changeset 7896:45dc3d4c5c24

8061802: REDO - Remove the generations array Summary: The _gens array is removed and replaced by explicit _young_gen and _old_gen variables. Reviewed-by: kbarrett, mgerdin
author jwilhelm
date Fri, 22 Aug 2014 10:10:08 +0200
parents a1676462e072
children b8ef0ff7b28d
files agent/src/share/classes/sun/jvm/hotspot/memory/GenCollectedHeap.java src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp src/share/vm/memory/defNewGeneration.cpp src/share/vm/memory/genCollectedHeap.cpp src/share/vm/memory/genCollectedHeap.hpp src/share/vm/memory/genMarkSweep.cpp src/share/vm/memory/generation.cpp src/share/vm/runtime/vmStructs.cpp test/gc/metaspace/CompressedClassSpaceSizeInJmapHeap.java
diffstat 9 files changed, 334 insertions(+), 337 deletions(-) [+]
line wrap: on
line diff
--- a/agent/src/share/classes/sun/jvm/hotspot/memory/GenCollectedHeap.java	Wed Feb 18 09:22:37 2015 +0100
+++ b/agent/src/share/classes/sun/jvm/hotspot/memory/GenCollectedHeap.java	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -35,7 +35,8 @@
 
 public class GenCollectedHeap extends SharedHeap {
   private static CIntegerField nGensField;
-  private static long gensOffset;
+  private static AddressField youngGenField;
+  private static AddressField oldGenField;
   private static AddressField genSpecsField;
 
   private static GenerationFactory genFactory;
@@ -52,7 +53,8 @@
     Type type = db.lookupType("GenCollectedHeap");
 
     nGensField = type.getCIntegerField("_n_gens");
-    gensOffset = type.getField("_gens").getOffset();
+    youngGenField = type.getAddressField("_young_gen");
+    oldGenField = type.getAddressField("_old_gen");
     genSpecsField = type.getAddressField("_gen_specs");
 
     genFactory = new GenerationFactory();
@@ -68,18 +70,19 @@
 
   public Generation getGen(int i) {
     if (Assert.ASSERTS_ENABLED) {
-      Assert.that((i >= 0) && (i < nGens()), "Index " + i +
-                  " out of range (should be between 0 and " + nGens() + ")");
+      Assert.that((i == 0) || (i == 1), "Index " + i +
+                  " out of range (should be 0 or 1)");
     }
 
-    if ((i < 0) || (i >= nGens())) {
+    switch (i) {
+    case 0:
+      return genFactory.newObject(youngGenField.getAddress());
+    case 1:
+      return genFactory.newObject(oldGenField.getAddress());
+    default:
+      // no generation for i, and assertions disabled.
       return null;
     }
-
-    Address genAddr = addr.getAddressAt(gensOffset +
-                                        (i * VM.getVM().getAddressSize()));
-    return genFactory.newObject(addr.getAddressAt(gensOffset +
-                                                  (i * VM.getVM().getAddressSize())));
   }
 
   public boolean isIn(Address a) {
--- a/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -869,7 +869,7 @@
       if (prev_level >= 0) {
         size_t prev_size = 0;
         GenCollectedHeap* gch = GenCollectedHeap::heap();
-        Generation* prev_gen = gch->_gens[prev_level];
+        Generation* prev_gen = gch->get_gen(prev_level);
         prev_size = prev_gen->capacity();
           gclog_or_tty->print_cr("  Younger gen size "SIZE_FORMAT,
                                  prev_size/1000);
--- a/src/share/vm/memory/defNewGeneration.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/memory/defNewGeneration.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -383,7 +383,7 @@
   assert(next_level < gch->_n_gens,
          "DefNewGeneration cannot be an oldest gen");
 
-  Generation* next_gen = gch->_gens[next_level];
+  Generation* next_gen = gch->get_gen(next_level);
   size_t old_size = next_gen->capacity();
   size_t new_size_before = _virtual_space.committed_size();
   size_t min_new_size = spec()->init_size();
--- a/src/share/vm/memory/genCollectedHeap.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/memory/genCollectedHeap.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -86,6 +86,7 @@
 
   int i;
   _n_gens = gen_policy()->number_of_generations();
+  assert(_n_gens == 2, "There is no support for more than two generations");
 
   // While there are no constraints in the GC code that HeapWordSize
   // be any particular value, there are multiple other areas in the
@@ -126,11 +127,12 @@
 
   _gch = this;
 
-  for (i = 0; i < _n_gens; i++) {
-    ReservedSpace this_rs = heap_rs.first_part(_gen_specs[i]->max_size(), false, false);
-    _gens[i] = _gen_specs[i]->init(this_rs, i, rem_set());
-    heap_rs = heap_rs.last_part(_gen_specs[i]->max_size());
-  }
+  ReservedSpace young_rs = heap_rs.first_part(_gen_specs[0]->max_size(), false, false);
+  _young_gen = _gen_specs[0]->init(young_rs, 0, rem_set());
+  heap_rs = heap_rs.last_part(_gen_specs[0]->max_size());
+
+  ReservedSpace old_rs = heap_rs.first_part(_gen_specs[1]->max_size(), false, false);
+  _old_gen = _gen_specs[1]->init(old_rs, 1, rem_set());
   clear_incremental_collection_failed();
 
 #if INCLUDE_ALL_GCS
@@ -145,7 +147,6 @@
   return JNI_OK;
 }
 
-
 char* GenCollectedHeap::allocate(size_t alignment,
                                  ReservedSpace* heap_rs){
   const char overflow_msg[] = "The size of the object heap + VM data exceeds "
@@ -172,7 +173,6 @@
   return heap_rs->base();
 }
 
-
 void GenCollectedHeap::post_initialize() {
   SharedHeap::post_initialize();
   GenCollectorPolicy *policy = (GenCollectorPolicy *)collector_policy();
@@ -195,41 +195,30 @@
 
 void GenCollectedHeap::ref_processing_init() {
   SharedHeap::ref_processing_init();
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->ref_processor_init();
-  }
+  _young_gen->ref_processor_init();
+  _old_gen->ref_processor_init();
 }
 
 size_t GenCollectedHeap::capacity() const {
-  size_t res = 0;
-  for (int i = 0; i < _n_gens; i++) {
-    res += _gens[i]->capacity();
-  }
-  return res;
+  return _young_gen->capacity() + _old_gen->capacity();
 }
 
 size_t GenCollectedHeap::used() const {
-  size_t res = 0;
-  for (int i = 0; i < _n_gens; i++) {
-    res += _gens[i]->used();
-  }
-  return res;
+  return _young_gen->used() + _old_gen->used();
 }
 
 // Save the "used_region" for generations level and lower.
 void GenCollectedHeap::save_used_regions(int level) {
+  assert(level >= 0, "Illegal level parameter");
   assert(level < _n_gens, "Illegal level parameter");
-  for (int i = level; i >= 0; i--) {
-    _gens[i]->save_used_region();
+  if (level == 1) {
+    _old_gen->save_used_region();
   }
+  _young_gen->save_used_region();
 }
 
 size_t GenCollectedHeap::max_capacity() const {
-  size_t res = 0;
-  for (int i = 0; i < _n_gens; i++) {
-    res += _gens[i]->max_capacity();
-  }
-  return res;
+  return _young_gen->max_capacity() + _old_gen->max_capacity();
 }
 
 // Update the _full_collections_completed counter
@@ -293,16 +282,20 @@
 HeapWord* GenCollectedHeap::attempt_allocation(size_t size,
                                                bool is_tlab,
                                                bool first_only) {
-  HeapWord* res;
-  for (int i = 0; i < _n_gens; i++) {
-    if (_gens[i]->should_allocate(size, is_tlab)) {
-      res = _gens[i]->allocate(size, is_tlab);
-      if (res != NULL) return res;
-      else if (first_only) break;
+  HeapWord* res = NULL;
+
+  if (_young_gen->should_allocate(size, is_tlab)) {
+    res = _young_gen->allocate(size, is_tlab);
+    if (res != NULL || first_only) {
+      return res;
     }
   }
-  // Otherwise...
-  return NULL;
+
+  if (_old_gen->should_allocate(size, is_tlab)) {
+    res = _old_gen->allocate(size, is_tlab);
+  }
+
+  return res;
 }
 
 HeapWord* GenCollectedHeap::mem_allocate(size_t size,
@@ -322,12 +315,107 @@
           (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));
 }
 
-void GenCollectedHeap::do_collection(bool  full,
+void GenCollectedHeap::collect_generation(Generation* gen, bool full, size_t size,
+                                          bool is_tlab, bool run_verification, bool clear_soft_refs,
+                                          bool restore_marks_for_biased_locking) {
+  // Timer for individual generations. Last argument is false: no CR
+  // FIXME: We should try to start the timing earlier to cover more of the GC pause
+  // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later
+  // so we can assume here that the next GC id is what we want.
+  GCTraceTime t1(gen->short_name(), PrintGCDetails, false, NULL, GCId::peek());
+  TraceCollectorStats tcs(gen->counters());
+  TraceMemoryManagerStats tmms(gen->kind(),gc_cause());
+
+  size_t prev_used = gen->used();
+  gen->stat_record()->invocations++;
+  gen->stat_record()->accumulated_time.start();
+
+  // Must be done anew before each collection because
+  // a previous collection will do mangling and will
+  // change top of some spaces.
+  record_gen_tops_before_GC();
+
+  if (PrintGC && Verbose) {
+    gclog_or_tty->print("level=%d invoke=%d size=" SIZE_FORMAT,
+                        gen->level(),
+                        gen->stat_record()->invocations,
+                        size * HeapWordSize);
+  }
+
+  if (run_verification && VerifyBeforeGC) {
+    HandleMark hm;  // Discard invalid handles created during verification
+    Universe::verify(" VerifyBeforeGC:");
+  }
+  COMPILER2_PRESENT(DerivedPointerTable::clear());
+
+  if (restore_marks_for_biased_locking) {
+    // We perform this mark word preservation work lazily
+    // because it's only at this point that we know whether we
+    // absolutely have to do it; we want to avoid doing it for
+    // scavenge-only collections where it's unnecessary
+    BiasedLocking::preserve_marks();
+  }
+
+  // Do collection work
+  {
+    // Note on ref discovery: For what appear to be historical reasons,
+    // GCH enables and disabled (by enqueing) refs discovery.
+    // In the future this should be moved into the generation's
+    // collect method so that ref discovery and enqueueing concerns
+    // are local to a generation. The collect method could return
+    // an appropriate indication in the case that notification on
+    // the ref lock was needed. This will make the treatment of
+    // weak refs more uniform (and indeed remove such concerns
+    // from GCH). XXX
+
+    HandleMark hm;  // Discard invalid handles created during gc
+    save_marks();   // save marks for all gens
+    // We want to discover references, but not process them yet.
+    // This mode is disabled in process_discovered_references if the
+    // generation does some collection work, or in
+    // enqueue_discovered_references if the generation returns
+    // without doing any work.
+    ReferenceProcessor* rp = gen->ref_processor();
+    // If the discovery of ("weak") refs in this generation is
+    // atomic wrt other collectors in this configuration, we
+    // are guaranteed to have empty discovered ref lists.
+    if (rp->discovery_is_atomic()) {
+      rp->enable_discovery();
+      rp->setup_policy(clear_soft_refs);
+    } else {
+      // collect() below will enable discovery as appropriate
+    }
+    gen->collect(full, clear_soft_refs, size, is_tlab);
+    if (!rp->enqueuing_is_done()) {
+      rp->enqueue_discovered_references();
+    } else {
+      rp->set_enqueuing_is_done(false);
+    }
+    rp->verify_no_references_recorded();
+  }
+
+  COMPILER2_PRESENT(DerivedPointerTable::update_pointers());
+
+  gen->stat_record()->accumulated_time.stop();
+
+  update_gc_stats(gen->level(), full);
+
+  if (run_verification && VerifyAfterGC) {
+    HandleMark hm;  // Discard invalid handles created during verification
+    Universe::verify(" VerifyAfterGC:");
+  }
+
+  if (PrintGCDetails) {
+    gclog_or_tty->print(":");
+    gen->print_heap_change(prev_used);
+  }
+}
+
+void GenCollectedHeap::do_collection(bool   full,
                                      bool   clear_all_soft_refs,
                                      size_t size,
                                      bool   is_tlab,
                                      int    max_level) {
-  bool prepared_for_verification = false;
   ResourceMark rm;
   DEBUG_ONLY(Thread* my_thread = Thread::current();)
 
@@ -367,141 +455,62 @@
     increment_total_collections(complete);
 
     size_t gch_prev_used = used();
+    bool run_verification = total_collections() >= VerifyGCStartAt;
 
-    int starting_level = 0;
-    if (full) {
-      // Search for the oldest generation which will collect all younger
-      // generations, and start collection loop there.
-      for (int i = max_level; i >= 0; i--) {
-        if (_gens[i]->full_collects_younger_generations()) {
-          starting_level = i;
-          break;
-        }
+    bool prepared_for_verification = false;
+    int max_level_collected = 0;
+    bool old_collects_young = (max_level == 1) &&
+                              full &&
+                              _old_gen->full_collects_younger_generations();
+    if (!old_collects_young &&
+        _young_gen->should_collect(full, size, is_tlab)) {
+      if (run_verification && VerifyGCLevel <= 0 && VerifyBeforeGC) {
+        prepare_for_verify();
+        prepared_for_verification = true;
+      }
+
+      assert(!_young_gen->performs_in_place_marking(), "No young generation do in place marking");
+      collect_generation(_young_gen,
+                         full,
+                         size,
+                         is_tlab,
+                         run_verification && VerifyGCLevel <= 0,
+                         do_clear_all_soft_refs,
+                         false);
+
+      if (size > 0 && (!is_tlab || _young_gen->supports_tlab_allocation()) &&
+          size * HeapWordSize <= _young_gen->unsafe_max_alloc_nogc()) {
+        // Allocation request was met by young GC.
+        size = 0;
       }
     }
 
     bool must_restore_marks_for_biased_locking = false;
 
-    int max_level_collected = starting_level;
-    for (int i = starting_level; i <= max_level; i++) {
-      if (_gens[i]->should_collect(full, size, is_tlab)) {
-        if (i == n_gens() - 1) {  // a major collection is to happen
-          if (!complete) {
-            // The full_collections increment was missed above.
-            increment_total_full_collections();
-          }
-          pre_full_gc_dump(NULL);    // do any pre full gc dumps
-        }
-        // Timer for individual generations. Last argument is false: no CR
-        // FIXME: We should try to start the timing earlier to cover more of the GC pause
-        // The PrintGCDetails logging starts before we have incremented the GC id. We will do that later
-        // so we can assume here that the next GC id is what we want.
-        GCTraceTime t1(_gens[i]->short_name(), PrintGCDetails, false, NULL, GCId::peek());
-        TraceCollectorStats tcs(_gens[i]->counters());
-        TraceMemoryManagerStats tmms(_gens[i]->kind(),gc_cause());
+    if (max_level == 1 && _old_gen->should_collect(full, size, is_tlab)) {
+      if (!complete) {
+        // The full_collections increment was missed above.
+        increment_total_full_collections();
+      }
 
-        size_t prev_used = _gens[i]->used();
-        _gens[i]->stat_record()->invocations++;
-        _gens[i]->stat_record()->accumulated_time.start();
+      pre_full_gc_dump(NULL);    // do any pre full gc dumps
 
-        // Must be done anew before each collection because
-        // a previous collection will do mangling and will
-        // change top of some spaces.
-        record_gen_tops_before_GC();
+      if (!prepared_for_verification && run_verification &&
+          VerifyGCLevel <= 1 && VerifyBeforeGC) {
+        prepare_for_verify();
+      }
 
-        if (PrintGC && Verbose) {
-          gclog_or_tty->print("level=%d invoke=%d size=" SIZE_FORMAT,
-                     i,
-                     _gens[i]->stat_record()->invocations,
-                     size*HeapWordSize);
-        }
+      assert(_old_gen->performs_in_place_marking(), "All old generations do in place marking");
+      collect_generation(_old_gen,
+                         full,
+                         size,
+                         is_tlab,
+                         run_verification && VerifyGCLevel <= 1,
+                         do_clear_all_soft_refs,
+                         true);
 
-        if (VerifyBeforeGC && i >= VerifyGCLevel &&
-            total_collections() >= VerifyGCStartAt) {
-          HandleMark hm;  // Discard invalid handles created during verification
-          if (!prepared_for_verification) {
-            prepare_for_verify();
-            prepared_for_verification = true;
-          }
-          Universe::verify(" VerifyBeforeGC:");
-        }
-        COMPILER2_PRESENT(DerivedPointerTable::clear());
-
-        if (!must_restore_marks_for_biased_locking &&
-            _gens[i]->performs_in_place_marking()) {
-          // We perform this mark word preservation work lazily
-          // because it's only at this point that we know whether we
-          // absolutely have to do it; we want to avoid doing it for
-          // scavenge-only collections where it's unnecessary
-          must_restore_marks_for_biased_locking = true;
-          BiasedLocking::preserve_marks();
-        }
-
-        // Do collection work
-        {
-          // Note on ref discovery: For what appear to be historical reasons,
-          // GCH enables and disabled (by enqueing) refs discovery.
-          // In the future this should be moved into the generation's
-          // collect method so that ref discovery and enqueueing concerns
-          // are local to a generation. The collect method could return
-          // an appropriate indication in the case that notification on
-          // the ref lock was needed. This will make the treatment of
-          // weak refs more uniform (and indeed remove such concerns
-          // from GCH). XXX
-
-          HandleMark hm;  // Discard invalid handles created during gc
-          save_marks();   // save marks for all gens
-          // We want to discover references, but not process them yet.
-          // This mode is disabled in process_discovered_references if the
-          // generation does some collection work, or in
-          // enqueue_discovered_references if the generation returns
-          // without doing any work.
-          ReferenceProcessor* rp = _gens[i]->ref_processor();
-          // If the discovery of ("weak") refs in this generation is
-          // atomic wrt other collectors in this configuration, we
-          // are guaranteed to have empty discovered ref lists.
-          if (rp->discovery_is_atomic()) {
-            rp->enable_discovery();
-            rp->setup_policy(do_clear_all_soft_refs);
-          } else {
-            // collect() below will enable discovery as appropriate
-          }
-          _gens[i]->collect(full, do_clear_all_soft_refs, size, is_tlab);
-          if (!rp->enqueuing_is_done()) {
-            rp->enqueue_discovered_references();
-          } else {
-            rp->set_enqueuing_is_done(false);
-          }
-          rp->verify_no_references_recorded();
-        }
-        max_level_collected = i;
-
-        // Determine if allocation request was met.
-        if (size > 0) {
-          if (!is_tlab || _gens[i]->supports_tlab_allocation()) {
-            if (size*HeapWordSize <= _gens[i]->unsafe_max_alloc_nogc()) {
-              size = 0;
-            }
-          }
-        }
-
-        COMPILER2_PRESENT(DerivedPointerTable::update_pointers());
-
-        _gens[i]->stat_record()->accumulated_time.stop();
-
-        update_gc_stats(i, full);
-
-        if (VerifyAfterGC && i >= VerifyGCLevel &&
-            total_collections() >= VerifyGCStartAt) {
-          HandleMark hm;  // Discard invalid handles created during verification
-          Universe::verify(" VerifyAfterGC:");
-        }
-
-        if (PrintGCDetails) {
-          gclog_or_tty->print(":");
-          _gens[i]->print_heap_change(prev_used);
-        }
-      }
+      must_restore_marks_for_biased_locking = true;
+      max_level_collected = 1;
     }
 
     // Update "complete" boolean wrt what actually transpired --
@@ -523,10 +532,11 @@
       }
     }
 
-    for (int j = max_level_collected; j >= 0; j -= 1) {
-      // Adjust generation sizes.
-      _gens[j]->compute_new_size();
+    // Adjust generation sizes.
+    if (max_level_collected == 1) {
+      _old_gen->compute_new_size();
     }
+    _young_gen->compute_new_size();
 
     if (complete) {
       // Delete metaspaces for unloaded class loaders and clean up loader_data graph
@@ -583,18 +593,18 @@
 
   if (younger_gens_as_roots) {
     if (!_gen_process_roots_tasks->is_task_claimed(GCH_PS_younger_gens)) {
-      for (int i = 0; i < level; i++) {
-        not_older_gens->set_generation(_gens[i]);
-        _gens[i]->oop_iterate(not_older_gens);
+      if (level == 1) {
+        not_older_gens->set_generation(_young_gen);
+        _young_gen->oop_iterate(not_older_gens);
       }
       not_older_gens->reset_generation();
     }
   }
   // When collection is parallel, all threads get to cooperate to do
   // older-gen scanning.
-  for (int i = level+1; i < _n_gens; i++) {
-    older_gens->set_generation(_gens[i]);
-    rem_set()->younger_refs_iterate(_gens[i], older_gens);
+  if (level == 0) {
+    older_gens->set_generation(_old_gen);
+    rem_set()->younger_refs_iterate(_old_gen, older_gens);
     older_gens->reset_generation();
   }
 
@@ -635,9 +645,8 @@
 void GenCollectedHeap::gen_process_weak_roots(OopClosure* root_closure) {
   SharedHeap::process_weak_roots(root_closure);
   // "Local" "weak" refs
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->ref_processor()->weak_oops_do(root_closure);
-  }
+  _young_gen->ref_processor()->weak_oops_do(root_closure);
+  _old_gen->ref_processor()->weak_oops_do(root_closure);
 }
 
 #define GCH_SINCE_SAVE_MARKS_ITERATE_DEFN(OopClosureType, nv_suffix)    \
@@ -645,9 +654,11 @@
 oop_since_save_marks_iterate(int level,                                 \
                              OopClosureType* cur,                       \
                              OopClosureType* older) {                   \
-  _gens[level]->oop_since_save_marks_iterate##nv_suffix(cur);           \
-  for (int i = level+1; i < n_gens(); i++) {                            \
-    _gens[i]->oop_since_save_marks_iterate##nv_suffix(older);           \
+  if (level == 0) {                                                     \
+    _young_gen->oop_since_save_marks_iterate##nv_suffix(cur);           \
+    _old_gen->oop_since_save_marks_iterate##nv_suffix(older);           \
+  } else {                                                              \
+    _old_gen->oop_since_save_marks_iterate##nv_suffix(cur);             \
   }                                                                     \
 }
 
@@ -656,22 +667,22 @@
 #undef GCH_SINCE_SAVE_MARKS_ITERATE_DEFN
 
 bool GenCollectedHeap::no_allocs_since_save_marks(int level) {
-  for (int i = level; i < _n_gens; i++) {
-    if (!_gens[i]->no_allocs_since_save_marks()) return false;
+  if (level == 0 && !_young_gen->no_allocs_since_save_marks()) {
+    return false;
   }
-  return true;
+  return _old_gen->no_allocs_since_save_marks();
 }
 
 bool GenCollectedHeap::supports_inline_contig_alloc() const {
-  return _gens[0]->supports_inline_contig_alloc();
+  return _young_gen->supports_inline_contig_alloc();
 }
 
 HeapWord** GenCollectedHeap::top_addr() const {
-  return _gens[0]->top_addr();
+  return _young_gen->top_addr();
 }
 
 HeapWord** GenCollectedHeap::end_addr() const {
-  return _gens[0]->end_addr();
+  return _young_gen->end_addr();
 }
 
 // public collection interfaces
@@ -734,12 +745,12 @@
 #if INCLUDE_ALL_GCS
 bool GenCollectedHeap::create_cms_collector() {
 
-  assert(_gens[1]->kind() == Generation::ConcurrentMarkSweep,
+  assert(_old_gen->kind() == Generation::ConcurrentMarkSweep,
          "Unexpected generation kinds");
   // Skip two header words in the block content verification
   NOT_PRODUCT(_skip_header_HeapWords = CMSCollector::skip_header_HeapWords();)
   CMSCollector* collector = new CMSCollector(
-    (ConcurrentMarkSweepGeneration*)_gens[1],
+    (ConcurrentMarkSweepGeneration*)_old_gen,
     _rem_set->as_CardTableRS(),
     (ConcurrentMarkSweepPolicy*) collector_policy());
 
@@ -806,8 +817,8 @@
 }
 
 bool GenCollectedHeap::is_in_young(oop p) {
-  bool result = ((HeapWord*)p) < _gens[_n_gens - 1]->reserved().start();
-  assert(result == _gens[0]->is_in_reserved(p),
+  bool result = ((HeapWord*)p) < _old_gen->reserved().start();
+  assert(result == _young_gen->is_in_reserved(p),
          err_msg("incorrect test - result=%d, p=" INTPTR_FORMAT, result, p2i((void*)p)));
   return result;
 }
@@ -825,13 +836,7 @@
     VMError::fatal_error_in_progress(), "too expensive");
 
   #endif
-  // This might be sped up with a cache of the last generation that
-  // answered yes.
-  for (int i = 0; i < _n_gens; i++) {
-    if (_gens[i]->is_in(p)) return true;
-  }
-  // Otherwise...
-  return false;
+  return _young_gen->is_in(p) || _old_gen->is_in(p);
 }
 
 #ifdef ASSERT
@@ -840,114 +845,97 @@
 bool GenCollectedHeap::is_in_partial_collection(const void* p) {
   assert(is_in_reserved(p) || p == NULL,
     "Does not work if address is non-null and outside of the heap");
-  return p < _gens[_n_gens - 2]->reserved().end() && p != NULL;
+  return p < _young_gen->reserved().end() && p != NULL;
 }
 #endif
 
 void GenCollectedHeap::oop_iterate(ExtendedOopClosure* cl) {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->oop_iterate(cl);
-  }
+  _young_gen->oop_iterate(cl);
+  _old_gen->oop_iterate(cl);
 }
 
 void GenCollectedHeap::object_iterate(ObjectClosure* cl) {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->object_iterate(cl);
-  }
+  _young_gen->object_iterate(cl);
+  _old_gen->object_iterate(cl);
 }
 
 void GenCollectedHeap::safe_object_iterate(ObjectClosure* cl) {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->safe_object_iterate(cl);
-  }
+  _young_gen->safe_object_iterate(cl);
+  _old_gen->safe_object_iterate(cl);
 }
 
 Space* GenCollectedHeap::space_containing(const void* addr) const {
-  for (int i = 0; i < _n_gens; i++) {
-    Space* res = _gens[i]->space_containing(addr);
-    if (res != NULL) return res;
+  Space* res = _young_gen->space_containing(addr);
+  if (res != NULL) {
+    return res;
   }
-  // Otherwise...
-  assert(false, "Could not find containing space");
-  return NULL;
+  res = _old_gen->space_containing(addr);
+  assert(res != NULL, "Could not find containing space");
+  return res;
 }
 
-
 HeapWord* GenCollectedHeap::block_start(const void* addr) const {
   assert(is_in_reserved(addr), "block_start of address outside of heap");
-  for (int i = 0; i < _n_gens; i++) {
-    if (_gens[i]->is_in_reserved(addr)) {
-      assert(_gens[i]->is_in(addr),
-             "addr should be in allocated part of generation");
-      return _gens[i]->block_start(addr);
-    }
+  if (_young_gen->is_in_reserved(addr)) {
+    assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
+    return _young_gen->block_start(addr);
   }
-  assert(false, "Some generation should contain the address");
-  return NULL;
+
+  assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
+  assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
+  return _old_gen->block_start(addr);
 }
 
 size_t GenCollectedHeap::block_size(const HeapWord* addr) const {
   assert(is_in_reserved(addr), "block_size of address outside of heap");
-  for (int i = 0; i < _n_gens; i++) {
-    if (_gens[i]->is_in_reserved(addr)) {
-      assert(_gens[i]->is_in(addr),
-             "addr should be in allocated part of generation");
-      return _gens[i]->block_size(addr);
-    }
+  if (_young_gen->is_in_reserved(addr)) {
+    assert(_young_gen->is_in(addr), "addr should be in allocated part of generation");
+    return _young_gen->block_size(addr);
   }
-  assert(false, "Some generation should contain the address");
-  return 0;
+
+  assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
+  assert(_old_gen->is_in(addr), "addr should be in allocated part of generation");
+  return _old_gen->block_size(addr);
 }
 
 bool GenCollectedHeap::block_is_obj(const HeapWord* addr) const {
   assert(is_in_reserved(addr), "block_is_obj of address outside of heap");
   assert(block_start(addr) == addr, "addr must be a block start");
-  for (int i = 0; i < _n_gens; i++) {
-    if (_gens[i]->is_in_reserved(addr)) {
-      return _gens[i]->block_is_obj(addr);
-    }
+  if (_young_gen->is_in_reserved(addr)) {
+    return _young_gen->block_is_obj(addr);
   }
-  assert(false, "Some generation should contain the address");
-  return false;
+
+  assert(_old_gen->is_in_reserved(addr), "Some generation should contain the address");
+  return _old_gen->block_is_obj(addr);
 }
 
 bool GenCollectedHeap::supports_tlab_allocation() const {
-  for (int i = 0; i < _n_gens; i += 1) {
-    if (_gens[i]->supports_tlab_allocation()) {
-      return true;
-    }
-  }
-  return false;
+  assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
+  return _young_gen->supports_tlab_allocation();
 }
 
 size_t GenCollectedHeap::tlab_capacity(Thread* thr) const {
-  size_t result = 0;
-  for (int i = 0; i < _n_gens; i += 1) {
-    if (_gens[i]->supports_tlab_allocation()) {
-      result += _gens[i]->tlab_capacity();
-    }
+  assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
+  if (_young_gen->supports_tlab_allocation()) {
+    return _young_gen->tlab_capacity();
   }
-  return result;
+  return 0;
 }
 
 size_t GenCollectedHeap::tlab_used(Thread* thr) const {
-  size_t result = 0;
-  for (int i = 0; i < _n_gens; i += 1) {
-    if (_gens[i]->supports_tlab_allocation()) {
-      result += _gens[i]->tlab_used();
-    }
+  assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
+  if (_young_gen->supports_tlab_allocation()) {
+    return _young_gen->tlab_used();
   }
-  return result;
+  return 0;
 }
 
 size_t GenCollectedHeap::unsafe_max_tlab_alloc(Thread* thr) const {
-  size_t result = 0;
-  for (int i = 0; i < _n_gens; i += 1) {
-    if (_gens[i]->supports_tlab_allocation()) {
-      result += _gens[i]->unsafe_max_tlab_alloc();
-    }
+  assert(!_old_gen->supports_tlab_allocation(), "Old gen supports TLAB allocation?!");
+  if (_young_gen->supports_tlab_allocation()) {
+    return _young_gen->unsafe_max_tlab_alloc();
   }
-  return result;
+  return 0;
 }
 
 HeapWord* GenCollectedHeap::allocate_new_tlab(size_t size) {
@@ -996,17 +984,15 @@
 ScratchBlock* GenCollectedHeap::gather_scratch(Generation* requestor,
                                                size_t max_alloc_words) {
   ScratchBlock* res = NULL;
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->contribute_scratch(res, requestor, max_alloc_words);
-  }
+  _young_gen->contribute_scratch(res, requestor, max_alloc_words);
+  _old_gen->contribute_scratch(res, requestor, max_alloc_words);
   sort_scratch_list(res);
   return res;
 }
 
 void GenCollectedHeap::release_scratch() {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->reset_scratch();
-  }
+  _young_gen->reset_scratch();
+  _old_gen->reset_scratch();
 }
 
 class GenPrepareForVerifyClosure: public GenCollectedHeap::GenClosure {
@@ -1021,39 +1007,29 @@
   generation_iterate(&blk, false);
 }
 
-
 void GenCollectedHeap::generation_iterate(GenClosure* cl,
                                           bool old_to_young) {
   if (old_to_young) {
-    for (int i = _n_gens-1; i >= 0; i--) {
-      cl->do_generation(_gens[i]);
-    }
+    cl->do_generation(_old_gen);
+    cl->do_generation(_young_gen);
   } else {
-    for (int i = 0; i < _n_gens; i++) {
-      cl->do_generation(_gens[i]);
-    }
+    cl->do_generation(_young_gen);
+    cl->do_generation(_old_gen);
   }
 }
 
 void GenCollectedHeap::space_iterate(SpaceClosure* cl) {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->space_iterate(cl, true);
-  }
+  _young_gen->space_iterate(cl, true);
+  _old_gen->space_iterate(cl, true);
 }
 
 bool GenCollectedHeap::is_maximal_no_gc() const {
-  for (int i = 0; i < _n_gens; i++) {
-    if (!_gens[i]->is_maximal_no_gc()) {
-      return false;
-    }
-  }
-  return true;
+  return _young_gen->is_maximal_no_gc() && _old_gen->is_maximal_no_gc();
 }
 
 void GenCollectedHeap::save_marks() {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->save_marks();
-  }
+  _young_gen->save_marks();
+  _old_gen->save_marks();
 }
 
 GenCollectedHeap* GenCollectedHeap::heap() {
@@ -1065,27 +1041,33 @@
 
 void GenCollectedHeap::prepare_for_compaction() {
   guarantee(_n_gens = 2, "Wrong number of generations");
-  Generation* old_gen = _gens[1];
   // Start by compacting into same gen.
-  CompactPoint cp(old_gen);
-  old_gen->prepare_for_compaction(&cp);
-  Generation* young_gen = _gens[0];
-  young_gen->prepare_for_compaction(&cp);
+  CompactPoint cp(_old_gen);
+  _old_gen->prepare_for_compaction(&cp);
+  _young_gen->prepare_for_compaction(&cp);
 }
 
 GCStats* GenCollectedHeap::gc_stats(int level) const {
-  return _gens[level]->gc_stats();
+  if (level == 0) {
+    return _young_gen->gc_stats();
+  } else {
+    return _old_gen->gc_stats();
+  }
 }
 
 void GenCollectedHeap::verify(bool silent, VerifyOption option /* ignored */) {
-  for (int i = _n_gens-1; i >= 0; i--) {
-    Generation* g = _gens[i];
-    if (!silent) {
-      gclog_or_tty->print("%s", g->name());
-      gclog_or_tty->print(" ");
-    }
-    g->verify();
+  if (!silent) {
+    gclog_or_tty->print("%s", _old_gen->name());
+    gclog_or_tty->print(" ");
   }
+  _old_gen->verify();
+
+  if (!silent) {
+    gclog_or_tty->print("%s", _young_gen->name());
+    gclog_or_tty->print(" ");
+  }
+  _young_gen->verify();
+
   if (!silent) {
     gclog_or_tty->print("remset ");
   }
@@ -1093,9 +1075,8 @@
 }
 
 void GenCollectedHeap::print_on(outputStream* st) const {
-  for (int i = 0; i < _n_gens; i++) {
-    _gens[i]->print_on(st);
-  }
+  _young_gen->print_on(st);
+  _old_gen->print_on(st);
   MetaspaceAux::print_on(st);
 }
 
--- a/src/share/vm/memory/genCollectedHeap.hpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/memory/genCollectedHeap.hpp	Fri Aug 22 10:10:08 2014 +0200
@@ -33,7 +33,7 @@
 class SubTasksDone;
 
 // A "GenCollectedHeap" is a SharedHeap that uses generational
-// collection.  It is represented with a sequence of Generation's.
+// collection.  It has two generations, young and old.
 class GenCollectedHeap : public SharedHeap {
   friend class GenCollectorPolicy;
   friend class Generation;
@@ -63,7 +63,10 @@
 
  private:
   int _n_gens;
-  Generation* _gens[max_gens];
+
+  Generation* _young_gen;
+  Generation* _old_gen;
+
   GenerationSpec** _gen_specs;
 
   // The singleton Gen Remembered Set.
@@ -85,6 +88,11 @@
   SubTasksDone* _gen_process_roots_tasks;
   SubTasksDone* gen_process_roots_tasks() { return _gen_process_roots_tasks; }
 
+  // Collects the given generation.
+  void collect_generation(Generation* gen, bool full, size_t size, bool is_tlab,
+                          bool run_verification, bool clear_soft_refs,
+                          bool restore_marks_for_biased_locking);
+
   // In block contents verification, the number of header words to skip
   NOT_PRODUCT(static size_t _skip_header_HeapWords;)
 
@@ -138,8 +146,12 @@
     return CollectedHeap::GenCollectedHeap;
   }
 
+  Generation* young_gen() { return _young_gen; }
+  Generation* old_gen()   { return _old_gen; }
+
   // The generational collector policy.
   GenCollectorPolicy* gen_policy() const { return _gen_policy; }
+
   virtual CollectorPolicy* collector_policy() const { return (CollectorPolicy*) gen_policy(); }
 
   // Adaptive size policy
@@ -309,20 +321,17 @@
   // Update above counter, as appropriate, at the end of a concurrent GC cycle
   unsigned int update_full_collections_completed(unsigned int count);
 
-  // Update "time of last gc" for all constituent generations
-  // to "now".
+  // Update "time of last gc" for all generations to "now".
   void update_time_of_last_gc(jlong now) {
-    for (int i = 0; i < _n_gens; i++) {
-      _gens[i]->update_time_of_last_gc(now);
-    }
+    _young_gen->update_time_of_last_gc(now);
+    _old_gen->update_time_of_last_gc(now);
   }
 
   // Update the gc statistics for each generation.
   // "level" is the level of the latest collection.
   void update_gc_stats(int current_level, bool full) {
-    for (int i = 0; i < _n_gens; i++) {
-      _gens[i]->update_gc_stats(current_level, full);
-    }
+    _young_gen->update_gc_stats(current_level, full);
+    _old_gen->update_gc_stats(current_level, full);
   }
 
   // Override.
@@ -366,21 +375,23 @@
 
   // Return the generation before "gen".
   Generation* prev_gen(Generation* gen) const {
-    int l = gen->level();
-    guarantee(l > 0, "Out of bounds");
-    return _gens[l-1];
+    guarantee(gen->level() == 1, "Out of bounds");
+    return _young_gen;
   }
 
   // Return the generation after "gen".
   Generation* next_gen(Generation* gen) const {
-    int l = gen->level() + 1;
-    guarantee(l < _n_gens, "Out of bounds");
-    return _gens[l];
+    guarantee(gen->level() == 0, "Out of bounds");
+    return _old_gen;
   }
 
   Generation* get_gen(int i) const {
-    guarantee(i >= 0 && i < _n_gens, "Out of bounds");
-    return _gens[i];
+    guarantee(i == 0 || i == 1, "Out of bounds");
+    if (i == 0) {
+      return _young_gen;
+    } else {
+      return _old_gen;
+    }
   }
 
   int n_gens() const {
--- a/src/share/vm/memory/genMarkSweep.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/memory/genMarkSweep.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -159,7 +159,7 @@
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   // Scratch request on behalf of oldest generation; will do no
   // allocation.
-  ScratchBlock* scratch = gch->gather_scratch(gch->_gens[gch->_n_gens-1], 0);
+  ScratchBlock* scratch = gch->gather_scratch(gch->get_gen(gch->_n_gens-1), 0);
 
   // $$$ To cut a corner, we'll only use the first scratch block, and then
   // revert to malloc.
--- a/src/share/vm/memory/generation.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/memory/generation.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -155,7 +155,7 @@
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   int next = level() + 1;
   if (next < gch->_n_gens) {
-    return gch->_gens[next];
+    return gch->get_gen(next);
   } else {
     return NULL;
   }
--- a/src/share/vm/runtime/vmStructs.cpp	Wed Feb 18 09:22:37 2015 +0100
+++ b/src/share/vm/runtime/vmStructs.cpp	Fri Aug 22 10:10:08 2014 +0200
@@ -552,8 +552,9 @@
   nonstatic_field(GenerationSpec,              _max_size,                                     size_t)                                \
                                                                                                                                      \
     static_field(GenCollectedHeap,             _gch,                                          GenCollectedHeap*)                     \
+  nonstatic_field(GenCollectedHeap,            _young_gen,                                    Generation*)                           \
+  nonstatic_field(GenCollectedHeap,            _old_gen,                                      Generation*)                           \
  nonstatic_field(GenCollectedHeap,             _n_gens,                                       int)                                   \
- unchecked_nonstatic_field(GenCollectedHeap,   _gens,                                         sizeof(GenCollectedHeap::_gens)) /* NOTE: no type */ \
   nonstatic_field(GenCollectedHeap,            _gen_specs,                                    GenerationSpec**)                      \
                                                                                                                                      \
   nonstatic_field(HeapWord,                    i,                                             char*)                                 \
--- a/test/gc/metaspace/CompressedClassSpaceSizeInJmapHeap.java	Wed Feb 18 09:22:37 2015 +0100
+++ b/test/gc/metaspace/CompressedClassSpaceSizeInJmapHeap.java	Fri Aug 22 10:10:08 2014 +0200
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2013, 2015, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -36,6 +36,7 @@
 import java.util.List;
 
 public class CompressedClassSpaceSizeInJmapHeap {
+    // Note that on some platforms it may require root privileges to run this test.
     public static void main(String[] args) throws Exception {
         if (!Platform.is64bit()) {
             // Compressed Class Space is only available on 64-bit JVMs