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_G1STRINGDEDUPTABLE_HPP
26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
27
28 #include "gc_implementation/g1/g1StringDedupStat.hpp"
29
30 class G1StringDedupEntryCache;
31
32 //
33 // Table entry in the deduplication hashtable. Points weakly to the
34 // character array. Can be chained in a linked list in case of hash
35 // collisions or when placed in a freelist in the entry cache.
36 //
37 class G1StringDedupEntry : public CHeapObj<mtGC> {
38 private:
39 G1StringDedupEntry* _next;
40 unsigned int _hash;
41 typeArrayOop _obj;
42
43 public:
44 G1StringDedupEntry() :
45 _next(NULL),
46 _hash(0),
47 _obj(NULL) {
48 }
49
50 G1StringDedupEntry* next() {
51 return _next;
52 }
53
54 G1StringDedupEntry** next_addr() {
55 return &_next;
56 }
57
58 void set_next(G1StringDedupEntry* next) {
59 _next = next;
60 }
61
62 unsigned int hash() {
63 return _hash;
64 }
65
66 void set_hash(unsigned int hash) {
67 _hash = hash;
68 }
69
70 typeArrayOop obj() {
71 return _obj;
72 }
73
74 typeArrayOop* obj_addr() {
75 return &_obj;
76 }
77
78 void set_obj(typeArrayOop obj) {
79 _obj = obj;
80 }
81 };
82
83 //
84 // The deduplication hashtable keeps track of all unique character arrays used
85 // by String objects. Each table entry weakly points to an character array, allowing
86 // otherwise unreachable character arrays to be declared dead and pruned from the
87 // table.
88 //
89 // The table is dynamically resized to accommodate the current number of table entries.
90 // The table has hash buckets with chains for hash collision. If the average chain
91 // length goes above or below given thresholds the table grows or shrinks accordingly.
92 //
93 // The table is also dynamically rehashed (using a new hash seed) if it becomes severely
94 // unbalanced, i.e., a hash chain is significantly longer than average.
95 //
96 // All access to the table is protected by the StringDedupTable_lock, except under
97 // safepoints in which case GC workers are allowed to access a table partitions they
98 // have claimed without first acquiring the lock. Note however, that this applies only
99 // the table partition (i.e. a range of elements in _buckets), not other parts of the
100 // table such as the _entries field, statistics counters, etc.
101 //
102 class G1StringDedupTable : public CHeapObj<mtGC> {
103 private:
104 // The currently active hashtable instance. Only modified when
105 // the table is resizes or rehashed.
106 static G1StringDedupTable* _table;
107
108 // Cache for reuse and fast alloc/free of table entries.
109 static G1StringDedupEntryCache* _entry_cache;
110
111 G1StringDedupEntry** _buckets;
112 size_t _size;
113 uintx _entries;
114 uintx _shrink_threshold;
115 uintx _grow_threshold;
116 bool _rehash_needed;
117
118 // The hash seed also dictates which hash function to use. A
119 // zero hash seed means we will use the Java compatible hash
120 // function (which doesn't use a seed), and a non-zero hash
121 // seed means we use the murmur3 hash function.
122 jint _hash_seed;
123
124 // Constants governing table resize/rehash/cache.
125 static const size_t _min_size;
126 static const size_t _max_size;
127 static const double _grow_load_factor;
128 static const double _shrink_load_factor;
129 static const uintx _rehash_multiple;
130 static const uintx _rehash_threshold;
131 static const double _max_cache_factor;
132
133 // Table statistics, only used for logging.
134 static uintx _entries_added;
135 static uintx _entries_removed;
136 static uintx _resize_count;
137 static uintx _rehash_count;
138
139 G1StringDedupTable(size_t size, jint hash_seed = 0);
140 ~G1StringDedupTable();
141
142 // Returns the hash bucket at the given index.
143 G1StringDedupEntry** bucket(size_t index) {
144 return _buckets + index;
145 }
146
147 // Returns the hash bucket index for the given hash code.
148 size_t hash_to_index(unsigned int hash) {
149 return (size_t)hash & (_size - 1);
150 }
151
152 // Adds a new table entry to the given hash bucket.
153 void add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list);
154
155 // Removes the given table entry from the table.
156 void remove(G1StringDedupEntry** pentry, uint worker_id);
157
158 // Transfers a table entry from the current table to the destination table.
159 void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest);
160
161 // Returns an existing character array in the given hash bucket, or NULL
162 // if no matching character array exists.
163 typeArrayOop lookup(typeArrayOop value, unsigned int hash,
164 G1StringDedupEntry** list, uintx &count);
165
166 // Returns an existing character array in the table, or inserts a new
167 // table entry if no matching character array exists.
168 typeArrayOop lookup_or_add_inner(typeArrayOop value, unsigned int hash);
169
170 // Thread safe lookup or add of table entry
171 static typeArrayOop lookup_or_add(typeArrayOop value, unsigned int hash) {
172 // Protect the table from concurrent access. Also note that this lock
173 // acts as a fence for _table, which could have been replaced by a new
174 // instance if the table was resized or rehashed.
175 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
176 return _table->lookup_or_add_inner(value, hash);
177 }
178
179 // Returns true if the hashtable is currently using a Java compatible
180 // hash function.
181 static bool use_java_hash() {
182 return _table->_hash_seed == 0;
183 }
184
185 static bool equals(typeArrayOop value1, typeArrayOop value2);
186
187 // Computes the hash code for the given character array, using the
188 // currently active hash function and hash seed.
189 static unsigned int hash_code(typeArrayOop value);
190
191 static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,
192 size_t partition_begin,
193 size_t partition_end,
194 uint worker_id);
195
196 public:
197 static void create();
198
199 // Deduplicates the given String object, or adds its backing
200 // character array to the deduplication hashtable.
201 static void deduplicate(oop java_string, G1StringDedupStat& stat);
202
203 // If a table resize is needed, returns a newly allocated empty
204 // hashtable of the proper size.
205 static G1StringDedupTable* prepare_resize();
206
207 // Installs a newly resized table as the currently active table
208 // and deletes the previously active table.
209 static void finish_resize(G1StringDedupTable* resized_table);
210
211 // If a table rehash is needed, returns a newly allocated empty
212 // hashtable and updates the hash seed.
213 static G1StringDedupTable* prepare_rehash();
214
215 // Transfers rehashed entries from the currently active table into
216 // the new table. Installs the new table as the currently active table
217 // and deletes the previously active table.
218 static void finish_rehash(G1StringDedupTable* rehashed_table);
219
220 // If the table entry cache has grown too large, trim it down according to policy
221 static void trim_entry_cache();
222
223 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);
224
225 static void print_statistics(outputStream* st);
226 static void verify();
227 };
228
229 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP