6937160: G1: should observe GCTimeRatio

Summary: Remove the G1GCPercent parameter, that specifies the desired GC overhead percentage in G1, and observe the GCTimeRatio parameter instead.
Reviewed-by: jmasa, johnc

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

@@ -198,7 +198,9 @@ G1CollectorPolicy::G1CollectorPolicy() :
   _recorded_survivor_regions(0),
   _recorded_survivor_head(NULL),
   _recorded_survivor_tail(NULL),
-  _survivors_age_table(true)
+  _survivors_age_table(true),
+
+  _gc_overhead_perc(0.0)
 
 {
   // Set up the region size and associated fields. Given that the
@@ -275,6 +277,11 @@ G1CollectorPolicy::G1CollectorPolicy() :
   // calculate_young_list_target_config during initialization
   _max_survivor_regions = G1FixedSurvivorSpaceSize / HeapRegion::GrainBytes;
 
+  assert(GCTimeRatio > 0,
+         "we should have set it to a default value set_g1_gc_flags() "
+         "if a user set it to 0");
+  _gc_overhead_perc = 100.0 * (1.0 / (1.0 + GCTimeRatio));
+
   initialize_all();
 }
 
@@ -2288,7 +2295,7 @@ G1CollectorPolicy::conservative_avg_survival_fraction_work(double avg,
 }
 
 size_t G1CollectorPolicy::expansion_amount() {
-  if ((int)(recent_avg_pause_time_ratio() * 100.0) > G1GCPercent) {
+  if ((recent_avg_pause_time_ratio() * 100.0) > _gc_overhead_perc) {
     // We will double the existing space, or take
     // G1ExpandByPercentOfAvailable % of the available expansion
     // space, whichever is smaller, bounded below by a minimum

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

@@ -215,6 +215,8 @@ protected:
   SurvRateGroup*        _survivor_surv_rate_group;
   // add here any more surv rate groups
 
+  double                _gc_overhead_perc;
+
   bool during_marking() {
     return _during_marking;
   }

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -40,9 +40,6 @@
   develop(bool, G1Gen, true,                                                \
           "If true, it will enable the generational G1")                    \
                                                                             \
-  develop(intx, G1GCPercent, 10,                                            \
-          "The desired percent time spent on GC")                           \
-                                                                            \
   develop(intx, G1PolicyVerbose, 0,                                         \
           "The verbosity level on G1 policy decisions")                     \
                                                                             \

src/share/vm/runtime/arguments.cpp

@@ -1353,6 +1353,16 @@ void Arguments::set_g1_gc_flags() {
       MarkStackSize / K, MarkStackSizeMax / K);
     tty->print_cr("ConcGCThreads: %u", ConcGCThreads);
   }
+
+  if (FLAG_IS_DEFAULT(GCTimeRatio) || GCTimeRatio == 0) {
+    // In G1, we want the default GC overhead goal to be higher than
+    // say in PS. So we set it here to 10%. Otherwise the heap might
+    // be expanded more aggressively than we would like it to. In
+    // fact, even 10% seems to not be high enough in some cases
+    // (especially small GC stress tests that the main thing they do
+    // is allocation). We might consider increase it further.
+    FLAG_SET_DEFAULT(GCTimeRatio, 9);
+  }
 }
 
 void Arguments::set_heap_size() {