms(pause_time_ms);
-        int active_gc_threads = workers()->active_workers();
-        g1_policy()->record_collection_pause_end(active_gc_threads);
+        int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
+                                workers()->active_workers() : 1);
+        g1_policy()->record_collection_pause_end(active_workers);
 
         MemoryService::track_memory_usage();
 
         // In prepare_for_verify() below we'll need to scan the deferred
         // update buffers to bring the RSets up-to-date if

@@ -5246,12 +5247,14 @@
   // object discovered by the STW ref processor.
 
   int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
                         workers()->active_workers() : 1);
 
-  assert(active_workers == workers()->active_workers(),
+  assert(!G1CollectedHeap::use_parallel_gc_threads() ||
+           active_workers == workers()->active_workers(),
          "Need to reset active_workers");
+
   set_par_threads(active_workers);
   G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
     workers()->run_task(&keep_cm_referents);

@@ -5385,17 +5388,17 @@
                                      workers()->active_workers(),
                                      Threads::number_of_non_daemon_threads());
     assert(UseDynamicNumberOfGCThreads ||
            n_workers == workers()->total_workers(),
            "If not dynamic should be using all the  workers");
+    workers()->set_active_workers(n_workers);
     set_par_threads(n_workers);
   } else {
     assert(n_par_threads() == 0,
            "Should be the original non-parallel value");
     n_workers = 1;
   }
-  workers()->set_active_workers(n_workers);
 
   G1ParTask g1_par_task(this, _task_queues);
 
   init_for_evac_failure(NULL);
 

@@ -5413,10 +5416,11 @@
            workers()->active_workers() == workers()->total_workers(),
            "If not dynamic should be using all the  workers");
     workers()->run_task(&g1_par_task);
   } else {
     StrongRootsScope srs(this);
+    g1_par_task.set_for_termination(n_workers);
     g1_par_task.work(0);
   }
 
   double par_time = (os::elapsedTime() - start_par) * 1000.0;
   g1_policy()->record_par_time(par_time);

@@ -6070,10 +6074,11 @@
 }
 
 void G1CollectedHeap::set_par_threads() {
   // Don't change the number of workers.  Use the value previously set
   // in the workgroup.
+  assert(G1CollectedHeap::use_parallel_gc_threads(), "shouldn't be here otherwise");
   int n_workers = workers()->active_workers();
     assert(UseDynamicNumberOfGCThreads ||
            n_workers == workers()->total_workers(),
       "Otherwise should be using the total number of workers");
   if (n_workers == 0) {