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.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 * accompanied this code).
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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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
95 //
96 // Main interface for interacting with string deduplication.
97 //
98 class G1StringDedup : public AllStatic {
99 private:
100 // Single state for checking if both G1 and string deduplication is enabled.
101 static bool _enabled;
102
103 // Candidate selection policies, returns true if the given object is
104 // candidate for string deduplication.
105 static bool is_candidate_from_mark(oop obj);
106 static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
107
108 public:
109 // Returns true if both G1 and string deduplication is enabled.
110 static bool is_enabled() {
111 return _enabled;
112 }
113
114 // Initialize string deduplication.
115 static void initialize();
116
117 // Stop the deduplication thread.
118 static void stop();
119
120 // Immediately deduplicates the given String object, bypassing the
121 // the deduplication queue.
122 static void deduplicate(oop java_string);
123
124 // Enqueues a deduplication candidate for later processing by the deduplication
125 // thread. Before enqueuing, these functions apply the appropriate candidate
126 // selection policy to filters out non-candidates.
127 static void enqueue_from_mark(oop java_string);
128 static void enqueue_from_evacuation(bool from_young, bool to_young,
129 unsigned int queue, oop java_string);
130
131 static void oops_do(OopClosure* keep_alive);
132 static void unlink(BoolObjectClosure* is_alive);
133 static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
134 bool allow_resize_and_rehash = true);
135
136 static void threads_do(ThreadClosure* tc);
137 static void print_worker_threads_on(outputStream* st);
138 static void verify();
139 };
140
141 //
142 // This closure encapsulates the state and the closures needed when scanning
143 // the deduplication queue and table during the unlink_or_oops_do() operation.
144 // A single instance of this closure is created and then shared by all worker
145 // threads participating in the scan. The _next_queue and _next_bucket fields
146 // provide a simple mechanism for GC workers to claim exclusive access to a
147 // queue or a table partition.
148 //
149 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
150 private:
151 BoolObjectClosure* _is_alive;
152 OopClosure* _keep_alive;
153 G1StringDedupTable* _resized_table;
154 G1StringDedupTable* _rehashed_table;
155 size_t _next_queue;
156 size_t _next_bucket;
157
158 public:
159 G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
160 OopClosure* keep_alive,
161 bool allow_resize_and_rehash);
162 ~G1StringDedupUnlinkOrOopsDoClosure();
163
164 bool is_resizing() {
165 return _resized_table != NULL;
166 }
167
168 G1StringDedupTable* resized_table() {
169 return _resized_table;
170 }
171
172 bool is_rehashing() {
173 return _rehashed_table != NULL;
174 }
175
176 // Atomically claims the next available queue for exclusive access by
177 // the current thread. Returns the queue number of the claimed queue.
178 size_t claim_queue();
179
180 // Atomically claims the next available table partition for exclusive
181 // access by the current thread. Returns the table bucket number where
182 // the claimed partition starts.
183 size_t claim_table_partition(size_t partition_size);
184
185 // Applies and returns the result from the is_alive closure, or
186 // returns true if no such closure was provided.
187 bool is_alive(oop o) {
188 if (_is_alive != NULL) {
189 return _is_alive->do_object_b(o);
190 }
191 return true;
192 }
193
194 // Applies the keep_alive closure, or does nothing if no such
195 // closure was provided.
196 void keep_alive(oop* p) {
197 if (_keep_alive != NULL) {
198 _keep_alive->do_oop(p);
199 }
200 }
201 };
202
203 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
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