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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 #include "precompiled.hpp"
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26 #include "classfile/altHashing.hpp"
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27 #include "classfile/javaClasses.hpp"
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28 #include "gc_implementation/g1/g1CollectedHeap.inline.hpp"
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29 #include "gc_implementation/g1/g1SATBCardTableModRefBS.hpp"
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30 #include "gc_implementation/g1/g1StringDedupTable.hpp"
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31 #include "memory/gcLocker.hpp"
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32 #include "memory/padded.inline.hpp"
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33 #include "oops/typeArrayOop.hpp"
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34 #include "runtime/mutexLocker.hpp"
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35
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36 //
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37 // Freelist in the deduplication table entry cache. Links table
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38 // entries together using their _next fields.
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39 //
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40 class G1StringDedupEntryFreeList : public CHeapObj<mtGC> {
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41 private:
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42 G1StringDedupEntry* _list;
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43 size_t _length;
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44
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45 public:
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46 G1StringDedupEntryFreeList() :
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47 _list(NULL),
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48 _length(0) {
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49 }
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50
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51 void add(G1StringDedupEntry* entry) {
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52 entry->set_next(_list);
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53 _list = entry;
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54 _length++;
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55 }
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56
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57 G1StringDedupEntry* remove() {
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58 G1StringDedupEntry* entry = _list;
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59 if (entry != NULL) {
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60 _list = entry->next();
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61 _length--;
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62 }
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63 return entry;
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64 }
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65
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66 size_t length() {
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67 return _length;
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68 }
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69 };
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70
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71 //
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72 // Cache of deduplication table entries. This cache provides fast allocation and
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73 // reuse of table entries to lower the pressure on the underlying allocator.
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74 // But more importantly, it provides fast/deferred freeing of table entries. This
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75 // is important because freeing of table entries is done during stop-the-world
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76 // phases and it is not uncommon for large number of entries to be freed at once.
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77 // Tables entries that are freed during these phases are placed onto a freelist in
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78 // the cache. The deduplication thread, which executes in a concurrent phase, will
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79 // later reuse or free the underlying memory for these entries.
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80 //
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81 // The cache allows for single-threaded allocations and multi-threaded frees.
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82 // Allocations are synchronized by StringDedupTable_lock as part of a table
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83 // modification.
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84 //
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85 class G1StringDedupEntryCache : public CHeapObj<mtGC> {
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86 private:
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87 // One freelist per GC worker to allow lock less freeing of
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88 // entries while doing a parallel scan of the table. Using
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89 // PaddedEnd to avoid false sharing.
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90 PaddedEnd<G1StringDedupEntryFreeList>* _lists;
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91 size_t _nlists;
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92
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93 public:
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94 G1StringDedupEntryCache();
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95 ~G1StringDedupEntryCache();
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96
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97 // Get a table entry from the cache freelist, or allocate a new
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98 // entry if the cache is empty.
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99 G1StringDedupEntry* alloc();
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100
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101 // Insert a table entry into the cache freelist.
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102 void free(G1StringDedupEntry* entry, uint worker_id);
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103
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104 // Returns current number of entries in the cache.
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105 size_t size();
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106
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107 // If the cache has grown above the given max size, trim it down
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108 // and deallocate the memory occupied by trimmed of entries.
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109 void trim(size_t max_size);
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110 };
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111
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112 G1StringDedupEntryCache::G1StringDedupEntryCache() {
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113 _nlists = MAX2(ParallelGCThreads, (size_t)1);
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114 _lists = PaddedArray<G1StringDedupEntryFreeList, mtGC>::create_unfreeable((uint)_nlists);
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115 }
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116
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117 G1StringDedupEntryCache::~G1StringDedupEntryCache() {
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118 ShouldNotReachHere();
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119 }
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120
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121 G1StringDedupEntry* G1StringDedupEntryCache::alloc() {
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122 for (size_t i = 0; i < _nlists; i++) {
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123 G1StringDedupEntry* entry = _lists[i].remove();
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124 if (entry != NULL) {
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125 return entry;
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126 }
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127 }
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128 return new G1StringDedupEntry();
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129 }
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130
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131 void G1StringDedupEntryCache::free(G1StringDedupEntry* entry, uint worker_id) {
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132 assert(entry->obj() != NULL, "Double free");
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133 assert(worker_id < _nlists, "Invalid worker id");
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134 entry->set_obj(NULL);
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135 entry->set_hash(0);
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136 _lists[worker_id].add(entry);
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137 }
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138
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139 size_t G1StringDedupEntryCache::size() {
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140 size_t size = 0;
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141 for (size_t i = 0; i < _nlists; i++) {
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142 size += _lists[i].length();
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143 }
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144 return size;
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145 }
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146
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147 void G1StringDedupEntryCache::trim(size_t max_size) {
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148 size_t cache_size = 0;
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149 for (size_t i = 0; i < _nlists; i++) {
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150 G1StringDedupEntryFreeList* list = &_lists[i];
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151 cache_size += list->length();
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152 while (cache_size > max_size) {
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153 G1StringDedupEntry* entry = list->remove();
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154 assert(entry != NULL, "Should not be null");
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155 cache_size--;
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156 delete entry;
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157 }
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158 }
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159 }
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160
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161 G1StringDedupTable* G1StringDedupTable::_table = NULL;
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162 G1StringDedupEntryCache* G1StringDedupTable::_entry_cache = NULL;
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163
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164 const size_t G1StringDedupTable::_min_size = (1 << 10); // 1024
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165 const size_t G1StringDedupTable::_max_size = (1 << 24); // 16777216
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166 const double G1StringDedupTable::_grow_load_factor = 2.0; // Grow table at 200% load
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167 const double G1StringDedupTable::_shrink_load_factor = _grow_load_factor / 3.0; // Shrink table at 67% load
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168 const double G1StringDedupTable::_max_cache_factor = 0.1; // Cache a maximum of 10% of the table size
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169 const uintx G1StringDedupTable::_rehash_multiple = 60; // Hash bucket has 60 times more collisions than expected
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170 const uintx G1StringDedupTable::_rehash_threshold = (uintx)(_rehash_multiple * _grow_load_factor);
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171
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172 uintx G1StringDedupTable::_entries_added = 0;
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173 uintx G1StringDedupTable::_entries_removed = 0;
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174 uintx G1StringDedupTable::_resize_count = 0;
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175 uintx G1StringDedupTable::_rehash_count = 0;
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176
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177 G1StringDedupTable::G1StringDedupTable(size_t size, jint hash_seed) :
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178 _size(size),
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179 _entries(0),
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180 _grow_threshold((uintx)(size * _grow_load_factor)),
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181 _shrink_threshold((uintx)(size * _shrink_load_factor)),
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182 _rehash_needed(false),
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183 _hash_seed(hash_seed) {
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184 assert(is_power_of_2(size), "Table size must be a power of 2");
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185 _buckets = NEW_C_HEAP_ARRAY(G1StringDedupEntry*, _size, mtGC);
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186 memset(_buckets, 0, _size * sizeof(G1StringDedupEntry*));
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187 }
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188
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189 G1StringDedupTable::~G1StringDedupTable() {
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190 FREE_C_HEAP_ARRAY(G1StringDedupEntry*, _buckets, mtGC);
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191 }
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192
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193 void G1StringDedupTable::create() {
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194 assert(_table == NULL, "One string deduplication table allowed");
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195 _entry_cache = new G1StringDedupEntryCache();
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196 _table = new G1StringDedupTable(_min_size);
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197 }
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198
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199 void G1StringDedupTable::add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list) {
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200 G1StringDedupEntry* entry = _entry_cache->alloc();
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201 entry->set_obj(value);
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202 entry->set_hash(hash);
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203 entry->set_next(*list);
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204 *list = entry;
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205 _entries++;
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206 }
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207
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208 void G1StringDedupTable::remove(G1StringDedupEntry** pentry, uint worker_id) {
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209 G1StringDedupEntry* entry = *pentry;
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210 *pentry = entry->next();
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211 _entry_cache->free(entry, worker_id);
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212 }
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213
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214 void G1StringDedupTable::transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest) {
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215 G1StringDedupEntry* entry = *pentry;
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216 *pentry = entry->next();
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217 unsigned int hash = entry->hash();
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218 size_t index = dest->hash_to_index(hash);
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219 G1StringDedupEntry** list = dest->bucket(index);
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220 entry->set_next(*list);
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221 *list = entry;
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222 }
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223
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224 bool G1StringDedupTable::equals(typeArrayOop value1, typeArrayOop value2) {
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225 return (value1 == value2 ||
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226 (value1->length() == value2->length() &&
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227 (!memcmp(value1->base(T_CHAR),
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228 value2->base(T_CHAR),
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229 value1->length() * sizeof(jchar)))));
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230 }
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231
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232 typeArrayOop G1StringDedupTable::lookup(typeArrayOop value, unsigned int hash,
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233 G1StringDedupEntry** list, uintx &count) {
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234 for (G1StringDedupEntry* entry = *list; entry != NULL; entry = entry->next()) {
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235 if (entry->hash() == hash) {
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236 typeArrayOop existing_value = entry->obj();
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237 if (equals(value, existing_value)) {
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238 // Match found
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239 return existing_value;
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240 }
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241 }
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242 count++;
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243 }
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244
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245 // Not found
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246 return NULL;
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247 }
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248
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249 typeArrayOop G1StringDedupTable::lookup_or_add_inner(typeArrayOop value, unsigned int hash) {
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250 size_t index = hash_to_index(hash);
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251 G1StringDedupEntry** list = bucket(index);
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252 uintx count = 0;
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253
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254 // Lookup in list
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255 typeArrayOop existing_value = lookup(value, hash, list, count);
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256
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257 // Check if rehash is needed
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258 if (count > _rehash_threshold) {
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259 _rehash_needed = true;
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260 }
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261
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262 if (existing_value == NULL) {
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263 // Not found, add new entry
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264 add(value, hash, list);
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265
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266 // Update statistics
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267 _entries_added++;
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268 }
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269
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270 return existing_value;
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271 }
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272
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273 unsigned int G1StringDedupTable::hash_code(typeArrayOop value) {
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274 unsigned int hash;
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275 int length = value->length();
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276 const jchar* data = (jchar*)value->base(T_CHAR);
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277
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278 if (use_java_hash()) {
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279 hash = java_lang_String::hash_code(data, length);
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280 } else {
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281 hash = AltHashing::murmur3_32(_table->_hash_seed, data, length);
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282 }
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283
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284 return hash;
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285 }
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286
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287 void G1StringDedupTable::deduplicate(oop java_string, G1StringDedupStat& stat) {
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288 assert(java_lang_String::is_instance(java_string), "Must be a string");
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289 No_Safepoint_Verifier nsv;
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290
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291 stat.inc_inspected();
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292
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293 typeArrayOop value = java_lang_String::value(java_string);
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294 if (value == NULL) {
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295 // String has no value
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296 stat.inc_skipped();
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297 return;
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298 }
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299
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300 unsigned int hash = 0;
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301
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302 if (use_java_hash()) {
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303 // Get hash code from cache
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304 hash = java_lang_String::hash(java_string);
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305 }
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306
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307 if (hash == 0) {
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308 // Compute hash
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309 hash = hash_code(value);
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310 stat.inc_hashed();
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311 }
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312
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313 if (use_java_hash() && hash != 0) {
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314 // Store hash code in cache
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315 java_lang_String::set_hash(java_string, hash);
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316 }
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317
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318 typeArrayOop existing_value = lookup_or_add(value, hash);
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319 if (existing_value == value) {
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320 // Same value, already known
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321 stat.inc_known();
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322 return;
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323 }
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324
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325 // Get size of value array
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326 uintx size_in_bytes = value->size() * HeapWordSize;
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327 stat.inc_new(size_in_bytes);
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328
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329 if (existing_value != NULL) {
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330 // Enqueue the reference to make sure it is kept alive. Concurrent mark might
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331 // otherwise declare it dead if there are no other strong references to this object.
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332 G1SATBCardTableModRefBS::enqueue(existing_value);
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333
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334 // Existing value found, deduplicate string
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335 java_lang_String::set_value(java_string, existing_value);
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336
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337 if (G1CollectedHeap::heap()->is_in_young(value)) {
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338 stat.inc_deduped_young(size_in_bytes);
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339 } else {
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340 stat.inc_deduped_old(size_in_bytes);
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341 }
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342 }
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343 }
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344
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345 G1StringDedupTable* G1StringDedupTable::prepare_resize() {
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346 size_t size = _table->_size;
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347
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348 // Check if the hashtable needs to be resized
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349 if (_table->_entries > _table->_grow_threshold) {
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350 // Grow table, double the size
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351 size *= 2;
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352 if (size > _max_size) {
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353 // Too big, don't resize
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354 return NULL;
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355 }
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356 } else if (_table->_entries < _table->_shrink_threshold) {
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357 // Shrink table, half the size
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358 size /= 2;
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359 if (size < _min_size) {
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360 // Too small, don't resize
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361 return NULL;
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362 }
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363 } else if (StringDeduplicationResizeALot) {
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364 // Force grow
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365 size *= 2;
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366 if (size > _max_size) {
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367 // Too big, force shrink instead
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368 size /= 4;
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369 }
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370 } else {
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371 // Resize not needed
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372 return NULL;
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373 }
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374
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375 // Update statistics
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376 _resize_count++;
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377
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378 // Allocate the new table. The new table will be populated by workers
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379 // calling unlink_or_oops_do() and finally installed by finish_resize().
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380 return new G1StringDedupTable(size, _table->_hash_seed);
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381 }
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382
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383 void G1StringDedupTable::finish_resize(G1StringDedupTable* resized_table) {
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384 assert(resized_table != NULL, "Invalid table");
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385
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386 resized_table->_entries = _table->_entries;
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387
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388 // Free old table
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389 delete _table;
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390
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391 // Install new table
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392 _table = resized_table;
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393 }
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394
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395 void G1StringDedupTable::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id) {
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396 // The table is divided into partitions to allow lock-less parallel processing by
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397 // multiple worker threads. A worker thread first claims a partition, which ensures
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398 // exclusive access to that part of the table, then continues to process it. To allow
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399 // shrinking of the table in parallel we also need to make sure that the same worker
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400 // thread processes all partitions where entries will hash to the same destination
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401 // partition. Since the table size is always a power of two and we always shrink by
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402 // dividing the table in half, we know that for a given partition there is only one
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403 // other partition whoes entries will hash to the same destination partition. That
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404 // other partition is always the sibling partition in the second half of the table.
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405 // For example, if the table is divided into 8 partitions, the sibling of partition 0
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406 // is partition 4, the sibling of partition 1 is partition 5, etc.
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407 size_t table_half = _table->_size / 2;
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408
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409 // Let each partition be one page worth of buckets
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410 size_t partition_size = MIN2(table_half, os::vm_page_size() / sizeof(G1StringDedupEntry*));
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411 assert(table_half % partition_size == 0, "Invalid partition size");
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412
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413 // Number of entries removed during the scan
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414 uintx removed = 0;
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415
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416 for (;;) {
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417 // Grab next partition to scan
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418 size_t partition_begin = cl->claim_table_partition(partition_size);
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419 size_t partition_end = partition_begin + partition_size;
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420 if (partition_begin >= table_half) {
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421 // End of table
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422 break;
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423 }
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424
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425 // Scan the partition followed by the sibling partition in the second half of the table
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426 removed += unlink_or_oops_do(cl, partition_begin, partition_end, worker_id);
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427 removed += unlink_or_oops_do(cl, table_half + partition_begin, table_half + partition_end, worker_id);
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428 }
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429
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430 // Delayed update avoid contention on the table lock
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431 if (removed > 0) {
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432 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
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433 _table->_entries -= removed;
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434 _entries_removed += removed;
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435 }
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436 }
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437
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438 uintx G1StringDedupTable::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,
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439 size_t partition_begin,
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440 size_t partition_end,
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441 uint worker_id) {
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442 uintx removed = 0;
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443 for (size_t bucket = partition_begin; bucket < partition_end; bucket++) {
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444 G1StringDedupEntry** entry = _table->bucket(bucket);
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445 while (*entry != NULL) {
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446 oop* p = (oop*)(*entry)->obj_addr();
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447 if (cl->is_alive(*p)) {
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448 cl->keep_alive(p);
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449 if (cl->is_resizing()) {
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450 // We are resizing the table, transfer entry to the new table
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451 _table->transfer(entry, cl->resized_table());
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452 } else {
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453 if (cl->is_rehashing()) {
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454 // We are rehashing the table, rehash the entry but keep it
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455 // in the table. We can't transfer entries into the new table
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456 // at this point since we don't have exclusive access to all
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457 // destination partitions. finish_rehash() will do a single
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458 // threaded transfer of all entries.
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459 typeArrayOop value = (typeArrayOop)*p;
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460 unsigned int hash = hash_code(value);
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461 (*entry)->set_hash(hash);
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462 }
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463
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464 // Move to next entry
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465 entry = (*entry)->next_addr();
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466 }
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467 } else {
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468 // Not alive, remove entry from table
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469 _table->remove(entry, worker_id);
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470 removed++;
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471 }
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472 }
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473 }
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474
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475 return removed;
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476 }
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477
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478 G1StringDedupTable* G1StringDedupTable::prepare_rehash() {
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479 if (!_table->_rehash_needed && !StringDeduplicationRehashALot) {
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480 // Rehash not needed
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481 return NULL;
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482 }
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483
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484 // Update statistics
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485 _rehash_count++;
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486
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487 // Compute new hash seed
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488 _table->_hash_seed = AltHashing::compute_seed();
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489
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490 // Allocate the new table, same size and hash seed
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491 return new G1StringDedupTable(_table->_size, _table->_hash_seed);
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492 }
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493
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494 void G1StringDedupTable::finish_rehash(G1StringDedupTable* rehashed_table) {
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495 assert(rehashed_table != NULL, "Invalid table");
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496
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497 // Move all newly rehashed entries into the correct buckets in the new table
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498 for (size_t bucket = 0; bucket < _table->_size; bucket++) {
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499 G1StringDedupEntry** entry = _table->bucket(bucket);
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500 while (*entry != NULL) {
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501 _table->transfer(entry, rehashed_table);
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502 }
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503 }
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504
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505 rehashed_table->_entries = _table->_entries;
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506
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507 // Free old table
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508 delete _table;
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509
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510 // Install new table
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511 _table = rehashed_table;
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512 }
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513
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514 void G1StringDedupTable::verify() {
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515 for (size_t bucket = 0; bucket < _table->_size; bucket++) {
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516 // Verify entries
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517 G1StringDedupEntry** entry = _table->bucket(bucket);
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518 while (*entry != NULL) {
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519 typeArrayOop value = (*entry)->obj();
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520 guarantee(value != NULL, "Object must not be NULL");
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521 guarantee(Universe::heap()->is_in_reserved(value), "Object must be on the heap");
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522 guarantee(!value->is_forwarded(), "Object must not be forwarded");
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523 guarantee(value->is_typeArray(), "Object must be a typeArrayOop");
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524 unsigned int hash = hash_code(value);
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525 guarantee((*entry)->hash() == hash, "Table entry has inorrect hash");
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526 guarantee(_table->hash_to_index(hash) == bucket, "Table entry has incorrect index");
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527 entry = (*entry)->next_addr();
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528 }
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529
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530 // Verify that we do not have entries with identical oops or identical arrays.
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531 // We only need to compare entries in the same bucket. If the same oop or an
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532 // identical array has been inserted more than once into different/incorrect
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533 // buckets the verification step above will catch that.
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534 G1StringDedupEntry** entry1 = _table->bucket(bucket);
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535 while (*entry1 != NULL) {
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536 typeArrayOop value1 = (*entry1)->obj();
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537 G1StringDedupEntry** entry2 = (*entry1)->next_addr();
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538 while (*entry2 != NULL) {
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539 typeArrayOop value2 = (*entry2)->obj();
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540 guarantee(!equals(value1, value2), "Table entries must not have identical arrays");
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541 entry2 = (*entry2)->next_addr();
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542 }
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543 entry1 = (*entry1)->next_addr();
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544 }
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545 }
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546 }
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547
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548 void G1StringDedupTable::trim_entry_cache() {
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549 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
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550 size_t max_cache_size = (size_t)(_table->_size * _max_cache_factor);
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551 _entry_cache->trim(max_cache_size);
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552 }
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553
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554 void G1StringDedupTable::print_statistics(outputStream* st) {
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555 st->print_cr(
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556 " [Table]\n"
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557 " [Memory Usage: "G1_STRDEDUP_BYTES_FORMAT_NS"]\n"
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558 " [Size: "SIZE_FORMAT", Min: "SIZE_FORMAT", Max: "SIZE_FORMAT"]\n"
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559 " [Entries: "UINTX_FORMAT", Load: "G1_STRDEDUP_PERCENT_FORMAT_NS", Cached: " UINTX_FORMAT ", Added: "UINTX_FORMAT", Removed: "UINTX_FORMAT"]\n"
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560 " [Resize Count: "UINTX_FORMAT", Shrink Threshold: "UINTX_FORMAT"("G1_STRDEDUP_PERCENT_FORMAT_NS"), Grow Threshold: "UINTX_FORMAT"("G1_STRDEDUP_PERCENT_FORMAT_NS")]\n"
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561 " [Rehash Count: "UINTX_FORMAT", Rehash Threshold: "UINTX_FORMAT", Hash Seed: 0x%x]\n"
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562 " [Age Threshold: "UINTX_FORMAT"]",
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563 G1_STRDEDUP_BYTES_PARAM(_table->_size * sizeof(G1StringDedupEntry*) + (_table->_entries + _entry_cache->size()) * sizeof(G1StringDedupEntry)),
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564 _table->_size, _min_size, _max_size,
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565 _table->_entries, (double)_table->_entries / (double)_table->_size * 100.0, _entry_cache->size(), _entries_added, _entries_removed,
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566 _resize_count, _table->_shrink_threshold, _shrink_load_factor * 100.0, _table->_grow_threshold, _grow_load_factor * 100.0,
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567 _rehash_count, _rehash_threshold, _table->_hash_seed,
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568 StringDeduplicationAgeThreshold);
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569 }
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