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 #include "runtime/mutexLocker.hpp" 30 31 class G1StringDedupEntryCache; 32 33 // 34 // Table entry in the deduplication hashtable. Points weakly to the 35 // character array. Can be chained in a linked list in case of hash 36 // collisions or when placed in a freelist in the entry cache. 37 // 38 class G1StringDedupEntry : public CHeapObj<mtGC> { 39 private: 40 G1StringDedupEntry* _next; 41 unsigned int _hash; 42 typeArrayOop _obj; 43 44 public: 45 G1StringDedupEntry() : 46 _next(NULL), 47 _hash(0), 48 _obj(NULL) { 49 } 50 51 G1StringDedupEntry* next() { 52 return _next; 53 } 54 55 G1StringDedupEntry** next_addr() { 56 return &_next; 57 } 58 59 void set_next(G1StringDedupEntry* next) { 60 _next = next; 61 } 62 63 unsigned int hash() { 64 return _hash; 65 } 66 67 void set_hash(unsigned int hash) { 68 _hash = hash; 69 } 70 71 typeArrayOop obj() { 72 return _obj; 73 } 74 75 typeArrayOop* obj_addr() { 76 return &_obj; 77 } 78 79 void set_obj(typeArrayOop obj) { 80 _obj = obj; 81 } 82 }; 83 84 // 85 // The deduplication hashtable keeps track of all unique character arrays used 86 // by String objects. Each table entry weakly points to an character array, allowing 87 // otherwise unreachable character arrays to be declared dead and pruned from the 88 // table. 89 // 90 // The table is dynamically resized to accommodate the current number of table entries. 91 // The table has hash buckets with chains for hash collision. If the average chain 92 // length goes above or below given thresholds the table grows or shrinks accordingly. 93 // 94 // The table is also dynamically rehashed (using a new hash seed) if it becomes severely 95 // unbalanced, i.e., a hash chain is significantly longer than average. 96 // 97 // All access to the table is protected by the StringDedupTable_lock, except under 98 // safepoints in which case GC workers are allowed to access a table partitions they 99 // have claimed without first acquiring the lock. Note however, that this applies only 100 // the table partition (i.e. a range of elements in _buckets), not other parts of the 101 // table such as the _entries field, statistics counters, etc. 102 // 103 class G1StringDedupTable : public CHeapObj<mtGC> { 104 private: 105 // The currently active hashtable instance. Only modified when 106 // the table is resizes or rehashed. 107 static G1StringDedupTable* _table; 108 109 // Cache for reuse and fast alloc/free of table entries. 110 static G1StringDedupEntryCache* _entry_cache; 111 112 G1StringDedupEntry** _buckets; 113 size_t _size; 114 uintx _entries; 115 uintx _shrink_threshold; 116 uintx _grow_threshold; 117 bool _rehash_needed; 118 119 // The hash seed also dictates which hash function to use. A 120 // zero hash seed means we will use the Java compatible hash 121 // function (which doesn't use a seed), and a non-zero hash 122 // seed means we use the murmur3 hash function. 123 jint _hash_seed; 124 125 // Constants governing table resize/rehash/cache. 126 static const size_t _min_size; 127 static const size_t _max_size; 128 static const double _grow_load_factor; 129 static const double _shrink_load_factor; 130 static const uintx _rehash_multiple; 131 static const uintx _rehash_threshold; 132 static const double _max_cache_factor; 133 134 // Table statistics, only used for logging. 135 static uintx _entries_added; 136 static uintx _entries_removed; 137 static uintx _resize_count; 138 static uintx _rehash_count; 139 140 G1StringDedupTable(size_t size, jint hash_seed = 0); 141 ~G1StringDedupTable(); 142 143 // Returns the hash bucket at the given index. 144 G1StringDedupEntry** bucket(size_t index) { 145 return _buckets + index; 146 } 147 148 // Returns the hash bucket index for the given hash code. 149 size_t hash_to_index(unsigned int hash) { 150 return (size_t)hash & (_size - 1); 151 } 152 153 // Adds a new table entry to the given hash bucket. 154 void add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list); 155 156 // Removes the given table entry from the table. 157 void remove(G1StringDedupEntry** pentry, uint worker_id); 158 159 // Transfers a table entry from the current table to the destination table. 160 void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest); 161 162 // Returns an existing character array in the given hash bucket, or NULL 163 // if no matching character array exists. 164 typeArrayOop lookup(typeArrayOop value, unsigned int hash, 165 G1StringDedupEntry** list, uintx &count); 166 167 // Returns an existing character array in the table, or inserts a new 168 // table entry if no matching character array exists. 169 typeArrayOop lookup_or_add_inner(typeArrayOop value, unsigned int hash); 170 171 // Thread safe lookup or add of table entry 172 static typeArrayOop lookup_or_add(typeArrayOop value, unsigned int hash) { 173 // Protect the table from concurrent access. Also note that this lock 174 // acts as a fence for _table, which could have been replaced by a new 175 // instance if the table was resized or rehashed. 176 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag); 177 return _table->lookup_or_add_inner(value, hash); 178 } 179 180 // Returns true if the hashtable is currently using a Java compatible 181 // hash function. 182 static bool use_java_hash() { 183 return _table->_hash_seed == 0; 184 } 185 186 static bool equals(typeArrayOop value1, typeArrayOop value2); 187 188 // Computes the hash code for the given character array, using the 189 // currently active hash function and hash seed. 190 static unsigned int hash_code(typeArrayOop value); 191 192 static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, 193 size_t partition_begin, 194 size_t partition_end, 195 uint worker_id); 196 197 public: 198 static void create(); 199 200 // Deduplicates the given String object, or adds its backing 201 // character array to the deduplication hashtable. 202 static void deduplicate(oop java_string, G1StringDedupStat& stat); 203 204 // If a table resize is needed, returns a newly allocated empty 205 // hashtable of the proper size. 206 static G1StringDedupTable* prepare_resize(); 207 208 // Installs a newly resized table as the currently active table 209 // and deletes the previously active table. 210 static void finish_resize(G1StringDedupTable* resized_table); 211 212 // If a table rehash is needed, returns a newly allocated empty 213 // hashtable and updates the hash seed. 214 static G1StringDedupTable* prepare_rehash(); 215 216 // Transfers rehashed entries from the currently active table into 217 // the new table. Installs the new table as the currently active table 218 // and deletes the previously active table. 219 static void finish_rehash(G1StringDedupTable* rehashed_table); 220 221 // If the table entry cache has grown too large, trim it down according to policy 222 static void trim_entry_cache(); 223 224 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id); 225 226 static void print_statistics(outputStream* st); 227 static void verify(); 228 }; 229 230 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP