1 /* 2 * Copyright (c) 2014, 2017, 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_G1_G1STRINGDEDUP_HPP 26 #define SHARE_VM_GC_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 "gc/shared/stringdedup/stringDedup.hpp" 86 #include "memory/allocation.hpp" 87 #include "oops/oop.hpp" 88 89 class OopClosure; 90 class BoolObjectClosure; 91 class G1GCPhaseTimes; 92 class G1StringDedupUnlinkOrOopsDoClosure; 93 94 // 95 // Candidate selection 96 // 97 // An object is considered a deduplication candidate if all of the following 98 // statements are true: 99 // 100 // - The object is an instance of java.lang.String 101 // 102 // - The object is being evacuated from a young heap region 103 // 104 // - The object is being evacuated to a young/survivor heap region and the 105 // object's age is equal to the deduplication age threshold 106 // 107 // or 108 // 109 // The object is being evacuated to an old heap region and the object's age is 110 // less than the deduplication age threshold 111 // 112 // Once an string object has been promoted to an old region, or its age is higher 113 // than the deduplication age threshold, is will never become a candidate again. 114 // This approach avoids making the same object a candidate more than once. 115 116 117 // 118 // G1 interface for interacting with string deduplication. 119 // 120 class G1StringDedup : public StringDedup { 121 private: 122 123 // Candidate selection policies, returns true if the given object is 124 // candidate for string deduplication. 125 static bool is_candidate_from_mark(oop obj); 126 static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj); 127 128 public: 129 // Initialize string deduplication. 130 static void initialize(); 131 132 // Enqueues a deduplication candidate for later processing by the deduplication 133 // thread. Before enqueuing, these functions apply the appropriate candidate 134 // selection policy to filters out non-candidates. 135 static void enqueue_from_mark(oop java_string, uint worker_id); 136 static void enqueue_from_evacuation(bool from_young, bool to_young, 137 unsigned int queue, oop java_string); 138 139 static void oops_do(OopClosure* keep_alive); 140 static void parallel_unlink(G1StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id); 141 static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive, 142 bool allow_resize_and_rehash, G1GCPhaseTimes* phase_times = NULL); 143 }; 144 145 // 146 // This closure encapsulates the state and the closures needed when scanning 147 // the deduplication queue and table during the unlink_or_oops_do() operation. 148 // A single instance of this closure is created and then shared by all worker 149 // threads participating in the scan. 150 // 151 class G1StringDedupUnlinkOrOopsDoClosure : public StringDedupUnlinkOrOopsDoClosure { 152 public: 153 G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive, 154 OopClosure* keep_alive, 155 bool allow_resize_and_rehash) : 156 StringDedupUnlinkOrOopsDoClosure(is_alive, keep_alive) { 157 if (G1StringDedup::is_enabled()) { 158 G1StringDedup::gc_prologue(allow_resize_and_rehash); 159 } 160 } 161 162 ~G1StringDedupUnlinkOrOopsDoClosure() { 163 if (G1StringDedup::is_enabled()) { 164 G1StringDedup::gc_epilogue(); 165 } 166 } 167 }; 168 169 #endif // SHARE_VM_GC_G1_G1STRINGDEDUP_HPP