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17764
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1 /*
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2 * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 *
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23 */
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24
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25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
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26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
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27
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28 #include "gc_implementation/g1/g1StringDedupStat.hpp"
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29 #include "runtime/mutexLocker.hpp"
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30
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31 class G1StringDedupEntryCache;
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32
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33 //
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34 // Table entry in the deduplication hashtable. Points weakly to the
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35 // character array. Can be chained in a linked list in case of hash
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36 // collisions or when placed in a freelist in the entry cache.
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37 //
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38 class G1StringDedupEntry : public CHeapObj<mtGC> {
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39 private:
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40 G1StringDedupEntry* _next;
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41 unsigned int _hash;
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42 typeArrayOop _obj;
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43
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44 public:
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45 G1StringDedupEntry() :
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46 _next(NULL),
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47 _hash(0),
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48 _obj(NULL) {
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49 }
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50
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51 G1StringDedupEntry* next() {
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52 return _next;
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53 }
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54
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55 G1StringDedupEntry** next_addr() {
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56 return &_next;
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57 }
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58
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59 void set_next(G1StringDedupEntry* next) {
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60 _next = next;
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61 }
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62
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63 unsigned int hash() {
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64 return _hash;
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65 }
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66
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67 void set_hash(unsigned int hash) {
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68 _hash = hash;
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69 }
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70
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71 typeArrayOop obj() {
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72 return _obj;
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73 }
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74
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75 typeArrayOop* obj_addr() {
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76 return &_obj;
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77 }
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78
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79 void set_obj(typeArrayOop obj) {
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80 _obj = obj;
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81 }
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82 };
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83
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84 //
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85 // The deduplication hashtable keeps track of all unique character arrays used
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86 // by String objects. Each table entry weakly points to an character array, allowing
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87 // otherwise unreachable character arrays to be declared dead and pruned from the
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88 // table.
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89 //
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90 // The table is dynamically resized to accommodate the current number of table entries.
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91 // The table has hash buckets with chains for hash collision. If the average chain
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92 // length goes above or below given thresholds the table grows or shrinks accordingly.
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93 //
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94 // The table is also dynamically rehashed (using a new hash seed) if it becomes severely
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95 // unbalanced, i.e., a hash chain is significantly longer than average.
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96 //
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97 // All access to the table is protected by the StringDedupTable_lock, except under
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98 // safepoints in which case GC workers are allowed to access a table partitions they
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99 // have claimed without first acquiring the lock. Note however, that this applies only
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100 // the table partition (i.e. a range of elements in _buckets), not other parts of the
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101 // table such as the _entries field, statistics counters, etc.
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102 //
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103 class G1StringDedupTable : public CHeapObj<mtGC> {
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104 private:
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105 // The currently active hashtable instance. Only modified when
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106 // the table is resizes or rehashed.
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107 static G1StringDedupTable* _table;
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108
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109 // Cache for reuse and fast alloc/free of table entries.
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110 static G1StringDedupEntryCache* _entry_cache;
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111
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112 G1StringDedupEntry** _buckets;
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113 size_t _size;
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114 uintx _entries;
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115 uintx _shrink_threshold;
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116 uintx _grow_threshold;
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117 bool _rehash_needed;
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118
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119 // The hash seed also dictates which hash function to use. A
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120 // zero hash seed means we will use the Java compatible hash
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121 // function (which doesn't use a seed), and a non-zero hash
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122 // seed means we use the murmur3 hash function.
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123 jint _hash_seed;
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124
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125 // Constants governing table resize/rehash/cache.
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126 static const size_t _min_size;
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127 static const size_t _max_size;
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128 static const double _grow_load_factor;
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129 static const double _shrink_load_factor;
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130 static const uintx _rehash_multiple;
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131 static const uintx _rehash_threshold;
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132 static const double _max_cache_factor;
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133
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134 // Table statistics, only used for logging.
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135 static uintx _entries_added;
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136 static uintx _entries_removed;
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137 static uintx _resize_count;
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138 static uintx _rehash_count;
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139
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140 G1StringDedupTable(size_t size, jint hash_seed = 0);
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141 ~G1StringDedupTable();
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142
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143 // Returns the hash bucket at the given index.
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144 G1StringDedupEntry** bucket(size_t index) {
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145 return _buckets + index;
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146 }
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147
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148 // Returns the hash bucket index for the given hash code.
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149 size_t hash_to_index(unsigned int hash) {
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150 return (size_t)hash & (_size - 1);
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151 }
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152
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153 // Adds a new table entry to the given hash bucket.
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154 void add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list);
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155
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156 // Removes the given table entry from the table.
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157 void remove(G1StringDedupEntry** pentry, uint worker_id);
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158
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159 // Transfers a table entry from the current table to the destination table.
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160 void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest);
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161
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162 // Returns an existing character array in the given hash bucket, or NULL
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163 // if no matching character array exists.
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164 typeArrayOop lookup(typeArrayOop value, unsigned int hash,
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165 G1StringDedupEntry** list, uintx &count);
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166
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167 // Returns an existing character array in the table, or inserts a new
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168 // table entry if no matching character array exists.
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169 typeArrayOop lookup_or_add_inner(typeArrayOop value, unsigned int hash);
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170
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171 // Thread safe lookup or add of table entry
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172 static typeArrayOop lookup_or_add(typeArrayOop value, unsigned int hash) {
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173 // Protect the table from concurrent access. Also note that this lock
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174 // acts as a fence for _table, which could have been replaced by a new
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175 // instance if the table was resized or rehashed.
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176 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
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177 return _table->lookup_or_add_inner(value, hash);
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178 }
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179
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180 // Returns true if the hashtable is currently using a Java compatible
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181 // hash function.
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182 static bool use_java_hash() {
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183 return _table->_hash_seed == 0;
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184 }
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185
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186 static bool equals(typeArrayOop value1, typeArrayOop value2);
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187
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188 // Computes the hash code for the given character array, using the
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189 // currently active hash function and hash seed.
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190 static unsigned int hash_code(typeArrayOop value);
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191
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192 static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,
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193 size_t partition_begin,
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194 size_t partition_end,
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195 uint worker_id);
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196
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197 public:
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198 static void create();
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199
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200 // Deduplicates the given String object, or adds its backing
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201 // character array to the deduplication hashtable.
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202 static void deduplicate(oop java_string, G1StringDedupStat& stat);
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203
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204 // If a table resize is needed, returns a newly allocated empty
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205 // hashtable of the proper size.
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206 static G1StringDedupTable* prepare_resize();
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207
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208 // Installs a newly resized table as the currently active table
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209 // and deletes the previously active table.
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210 static void finish_resize(G1StringDedupTable* resized_table);
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211
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212 // If a table rehash is needed, returns a newly allocated empty
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213 // hashtable and updates the hash seed.
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214 static G1StringDedupTable* prepare_rehash();
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215
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216 // Transfers rehashed entries from the currently active table into
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217 // the new table. Installs the new table as the currently active table
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218 // and deletes the previously active table.
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219 static void finish_rehash(G1StringDedupTable* rehashed_table);
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220
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221 // If the table entry cache has grown too large, trim it down according to policy
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222 static void trim_entry_cache();
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223
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224 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);
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225
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226 static void print_statistics(outputStream* st);
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227 static void verify();
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228 };
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229
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230 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
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