17764
|
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
|
|
88 class OopClosure;
|
|
89 class BoolObjectClosure;
|
|
90 class ThreadClosure;
|
|
91 class outputStream;
|
|
92 class G1StringDedupTable;
|
|
93
|
|
94 //
|
|
95 // Main interface for interacting with string deduplication.
|
|
96 //
|
|
97 class G1StringDedup : public AllStatic {
|
|
98 private:
|
|
99 // Single state for checking if both G1 and string deduplication is enabled.
|
|
100 static bool _enabled;
|
|
101
|
|
102 // Candidate selection policies, returns true if the given object is
|
|
103 // candidate for string deduplication.
|
|
104 static bool is_candidate_from_mark(oop obj);
|
|
105 static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
|
|
106
|
|
107 public:
|
|
108 // Returns true if both G1 and string deduplication is enabled.
|
|
109 static bool is_enabled() {
|
|
110 return _enabled;
|
|
111 }
|
|
112
|
|
113 static void initialize();
|
|
114
|
|
115 // Immediately deduplicates the given String object, bypassing the
|
|
116 // the deduplication queue.
|
|
117 static void deduplicate(oop java_string);
|
|
118
|
|
119 // Enqueues a deduplication candidate for later processing by the deduplication
|
|
120 // thread. Before enqueuing, these functions apply the appropriate candidate
|
|
121 // selection policy to filters out non-candidates.
|
|
122 static void enqueue_from_mark(oop java_string);
|
|
123 static void enqueue_from_evacuation(bool from_young, bool to_young,
|
|
124 unsigned int queue, oop java_string);
|
|
125
|
|
126 static void oops_do(OopClosure* keep_alive);
|
|
127 static void unlink(BoolObjectClosure* is_alive);
|
|
128 static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
|
|
129 bool allow_resize_and_rehash = true);
|
|
130
|
|
131 static void threads_do(ThreadClosure* tc);
|
|
132 static void print_worker_threads_on(outputStream* st);
|
|
133 static void verify();
|
|
134 };
|
|
135
|
|
136 //
|
|
137 // This closure encapsulates the state and the closures needed when scanning
|
|
138 // the deduplication queue and table during the unlink_or_oops_do() operation.
|
|
139 // A single instance of this closure is created and then shared by all worker
|
|
140 // threads participating in the scan. The _next_queue and _next_bucket fields
|
|
141 // provide a simple mechanism for GC workers to claim exclusive access to a
|
|
142 // queue or a table partition.
|
|
143 //
|
|
144 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
|
|
145 private:
|
|
146 BoolObjectClosure* _is_alive;
|
|
147 OopClosure* _keep_alive;
|
|
148 G1StringDedupTable* _resized_table;
|
|
149 G1StringDedupTable* _rehashed_table;
|
|
150 size_t _next_queue;
|
|
151 size_t _next_bucket;
|
|
152
|
|
153 public:
|
|
154 G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
|
|
155 OopClosure* keep_alive,
|
|
156 bool allow_resize_and_rehash);
|
|
157 ~G1StringDedupUnlinkOrOopsDoClosure();
|
|
158
|
|
159 bool is_resizing() {
|
|
160 return _resized_table != NULL;
|
|
161 }
|
|
162
|
|
163 G1StringDedupTable* resized_table() {
|
|
164 return _resized_table;
|
|
165 }
|
|
166
|
|
167 bool is_rehashing() {
|
|
168 return _rehashed_table != NULL;
|
|
169 }
|
|
170
|
|
171 // Atomically claims the next available queue for exclusive access by
|
|
172 // the current thread. Returns the queue number of the claimed queue.
|
|
173 size_t claim_queue() {
|
|
174 return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;
|
|
175 }
|
|
176
|
|
177 // Atomically claims the next available table partition for exclusive
|
|
178 // access by the current thread. Returns the table bucket number where
|
|
179 // the claimed partition starts.
|
|
180 size_t claim_table_partition(size_t partition_size) {
|
|
181 return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;
|
|
182 }
|
|
183
|
|
184 // Applies and returns the result from the is_alive closure, or
|
|
185 // returns true if no such closure was provided.
|
|
186 bool is_alive(oop o) {
|
|
187 if (_is_alive != NULL) {
|
|
188 return _is_alive->do_object_b(o);
|
|
189 }
|
|
190 return true;
|
|
191 }
|
|
192
|
|
193 // Applies the keep_alive closure, or does nothing if no such
|
|
194 // closure was provided.
|
|
195 void keep_alive(oop* p) {
|
|
196 if (_keep_alive != NULL) {
|
|
197 _keep_alive->do_oop(p);
|
|
198 }
|
|
199 }
|
|
200 };
|
|
201
|
|
202 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
|