1 /*
2 * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
27
28 //
29 // String Deduplication
30 //
31 // String deduplication aims to reduce the heap live-set by deduplicating identical
32 // instances of String so that they share the same backing character array.
33 //
34 // The deduplication process is divided in two main parts, 1) finding the objects to
35 // deduplicate, and 2) deduplicating those objects. The first part is done as part of
36 // a normal GC cycle when objects are marked or evacuated. At this time a check is
37 // applied on each object to check if it is a candidate for deduplication. If so, the
38 // object is placed on the deduplication queue for later processing. The second part,
39 // processing the objects on the deduplication queue, is a concurrent phase which
40 // starts right after the stop-the-wold marking/evacuation phase. This phase is
41 // executed by the deduplication thread, which pulls deduplication candidates of the
42 // deduplication queue and tries to deduplicate them.
43 //
44 // A deduplication hashtable is used to keep track of all unique character arrays
45 // used by String objects. When deduplicating, a lookup is made in this table to see
46 // if there is already an identical character array somewhere on the heap. If so, the
47 // String object is adjusted to point to that character array, releasing the reference
48 // to the original array allowing it to eventually be garbage collected. If the lookup
49 // fails the character array is instead inserted into the hashtable so that this array
50 // can be shared at some point in the future.
51 //
52 // Candidate selection
53 //
54 // An object is considered a deduplication candidate if all of the following
55 // statements are true:
56 //
57 // - The object is an instance of java.lang.String
58 //
59 // - The object is being evacuated from a young heap region
60 //
61 // - The object is being evacuated to a young/survivor heap region and the
62 // object's age is equal to the deduplication age threshold
63 //
64 // or
65 //
66 // The object is being evacuated to an old heap region and the object's age is
67 // less than the deduplication age threshold
68 //
69 // Once an string object has been promoted to an old region, or its age is higher
70 // than the deduplication age threshold, is will never become a candidate again.
71 // This approach avoids making the same object a candidate more than once.
72 //
73 // Interned strings are a bit special. They are explicitly deduplicated just before
74 // being inserted into the StringTable (to avoid counteracting C2 optimizations done
75 // on string literals), then they also become deduplication candidates if they reach
76 // the deduplication age threshold or are evacuated to an old heap region. The second
77 // attempt to deduplicate such strings will be in vain, but we have no fast way of
78 // filtering them out. This has not shown to be a problem, as the number of interned
79 // strings is usually dwarfed by the number of normal (non-interned) strings.
80 //
81 // For additional information on string deduplication, please see JEP 192,
82 // http://openjdk.java.net/jeps/192
83 //
84
85 #include "memory/allocation.hpp"
86 #include "oops/oop.hpp"
87 #include "runtime/atomic.hpp"
88
89 class OopClosure;
90 class BoolObjectClosure;
91 class ThreadClosure;
92 class outputStream;
93 class G1StringDedupTable;
94 class G1GCPhaseTimes;
95
96 //
97 // Main interface for interacting with string deduplication.
98 //
99 class G1StringDedup : public AllStatic {
100 private:
101 // Single state for checking if both G1 and string deduplication is enabled.
102 static bool _enabled;
103
104 // Candidate selection policies, returns true if the given object is
105 // candidate for string deduplication.
106 static bool is_candidate_from_mark(oop obj);
107 static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
108
109 public:
110 // Returns true if both G1 and string deduplication is enabled.
111 static bool is_enabled() {
112 return _enabled;
113 }
114
115 // Initialize string deduplication.
116 static void initialize();
117
118 // Stop the deduplication thread.
119 static void stop();
120
121 // Immediately deduplicates the given String object, bypassing the
122 // the deduplication queue.
123 static void deduplicate(oop java_string);
124
125 // Enqueues a deduplication candidate for later processing by the deduplication
126 // thread. Before enqueuing, these functions apply the appropriate candidate
127 // selection policy to filters out non-candidates.
128 static void enqueue_from_mark(oop java_string);
129 static void enqueue_from_evacuation(bool from_young, bool to_young,
130 unsigned int queue, oop java_string);
131
132 static void oops_do(OopClosure* keep_alive);
133 static void unlink(BoolObjectClosure* is_alive);
134 static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
135 bool allow_resize_and_rehash, G1GCPhaseTimes* phase_times = NULL);
136
137 static void threads_do(ThreadClosure* tc);
138 static void print_worker_threads_on(outputStream* st);
139 static void verify();
140 };
141
142 //
143 // This closure encapsulates the state and the closures needed when scanning
144 // the deduplication queue and table during the unlink_or_oops_do() operation.
145 // A single instance of this closure is created and then shared by all worker
146 // threads participating in the scan. The _next_queue and _next_bucket fields
147 // provide a simple mechanism for GC workers to claim exclusive access to a
148 // queue or a table partition.
149 //
150 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
151 private:
152 BoolObjectClosure* _is_alive;
153 OopClosure* _keep_alive;
154 G1StringDedupTable* _resized_table;
155 G1StringDedupTable* _rehashed_table;
156 size_t _next_queue;
157 size_t _next_bucket;
158
159 public:
160 G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
161 OopClosure* keep_alive,
162 bool allow_resize_and_rehash);
163 ~G1StringDedupUnlinkOrOopsDoClosure();
164
165 bool is_resizing() {
166 return _resized_table != NULL;
167 }
168
169 G1StringDedupTable* resized_table() {
170 return _resized_table;
171 }
172
173 bool is_rehashing() {
174 return _rehashed_table != NULL;
175 }
176
177 // Atomically claims the next available queue for exclusive access by
178 // the current thread. Returns the queue number of the claimed queue.
179 size_t claim_queue();
180
181 // Atomically claims the next available table partition for exclusive
182 // access by the current thread. Returns the table bucket number where
183 // the claimed partition starts.
184 size_t claim_table_partition(size_t partition_size);
185
186 // Applies and returns the result from the is_alive closure, or
187 // returns true if no such closure was provided.
188 bool is_alive(oop o) {
189 if (_is_alive != NULL) {
190 return _is_alive->do_object_b(o);
191 }
192 return true;
193 }
194
195 // Applies the keep_alive closure, or does nothing if no such
196 // closure was provided.
197 void keep_alive(oop* p) {
198 if (_keep_alive != NULL) {
199 _keep_alive->do_oop(p);
200 }
201 }
202 };
203
204 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP