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.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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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 *
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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 class ThreadClosure;
73
74 //
75 // Main interface for interacting with string deduplication.
76 //
77 class StringDedup : public AllStatic {
78 private:
79 // Single state for checking if string deduplication is enabled.
80 static bool _enabled;
81
82 public:
83 // Returns true if string deduplication is enabled.
84 static bool is_enabled() {
85 return _enabled;
86 }
87
88 // Stop the deduplication thread.
89 static void stop();
90
91 // Immediately deduplicates the given String object, bypassing the
92 // the deduplication queue.
93 static void deduplicate(oop java_string);
94
95 static void parallel_unlink(StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id);
96
97 static void threads_do(ThreadClosure* tc);
98 static void print_worker_threads_on(outputStream* st);
99 static void verify();
100
101 // GC support
102 static void gc_prologue(bool resize_and_rehash_table);
103 static void gc_epilogue();
104
105 protected:
106 // Initialize string deduplication.
107 // Q: String Dedup Queue implementation
108 // S: String Dedup Stat implementation
109 template <typename Q, typename S>
110 static void initialize_impl();
111 };
112
113 //
114 // This closure encapsulates the closures needed when scanning
115 // the deduplication queue and table during the unlink_or_oops_do() operation.
116 //
117 class StringDedupUnlinkOrOopsDoClosure : public StackObj {
118 AlwaysTrueClosure _always_true;
119 DoNothingClosure _do_nothing;
120 BoolObjectClosure* _is_alive;
121 OopClosure* _keep_alive;
122
123 public:
124 StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
125 OopClosure* keep_alive);
126
127 bool is_alive(oop o) { return _is_alive->do_object_b(o); }
128
129 void keep_alive(oop* p) { _keep_alive->do_oop(p); }
130 };
131
132 #endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP