1 /*
2 * Copyright (c) 2018, 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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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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13 * accompanied this code).
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24
25 #ifndef SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
26 #define SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP
27
28 #include "gc/shared/oopStorage.hpp"
29 #include "utilities/macros.hpp"
30
31 //////////////////////////////////////////////////////////////////////////////
32 // Support for parallel and optionally concurrent state iteration.
33 //
34 // Parallel iteration is for the exclusive use of the GC. Other iteration
35 // clients must use serial iteration.
36 //
37 // Concurrent Iteration
38 //
39 // Iteration involves the _active_array (an ActiveArray), which contains all
40 // of the blocks owned by a storage object.
41 //
42 // At most one concurrent ParState can exist at a time for a given storage
43 // object.
44 //
45 // A concurrent ParState sets the associated storage's
46 // _concurrent_iteration_active flag true when the state is constructed, and
47 // sets it false when the state is destroyed. These assignments are made with
48 // _active_mutex locked. Meanwhile, empty block deletion is not done while
49 // _concurrent_iteration_active is true. The flag check and the dependent
50 // removal of a block from the _active_array is performed with _active_mutex
51 // locked. This prevents concurrent iteration and empty block deletion from
52 // interfering with with each other.
53 //
54 // Both allocate() and delete_empty_blocks_concurrent() lock the
55 // _allocate_mutex while performing their respective list and array
56 // manipulations, preventing them from interfering with each other.
57 //
58 // When allocate() creates a new block, it is added to the end of the
59 // _active_array. Then _active_array's _block_count is incremented to account
60 // for the new block. When concurrent iteration is started (by a parallel
61 // worker thread calling the state's iterate() function), the current
62 // _active_array and its _block_count are captured for use by the iteration,
63 // with iteration processing all blocks in that array up to that block count.
64 //
65 // As a result, the sequence over which concurrent iteration operates is
66 // stable. However, once the iteration is started, later allocations may add
67 // blocks to the end of the array that won't be examined by the iteration.
68 // An allocation may even require expansion of the array, so the iteration is
69 // no longer processing the current array, but rather the previous one.
70 // And while the sequence is stable, concurrent allocate() and release()
71 // operations may change the set of allocated entries in a block at any time
72 // during the iteration.
73 //
74 // As a result, a concurrent iteration handler must accept that some
75 // allocations and releases that occur after the iteration started will not be
76 // seen by the iteration. Further, some may overlap examination by the
77 // iteration. To help with this, allocate() and release() have an invariant
78 // that an entry's value must be NULL when it is not in use.
79 //
80 // An in-progress delete_empty_blocks_concurrent() operation can contend with
81 // the start of a concurrent iteration over the _active_mutex. Since both are
82 // under GC control, that potential contention can be eliminated by never
83 // scheduling both operations to run at the same time.
84 //
85 // ParState<concurrent, is_const>
86 // concurrent must be true if iteration is concurrent with the
87 // mutator, false if iteration is at a safepoint.
88 //
89 // is_const must be true if the iteration is over a constant storage
90 // object, false if the iteration may modify the storage object.
91 //
92 // ParState([const] OopStorage* storage)
93 // Construct an object for managing an iteration over storage. For a
94 // concurrent ParState, empty block deletion for the associated storage
95 // is inhibited for the life of the ParState. There can be no more
96 // than one live concurrent ParState at a time for a given storage object.
97 //
98 // template<typename F> void iterate(F f)
99 // Repeatedly claims a block from the associated storage that has
100 // not been processed by this iteration (possibly by other threads),
101 // and applies f to each entry in the claimed block. Assume p is of
102 // type const oop* or oop*, according to is_const. Then f(p) must be
103 // a valid expression whose value is ignored. Concurrent uses must
104 // be prepared for an entry's value to change at any time, due to
105 // mutator activity.
106 //
107 // template<typename Closure> void oops_do(Closure* cl)
108 // Wrapper around iterate, providing an adaptation layer allowing
109 // the use of OopClosures and similar objects for iteration. Assume
110 // p is of type const oop* or oop*, according to is_const. Then
111 // cl->do_oop(p) must be a valid expression whose value is ignored.
112 // Concurrent uses must be prepared for the entry's value to change
113 // at any time, due to mutator activity.
114 //
115 // Optional operations, provided only if !concurrent && !is_const.
116 // These are not provided when is_const, because the storage object
117 // may be modified by the iteration infrastructure, even if the
118 // provided closure doesn't modify the storage object. These are not
119 // provided when concurrent because any pre-filtering behavior by the
120 // iteration infrastructure is inappropriate for concurrent iteration;
121 // modifications of the storage by the mutator could result in the
122 // pre-filtering being applied (successfully or not) to objects that
123 // are unrelated to what the closure finds in the entry.
124 //
125 // template<typename Closure> void weak_oops_do(Closure* cl)
126 // template<typename IsAliveClosure, typename Closure>
127 // void weak_oops_do(IsAliveClosure* is_alive, Closure* cl)
128 // Wrappers around iterate, providing an adaptation layer allowing
129 // the use of is-alive closures and OopClosures for iteration.
130 // Assume p is of type oop*. Then
131 //
132 // - cl->do_oop(p) must be a valid expression whose value is ignored.
133 //
134 // - is_alive->do_object_b(*p) must be a valid expression whose value
135 // is convertible to bool.
136 //
137 // If *p == NULL then neither is_alive nor cl will be invoked for p.
138 // If is_alive->do_object_b(*p) is false, then cl will not be
139 // invoked on p.
140
141 class OopStorage::BasicParState {
142 const OopStorage* _storage;
143 ActiveArray* _active_array;
144 size_t _block_count;
145 volatile size_t _next_block;
146 uint _estimated_thread_count;
147 bool _concurrent;
148
149 // Noncopyable.
150 BasicParState(const BasicParState&);
151 BasicParState& operator=(const BasicParState&);
152
153 struct IterationData;
154
155 void update_iteration_state(bool value);
156 bool claim_next_segment(IterationData* data);
157 bool finish_iteration(const IterationData* data) const;
158
159 // Wrapper for iteration handler; ignore handler result and return true.
160 template<typename F> class AlwaysTrueFn;
161
162 public:
163 BasicParState(const OopStorage* storage,
164 uint estimated_thread_count,
165 bool concurrent);
166 ~BasicParState();
167
168 template<bool is_const, typename F> void iterate(F f);
169
170 static uint default_estimated_thread_count(bool concurrent);
171 };
172
173 template<bool concurrent, bool is_const>
174 class OopStorage::ParState {
175 BasicParState _basic_state;
176
177 typedef typename Conditional<is_const,
178 const OopStorage*,
179 OopStorage*>::type StoragePtr;
180
181 public:
182 ParState(StoragePtr storage,
183 uint estimated_thread_count = BasicParState::default_estimated_thread_count(concurrent)) :
184 _basic_state(storage, estimated_thread_count, concurrent)
185 {}
186
187 template<typename F> void iterate(F f);
188 template<typename Closure> void oops_do(Closure* cl);
189 };
190
191 template<>
192 class OopStorage::ParState<false, false> {
193 BasicParState _basic_state;
194
195 public:
196 ParState(OopStorage* storage,
197 uint estimated_thread_count = BasicParState::default_estimated_thread_count(false)) :
198 _basic_state(storage, estimated_thread_count, false)
199 {}
200
201 template<typename F> void iterate(F f);
202 template<typename Closure> void oops_do(Closure* cl);
203 template<typename Closure> void weak_oops_do(Closure* cl);
204 template<typename IsAliveClosure, typename Closure>
205 void weak_oops_do(IsAliveClosure* is_alive, Closure* cl);
206 };
207
208 #endif // SHARE_GC_SHARED_OOPSTORAGEPARSTATE_HPP