hotspot/src/share/vm/memory/genMarkSweep.cpp - platform/libcore - Git at Google

 /*
  * Copyright (c) 2001, 2012, 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
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
  *
  * This code is distributed in the hope that it will be useful, but WITHOUT
  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  * version 2 for more details (a copy is included in the LICENSE file that
  * accompanied this code).
  *
  * You should have received a copy of the GNU General Public License version
  * 2 along with this work; if not, write to the Free Software Foundation,
  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  *
  */

 #include "precompiled.hpp"
 #include "classfile/javaClasses.hpp"
 #include "classfile/symbolTable.hpp"
 #include "classfile/systemDictionary.hpp"
 #include "classfile/vmSymbols.hpp"
 #include "code/codeCache.hpp"
 #include "code/icBuffer.hpp"
 #include "gc_interface/collectedHeap.inline.hpp"
 #include "memory/genCollectedHeap.hpp"
 #include "memory/genMarkSweep.hpp"
 #include "memory/genOopClosures.inline.hpp"
 #include "memory/generation.inline.hpp"
 #include "memory/modRefBarrierSet.hpp"
 #include "memory/referencePolicy.hpp"
 #include "memory/space.hpp"
 #include "oops/instanceRefKlass.hpp"
 #include "oops/oop.inline.hpp"
 #include "prims/jvmtiExport.hpp"
 #include "runtime/fprofiler.hpp"
 #include "runtime/handles.inline.hpp"
 #include "runtime/synchronizer.hpp"
 #include "runtime/thread.inline.hpp"
 #include "runtime/vmThread.hpp"
 #include "utilities/copy.hpp"
 #include "utilities/events.hpp"

 void GenMarkSweep::invoke_at_safepoint(int level, ReferenceProcessor* rp,
   bool clear_all_softrefs) {
   assert(SafepointSynchronize::is_at_safepoint(), "must be at a safepoint");

   GenCollectedHeap* gch = GenCollectedHeap::heap();
 #ifdef ASSERT
   if (gch->collector_policy()->should_clear_all_soft_refs()) {
     assert(clear_all_softrefs, "Policy should have been checked earlier");
   }
 #endif

   // hook up weak ref data so it can be used during Mark-Sweep
   assert(ref_processor() == NULL, "no stomping");
   assert(rp != NULL, "should be non-NULL");
   _ref_processor = rp;
   rp->setup_policy(clear_all_softrefs);

   TraceTime t1(GCCauseString("Full GC", gch->gc_cause()), PrintGC && !PrintGCDetails, true, gclog_or_tty);

   // When collecting the permanent generation Method*s may be moving,
   // so we either have to flush all bcp data or convert it into bci.
   CodeCache::gc_prologue();
   Threads::gc_prologue();

   // Increment the invocation count
   _total_invocations++;

   // Capture heap size before collection for printing.
   size_t gch_prev_used = gch->used();

   // Some of the card table updates below assume that the perm gen is
   // also being collected.
   assert(level == gch->n_gens() - 1,
          "All generations are being collected, ergo perm gen too.");

   // Capture used regions for each generation that will be
   // subject to collection, so that card table adjustments can
   // be made intelligently (see clear / invalidate further below).
   gch->save_used_regions(level);

   allocate_stacks();

   mark_sweep_phase1(level, clear_all_softrefs);

   mark_sweep_phase2();

   // Don't add any more derived pointers during phase3
   COMPILER2_PRESENT(assert(DerivedPointerTable::is_active(), "Sanity"));
   COMPILER2_PRESENT(DerivedPointerTable::set_active(false));

   mark_sweep_phase3(level);

   mark_sweep_phase4();

   restore_marks();

   // Set saved marks for allocation profiler (and other things? -- dld)
   // (Should this be in general part?)
   gch->save_marks();

   deallocate_stacks();

   // If compaction completely evacuated all generations younger than this
   // one, then we can clear the card table.  Otherwise, we must invalidate
   // it (consider all cards dirty).  In the future, we might consider doing
   // compaction within generations only, and doing card-table sliding.
   bool all_empty = true;
   for (int i = 0; all_empty && i < level; i++) {
     Generation* g = gch->get_gen(i);
     all_empty = all_empty && gch->get_gen(i)->used() == 0;
   }
   GenRemSet* rs = gch->rem_set();
   // Clear/invalidate below make use of the "prev_used_regions" saved earlier.
   if (all_empty) {
     // We've evacuated all generations below us.
     Generation* g = gch->get_gen(level);
     rs->clear_into_younger(g);
   } else {
     // Invalidate the cards corresponding to the currently used
     // region and clear those corresponding to the evacuated region
     // of all generations just collected (i.e. level and younger).
     rs->invalidate_or_clear(gch->get_gen(level),
                             true /* younger */);
   }

   Threads::gc_epilogue();
   CodeCache::gc_epilogue();
   JvmtiExport::gc_epilogue();

   if (PrintGC && !PrintGCDetails) {
     gch->print_heap_change(gch_prev_used);
   }

   // refs processing: clean slate
   _ref_processor = NULL;

   // Update heap occupancy information which is used as
   // input to soft ref clearing policy at the next gc.
   Universe::update_heap_info_at_gc();

   // Update time of last gc for all generations we collected
   // (which curently is all the generations in the heap).
   // We need to use a monotonically non-deccreasing time in ms
   // or we will see time-warp warnings and os::javaTimeMillis()
   // does not guarantee monotonicity.
   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
   gch->update_time_of_last_gc(now);
 }

 void GenMarkSweep::allocate_stacks() {
   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);

   // $$$ To cut a corner, we'll only use the first scratch block, and then
   // revert to malloc.
   if (scratch != NULL) {
     _preserved_count_max =
       scratch->num_words * HeapWordSize / sizeof(PreservedMark);
   } else {
     _preserved_count_max = 0;
   }

   _preserved_marks = (PreservedMark*)scratch;
   _preserved_count = 0;
 }


 void GenMarkSweep::deallocate_stacks() {
   if (!UseG1GC) {
     GenCollectedHeap* gch = GenCollectedHeap::heap();
     gch->release_scratch();
   }

   _preserved_mark_stack.clear(true);
   _preserved_oop_stack.clear(true);
   _marking_stack.clear();
   _objarray_stack.clear(true);
 }

 void GenMarkSweep::mark_sweep_phase1(int level,
                                   bool clear_all_softrefs) {
   // Recursively traverse all live objects and mark them
   TraceTime tm("phase 1", PrintGC && Verbose, true, gclog_or_tty);
   trace(" 1");

   GenCollectedHeap* gch = GenCollectedHeap::heap();

   // Because follow_root_closure is created statically, cannot
   // use OopsInGenClosure constructor which takes a generation,
   // as the Universe has not been created when the static constructors
   // are run.
   follow_root_closure.set_orig_generation(gch->get_gen(level));

   // Need new claim bits before marking starts.
   ClassLoaderDataGraph::clear_claimed_marks();

   gch->gen_process_strong_roots(level,
                                 false, // Younger gens are not roots.
                                 true,  // activate StrongRootsScope
                                 false, // not scavenging
                                 SharedHeap::SO_SystemClasses,
                                 &follow_root_closure,
                                 true,   // walk code active on stacks
                                 &follow_root_closure,
                                 &follow_klass_closure);

   // Process reference objects found during marking
   {
     ref_processor()->setup_policy(clear_all_softrefs);
     ref_processor()->process_discovered_references(
       &is_alive, &keep_alive, &follow_stack_closure, NULL);
   }

   // Follow system dictionary roots and unload classes
   bool purged_class = SystemDictionary::do_unloading(&is_alive);

   // Follow code cache roots
   CodeCache::do_unloading(&is_alive, purged_class);
   follow_stack(); // Flush marking stack

   // Update subklass/sibling/implementor links of live klasses
   Klass::clean_weak_klass_links(&is_alive);
   assert(_marking_stack.is_empty(), "just drained");

   // Visit interned string tables and delete unmarked oops
   StringTable::unlink(&is_alive);
   // Clean up unreferenced symbols in symbol table.
   SymbolTable::unlink();

   assert(_marking_stack.is_empty(), "stack should be empty by now");
 }


 void GenMarkSweep::mark_sweep_phase2() {
   // Now all live objects are marked, compute the new object addresses.

   // It is imperative that we traverse perm_gen LAST. If dead space is
   // allowed a range of dead object may get overwritten by a dead int
   // array. If perm_gen is not traversed last a Klass* may get
   // overwritten. This is fine since it is dead, but if the class has dead
   // instances we have to skip them, and in order to find their size we
   // need the Klass*!
   //
   // It is not required that we traverse spaces in the same order in
   // phase2, phase3 and phase4, but the ValidateMarkSweep live oops
   // tracking expects us to do so. See comment under phase4.

   GenCollectedHeap* gch = GenCollectedHeap::heap();

   TraceTime tm("phase 2", PrintGC && Verbose, true, gclog_or_tty);
   trace("2");

   gch->prepare_for_compaction();
 }

 class GenAdjustPointersClosure: public GenCollectedHeap::GenClosure {
 public:
   void do_generation(Generation* gen) {
     gen->adjust_pointers();
   }
 };

 void GenMarkSweep::mark_sweep_phase3(int level) {
   GenCollectedHeap* gch = GenCollectedHeap::heap();

   // Adjust the pointers to reflect the new locations
   TraceTime tm("phase 3", PrintGC && Verbose, true, gclog_or_tty);
   trace("3");

   // Need new claim bits for the pointer adjustment tracing.
   ClassLoaderDataGraph::clear_claimed_marks();

   // Because the two closures below are created statically, cannot
   // use OopsInGenClosure constructor which takes a generation,
   // as the Universe has not been created when the static constructors
   // are run.
   adjust_root_pointer_closure.set_orig_generation(gch->get_gen(level));
   adjust_pointer_closure.set_orig_generation(gch->get_gen(level));

   gch->gen_process_strong_roots(level,
                                 false, // Younger gens are not roots.
                                 true,  // activate StrongRootsScope
                                 false, // not scavenging
                                 SharedHeap::SO_AllClasses,
                                 &adjust_root_pointer_closure,
                                 false, // do not walk code
                                 &adjust_root_pointer_closure,
                                 &adjust_klass_closure);

   // Now adjust pointers in remaining weak roots.  (All of which should
   // have been cleared if they pointed to non-surviving objects.)
   CodeBlobToOopClosure adjust_code_pointer_closure(&adjust_pointer_closure,
                                                    /*do_marking=*/ false);
   gch->gen_process_weak_roots(&adjust_root_pointer_closure,
                               &adjust_code_pointer_closure,
                               &adjust_pointer_closure);

   adjust_marks();
   GenAdjustPointersClosure blk;
   gch->generation_iterate(&blk, true);
 }

 class GenCompactClosure: public GenCollectedHeap::GenClosure {
 public:
   void do_generation(Generation* gen) {
     gen->compact();
   }
 };

 void GenMarkSweep::mark_sweep_phase4() {
   // All pointers are now adjusted, move objects accordingly

   // It is imperative that we traverse perm_gen first in phase4. All
   // classes must be allocated earlier than their instances, and traversing
   // perm_gen first makes sure that all Klass*s have moved to their new
   // location before any instance does a dispatch through it's klass!

   // The ValidateMarkSweep live oops tracking expects us to traverse spaces
   // in the same order in phase2, phase3 and phase4. We don't quite do that
   // here (perm_gen first rather than last), so we tell the validate code
   // to use a higher index (saved from phase2) when verifying perm_gen.
   GenCollectedHeap* gch = GenCollectedHeap::heap();

   TraceTime tm("phase 4", PrintGC && Verbose, true, gclog_or_tty);
   trace("4");

   GenCompactClosure blk;
   gch->generation_iterate(&blk, true);
 }
	/*
	* Copyright (c) 2001, 2012, 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
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*
	*/

	#include "precompiled.hpp"
	#include "classfile/javaClasses.hpp"
	#include "classfile/symbolTable.hpp"
	#include "classfile/systemDictionary.hpp"
	#include "classfile/vmSymbols.hpp"
	#include "code/codeCache.hpp"
	#include "code/icBuffer.hpp"
	#include "gc_interface/collectedHeap.inline.hpp"
	#include "memory/genCollectedHeap.hpp"
	#include "memory/genMarkSweep.hpp"
	#include "memory/genOopClosures.inline.hpp"
	#include "memory/generation.inline.hpp"
	#include "memory/modRefBarrierSet.hpp"
	#include "memory/referencePolicy.hpp"
	#include "memory/space.hpp"
	#include "oops/instanceRefKlass.hpp"
	#include "oops/oop.inline.hpp"
	#include "prims/jvmtiExport.hpp"
	#include "runtime/fprofiler.hpp"
	#include "runtime/handles.inline.hpp"
	#include "runtime/synchronizer.hpp"
	#include "runtime/thread.inline.hpp"
	#include "runtime/vmThread.hpp"
	#include "utilities/copy.hpp"
	#include "utilities/events.hpp"

	void GenMarkSweep::invoke_at_safepoint(int level, ReferenceProcessor* rp,
	bool clear_all_softrefs) {
	assert(SafepointSynchronize::is_at_safepoint(), "must be at a safepoint");

	GenCollectedHeap* gch = GenCollectedHeap::heap();
	#ifdef ASSERT
	if (gch->collector_policy()->should_clear_all_soft_refs()) {
	assert(clear_all_softrefs, "Policy should have been checked earlier");
	}
	#endif

	// hook up weak ref data so it can be used during Mark-Sweep
	assert(ref_processor() == NULL, "no stomping");
	assert(rp != NULL, "should be non-NULL");
	_ref_processor = rp;
	rp->setup_policy(clear_all_softrefs);

	TraceTime t1(GCCauseString("Full GC", gch->gc_cause()), PrintGC && !PrintGCDetails, true, gclog_or_tty);

	// When collecting the permanent generation Method*s may be moving,
	// so we either have to flush all bcp data or convert it into bci.
	CodeCache::gc_prologue();
	Threads::gc_prologue();

	// Increment the invocation count
	_total_invocations++;

	// Capture heap size before collection for printing.
	size_t gch_prev_used = gch->used();

	// Some of the card table updates below assume that the perm gen is
	// also being collected.
	assert(level == gch->n_gens() - 1,
	"All generations are being collected, ergo perm gen too.");

	// Capture used regions for each generation that will be
	// subject to collection, so that card table adjustments can
	// be made intelligently (see clear / invalidate further below).
	gch->save_used_regions(level);

	allocate_stacks();

	mark_sweep_phase1(level, clear_all_softrefs);

	mark_sweep_phase2();

	// Don't add any more derived pointers during phase3
	COMPILER2_PRESENT(assert(DerivedPointerTable::is_active(), "Sanity"));
	COMPILER2_PRESENT(DerivedPointerTable::set_active(false));

	mark_sweep_phase3(level);

	mark_sweep_phase4();

	restore_marks();

	// Set saved marks for allocation profiler (and other things? -- dld)
	// (Should this be in general part?)
	gch->save_marks();

	deallocate_stacks();

	// If compaction completely evacuated all generations younger than this
	// one, then we can clear the card table. Otherwise, we must invalidate
	// it (consider all cards dirty). In the future, we might consider doing
	// compaction within generations only, and doing card-table sliding.
	bool all_empty = true;
	for (int i = 0; all_empty && i < level; i++) {
	Generation* g = gch->get_gen(i);
	all_empty = all_empty && gch->get_gen(i)->used() == 0;
	}
	GenRemSet* rs = gch->rem_set();
	// Clear/invalidate below make use of the "prev_used_regions" saved earlier.
	if (all_empty) {
	// We've evacuated all generations below us.
	Generation* g = gch->get_gen(level);
	rs->clear_into_younger(g);
	} else {
	// Invalidate the cards corresponding to the currently used
	// region and clear those corresponding to the evacuated region
	// of all generations just collected (i.e. level and younger).
	rs->invalidate_or_clear(gch->get_gen(level),
	true /* younger */);
	}

	Threads::gc_epilogue();
	CodeCache::gc_epilogue();
	JvmtiExport::gc_epilogue();

	if (PrintGC && !PrintGCDetails) {
	gch->print_heap_change(gch_prev_used);
	}

	// refs processing: clean slate
	_ref_processor = NULL;

	// Update heap occupancy information which is used as
	// input to soft ref clearing policy at the next gc.
	Universe::update_heap_info_at_gc();

	// Update time of last gc for all generations we collected
	// (which curently is all the generations in the heap).
	// We need to use a monotonically non-deccreasing time in ms
	// or we will see time-warp warnings and os::javaTimeMillis()
	// does not guarantee monotonicity.
	jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
	gch->update_time_of_last_gc(now);
	}

	void GenMarkSweep::allocate_stacks() {
	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);

	// $$$ To cut a corner, we'll only use the first scratch block, and then
	// revert to malloc.
	if (scratch != NULL) {
	_preserved_count_max =
	scratch->num_words * HeapWordSize / sizeof(PreservedMark);
	} else {
	_preserved_count_max = 0;
	}

	_preserved_marks = (PreservedMark*)scratch;
	_preserved_count = 0;
	}


	void GenMarkSweep::deallocate_stacks() {
	if (!UseG1GC) {
	GenCollectedHeap* gch = GenCollectedHeap::heap();
	gch->release_scratch();
	}

	_preserved_mark_stack.clear(true);
	_preserved_oop_stack.clear(true);
	_marking_stack.clear();
	_objarray_stack.clear(true);
	}

	void GenMarkSweep::mark_sweep_phase1(int level,
	bool clear_all_softrefs) {
	// Recursively traverse all live objects and mark them
	TraceTime tm("phase 1", PrintGC && Verbose, true, gclog_or_tty);
	trace(" 1");

	GenCollectedHeap* gch = GenCollectedHeap::heap();

	// Because follow_root_closure is created statically, cannot
	// use OopsInGenClosure constructor which takes a generation,
	// as the Universe has not been created when the static constructors
	// are run.
	follow_root_closure.set_orig_generation(gch->get_gen(level));

	// Need new claim bits before marking starts.
	ClassLoaderDataGraph::clear_claimed_marks();

	gch->gen_process_strong_roots(level,
	false, // Younger gens are not roots.
	true, // activate StrongRootsScope
	false, // not scavenging
	SharedHeap::SO_SystemClasses,
	&follow_root_closure,
	true, // walk code active on stacks
	&follow_root_closure,
	&follow_klass_closure);

	// Process reference objects found during marking
	{
	ref_processor()->setup_policy(clear_all_softrefs);
	ref_processor()->process_discovered_references(
	&is_alive, &keep_alive, &follow_stack_closure, NULL);
	}

	// Follow system dictionary roots and unload classes
	bool purged_class = SystemDictionary::do_unloading(&is_alive);

	// Follow code cache roots
	CodeCache::do_unloading(&is_alive, purged_class);
	follow_stack(); // Flush marking stack

	// Update subklass/sibling/implementor links of live klasses
	Klass::clean_weak_klass_links(&is_alive);
	assert(_marking_stack.is_empty(), "just drained");

	// Visit interned string tables and delete unmarked oops
	StringTable::unlink(&is_alive);
	// Clean up unreferenced symbols in symbol table.
	SymbolTable::unlink();

	assert(_marking_stack.is_empty(), "stack should be empty by now");
	}


	void GenMarkSweep::mark_sweep_phase2() {
	// Now all live objects are marked, compute the new object addresses.

	// It is imperative that we traverse perm_gen LAST. If dead space is
	// allowed a range of dead object may get overwritten by a dead int
	// array. If perm_gen is not traversed last a Klass* may get
	// overwritten. This is fine since it is dead, but if the class has dead
	// instances we have to skip them, and in order to find their size we
	// need the Klass*!
	//
	// It is not required that we traverse spaces in the same order in
	// phase2, phase3 and phase4, but the ValidateMarkSweep live oops
	// tracking expects us to do so. See comment under phase4.

	GenCollectedHeap* gch = GenCollectedHeap::heap();

	TraceTime tm("phase 2", PrintGC && Verbose, true, gclog_or_tty);
	trace("2");

	gch->prepare_for_compaction();
	}

	class GenAdjustPointersClosure: public GenCollectedHeap::GenClosure {
	public:
	void do_generation(Generation* gen) {
	gen->adjust_pointers();
	}
	};

	void GenMarkSweep::mark_sweep_phase3(int level) {
	GenCollectedHeap* gch = GenCollectedHeap::heap();

	// Adjust the pointers to reflect the new locations
	TraceTime tm("phase 3", PrintGC && Verbose, true, gclog_or_tty);
	trace("3");

	// Need new claim bits for the pointer adjustment tracing.
	ClassLoaderDataGraph::clear_claimed_marks();

	// Because the two closures below are created statically, cannot
	// use OopsInGenClosure constructor which takes a generation,
	// as the Universe has not been created when the static constructors
	// are run.
	adjust_root_pointer_closure.set_orig_generation(gch->get_gen(level));
	adjust_pointer_closure.set_orig_generation(gch->get_gen(level));

	gch->gen_process_strong_roots(level,
	false, // Younger gens are not roots.
	true, // activate StrongRootsScope
	false, // not scavenging
	SharedHeap::SO_AllClasses,
	&adjust_root_pointer_closure,
	false, // do not walk code
	&adjust_root_pointer_closure,
	&adjust_klass_closure);

	// Now adjust pointers in remaining weak roots. (All of which should
	// have been cleared if they pointed to non-surviving objects.)
	CodeBlobToOopClosure adjust_code_pointer_closure(&adjust_pointer_closure,
	/do_marking=/ false);
	gch->gen_process_weak_roots(&adjust_root_pointer_closure,
	&adjust_code_pointer_closure,
	&adjust_pointer_closure);

	adjust_marks();
	GenAdjustPointersClosure blk;
	gch->generation_iterate(&blk, true);
	}

	class GenCompactClosure: public GenCollectedHeap::GenClosure {
	public:
	void do_generation(Generation* gen) {
	gen->compact();
	}
	};

	void GenMarkSweep::mark_sweep_phase4() {
	// All pointers are now adjusted, move objects accordingly

	// It is imperative that we traverse perm_gen first in phase4. All
	// classes must be allocated earlier than their instances, and traversing
	// perm_gen first makes sure that all Klass*s have moved to their new
	// location before any instance does a dispatch through it's klass!

	// The ValidateMarkSweep live oops tracking expects us to traverse spaces
	// in the same order in phase2, phase3 and phase4. We don't quite do that
	// here (perm_gen first rather than last), so we tell the validate code
	// to use a higher index (saved from phase2) when verifying perm_gen.
	GenCollectedHeap* gch = GenCollectedHeap::heap();

	TraceTime tm("phase 4", PrintGC && Verbose, true, gclog_or_tty);
	trace("4");

	GenCompactClosure blk;
	gch->generation_iterate(&blk, true);
	}