1 /* 2 * Copyright (c) 2014, 2019, 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_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP 26 #define SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_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 criteria is GC specific. 53 // 54 // Interned strings are a bit special. They are explicitly deduplicated just before 55 // being inserted into the StringTable (to avoid counteracting C2 optimizations done 56 // on string literals), then they also become deduplication candidates if they reach 57 // the deduplication age threshold or are evacuated to an old heap region. The second 58 // attempt to deduplicate such strings will be in vain, but we have no fast way of 59 // filtering them out. This has not shown to be a problem, as the number of interned 60 // strings is usually dwarfed by the number of normal (non-interned) strings. 61 // 62 // For additional information on string deduplication, please see JEP 192, 63 // http://openjdk.java.net/jeps/192 64 // 65 66 #include "gc/shared/stringdedup/stringDedupQueue.hpp" 67 #include "gc/shared/stringdedup/stringDedupStat.hpp" 68 #include "gc/shared/stringdedup/stringDedupTable.hpp" 69 #include "memory/allocation.hpp" 70 #include "runtime/thread.hpp" 71 72 // 73 // Main interface for interacting with string deduplication. 74 // 75 class StringDedup : public AllStatic { 76 private: 77 // Single state for checking if string deduplication is enabled. 78 static bool _enabled; 79 80 public: 81 // Returns true if string deduplication is enabled. is_enabled()82 static bool is_enabled() { 83 return _enabled; 84 } 85 86 // Stop the deduplication thread. 87 static void stop(); 88 89 // Immediately deduplicates the given String object, bypassing the 90 // the deduplication queue. 91 static void deduplicate(oop java_string); 92 93 static void parallel_unlink(StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id); 94 95 static void threads_do(ThreadClosure* tc); 96 static void print_worker_threads_on(outputStream* st); 97 static void verify(); 98 99 // GC support 100 static void gc_prologue(bool resize_and_rehash_table); 101 static void gc_epilogue(); 102 103 protected: 104 // Initialize string deduplication. 105 // Q: String Dedup Queue implementation 106 // S: String Dedup Stat implementation 107 template <typename Q, typename S> 108 static void initialize_impl(); 109 }; 110 111 // 112 // This closure encapsulates the closures needed when scanning 113 // the deduplication queue and table during the unlink_or_oops_do() operation. 114 // 115 class StringDedupUnlinkOrOopsDoClosure : public StackObj { 116 AlwaysTrueClosure _always_true; 117 DoNothingClosure _do_nothing; 118 BoolObjectClosure* _is_alive; 119 OopClosure* _keep_alive; 120 121 public: 122 StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive, 123 OopClosure* keep_alive); 124 is_alive(oop o)125 bool is_alive(oop o) { return _is_alive->do_object_b(o); } 126 keep_alive(oop * p)127 void keep_alive(oop* p) { _keep_alive->do_oop(p); } 128 }; 129 130 #endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP 131