Tue, 24 Aug 2010 17:24:33 -0400
6974966: G1: unnecessary direct-to-old allocations
Summary: This change revamps the slow allocation path of G1. Improvements include the following: a) Allocations directly to old regions are now totally banned. G1 now only allows allocations out of young regions (with the only exception being humongous regions). b) The thread that allocates a new region (which is now guaranteed to be young) does not dirty all its cards. Each thread that successfully allocates out of a young region is now responsible for dirtying the cards that corresponding to the "block" that just got allocated. c) allocate_new_tlab() and mem_allocate() are now implemented differently and TLAB allocations are only done by allocate_new_tlab(). d) If a thread schedules an evacuation pause in order to satisfy an allocation request, it will perform the allocation at the end of the safepoint so that the thread that initiated the GC also gets "first pick" of any space made available by the GC. e) If a thread is unable to allocate a humongous object it will schedule an evacuation pause in case it reclaims enough regions so that the humongous allocation can be satisfied aftewards. f) The G1 policy is more careful to set the young list target length to be the survivor number +1. g) Lots of code tidy up, removal, refactoring to make future changes easier.
Reviewed-by: johnc, ysr
1 /*
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25 #include "precompiled.hpp"
26 #include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
27 #include "gc_implementation/g1/g1CollectorPolicy.hpp"
28 #include "gc_implementation/g1/vm_operations_g1.hpp"
29 #include "gc_implementation/shared/isGCActiveMark.hpp"
30 #include "gc_implementation/g1/vm_operations_g1.hpp"
31 #include "runtime/interfaceSupport.hpp"
33 VM_G1CollectForAllocation::VM_G1CollectForAllocation(
34 unsigned int gc_count_before,
35 size_t word_size)
36 : VM_G1OperationWithAllocRequest(gc_count_before, word_size) {
37 guarantee(word_size > 0, "an allocation should always be requested");
38 }
40 void VM_G1CollectForAllocation::doit() {
41 JvmtiGCForAllocationMarker jgcm;
42 G1CollectedHeap* g1h = G1CollectedHeap::heap();
43 _result = g1h->satisfy_failed_allocation(_word_size, &_pause_succeeded);
44 assert(_result == NULL || _pause_succeeded,
45 "if we get back a result, the pause should have succeeded");
46 }
48 void VM_G1CollectFull::doit() {
49 JvmtiGCFullMarker jgcm;
50 G1CollectedHeap* g1h = G1CollectedHeap::heap();
51 GCCauseSetter x(g1h, _gc_cause);
52 g1h->do_full_collection(false /* clear_all_soft_refs */);
53 }
55 VM_G1IncCollectionPause::VM_G1IncCollectionPause(
56 unsigned int gc_count_before,
57 size_t word_size,
58 bool should_initiate_conc_mark,
59 double target_pause_time_ms,
60 GCCause::Cause gc_cause)
61 : VM_G1OperationWithAllocRequest(gc_count_before, word_size),
62 _should_initiate_conc_mark(should_initiate_conc_mark),
63 _target_pause_time_ms(target_pause_time_ms),
64 _full_collections_completed_before(0) {
65 guarantee(target_pause_time_ms > 0.0,
66 err_msg("target_pause_time_ms = %1.6lf should be positive",
67 target_pause_time_ms));
68 guarantee(word_size == 0 || gc_cause == GCCause::_g1_inc_collection_pause,
69 "we can only request an allocation if the GC cause is for "
70 "an incremental GC pause");
71 _gc_cause = gc_cause;
72 }
74 void VM_G1IncCollectionPause::doit() {
75 JvmtiGCForAllocationMarker jgcm;
76 G1CollectedHeap* g1h = G1CollectedHeap::heap();
77 assert(!_should_initiate_conc_mark ||
78 ((_gc_cause == GCCause::_gc_locker && GCLockerInvokesConcurrent) ||
79 (_gc_cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent)),
80 "only a GC locker or a System.gc() induced GC should start a cycle");
82 if (_word_size > 0) {
83 // An allocation has been requested. So, try to do that first.
84 _result = g1h->attempt_allocation_at_safepoint(_word_size,
85 false /* expect_null_cur_alloc_region */);
86 if (_result != NULL) {
87 // If we can successfully allocate before we actually do the
88 // pause then we will consider this pause successful.
89 _pause_succeeded = true;
90 return;
91 }
92 }
94 GCCauseSetter x(g1h, _gc_cause);
95 if (_should_initiate_conc_mark) {
96 // It's safer to read full_collections_completed() here, given
97 // that noone else will be updating it concurrently. Since we'll
98 // only need it if we're initiating a marking cycle, no point in
99 // setting it earlier.
100 _full_collections_completed_before = g1h->full_collections_completed();
102 // At this point we are supposed to start a concurrent cycle. We
103 // will do so if one is not already in progress.
104 bool res = g1h->g1_policy()->force_initial_mark_if_outside_cycle();
105 }
107 _pause_succeeded =
108 g1h->do_collection_pause_at_safepoint(_target_pause_time_ms);
109 if (_pause_succeeded && _word_size > 0) {
110 // An allocation had been requested.
111 _result = g1h->attempt_allocation_at_safepoint(_word_size,
112 true /* expect_null_cur_alloc_region */);
113 } else {
114 assert(_result == NULL, "invariant");
115 }
116 }
118 void VM_G1IncCollectionPause::doit_epilogue() {
119 VM_GC_Operation::doit_epilogue();
121 // If the pause was initiated by a System.gc() and
122 // +ExplicitGCInvokesConcurrent, we have to wait here for the cycle
123 // that just started (or maybe one that was already in progress) to
124 // finish.
125 if (_gc_cause == GCCause::_java_lang_system_gc &&
126 _should_initiate_conc_mark) {
127 assert(ExplicitGCInvokesConcurrent,
128 "the only way to be here is if ExplicitGCInvokesConcurrent is set");
130 G1CollectedHeap* g1h = G1CollectedHeap::heap();
132 // In the doit() method we saved g1h->full_collections_completed()
133 // in the _full_collections_completed_before field. We have to
134 // wait until we observe that g1h->full_collections_completed()
135 // has increased by at least one. This can happen if a) we started
136 // a cycle and it completes, b) a cycle already in progress
137 // completes, or c) a Full GC happens.
139 // If the condition has already been reached, there's no point in
140 // actually taking the lock and doing the wait.
141 if (g1h->full_collections_completed() <=
142 _full_collections_completed_before) {
143 // The following is largely copied from CMS
145 Thread* thr = Thread::current();
146 assert(thr->is_Java_thread(), "invariant");
147 JavaThread* jt = (JavaThread*)thr;
148 ThreadToNativeFromVM native(jt);
150 MutexLockerEx x(FullGCCount_lock, Mutex::_no_safepoint_check_flag);
151 while (g1h->full_collections_completed() <=
152 _full_collections_completed_before) {
153 FullGCCount_lock->wait(Mutex::_no_safepoint_check_flag);
154 }
155 }
156 }
157 }
159 void VM_CGC_Operation::doit() {
160 gclog_or_tty->date_stamp(PrintGC && PrintGCDateStamps);
161 TraceCPUTime tcpu(PrintGCDetails, true, gclog_or_tty);
162 TraceTime t(_printGCMessage, PrintGC, true, gclog_or_tty);
163 SharedHeap* sh = SharedHeap::heap();
164 // This could go away if CollectedHeap gave access to _gc_is_active...
165 if (sh != NULL) {
166 IsGCActiveMark x;
167 _cl->do_void();
168 } else {
169 _cl->do_void();
170 }
171 }
173 bool VM_CGC_Operation::doit_prologue() {
174 Heap_lock->lock();
175 SharedHeap::heap()->_thread_holds_heap_lock_for_gc = true;
176 return true;
177 }
179 void VM_CGC_Operation::doit_epilogue() {
180 SharedHeap::heap()->_thread_holds_heap_lock_for_gc = false;
181 Heap_lock->unlock();
182 }