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_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 88 class OopClosure; 89 class BoolObjectClosure; 90 class ThreadClosure; 91 class outputStream; 92 class G1StringDedupTable; 93 class G1GCPhaseTimes; 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, G1GCPhaseTimes* phase_times = NULL); 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 return (size_t)Atomic::add_ptr(1, &_next_queue) - 1; 180 } 181 182 // Atomically claims the next available table partition for exclusive 183 // access by the current thread. Returns the table bucket number where 184 // the claimed partition starts. 185 size_t claim_table_partition(size_t partition_size) { 186 return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size; 187 } 188 189 // Applies and returns the result from the is_alive closure, or 190 // returns true if no such closure was provided. 191 bool is_alive(oop o) { 192 if (_is_alive != NULL) { 193 return _is_alive->do_object_b(o); 194 } 195 return true; 196 } 197 198 // Applies the keep_alive closure, or does nothing if no such 199 // closure was provided. 200 void keep_alive(oop* p) { 201 if (_keep_alive != NULL) { 202 _keep_alive->do_oop(p); 203 } 204 } 205 }; 206 207 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP