1/*
2 * Copyright (c) 2018, 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).
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24
25#ifndef SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP
26#define SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP
27
28#include "memory/allocation.hpp"
29#include "oops/oop.hpp"
30
31class StringDedupUnlinkOrOopsDoClosure;
32
33//
34// The deduplication queue acts as the communication channel between mark/evacuation
35// phase and the concurrent deduplication phase. Deduplication candidates
36// found during mark/evacuation are placed on this queue for later processing in the
37// deduplication thread. A queue entry is an oop pointing to a String object (as opposed
38// to entries in the deduplication hashtable which points to character arrays).
39//
40// While users of the queue treat it as a single queue, it is implemented as a set of
41// queues, one queue per GC worker thread, to allow lock-free and cache-friendly enqueue
42// operations by the GC workers.
43//
44// The oops in the queue are treated as weak pointers, meaning the objects they point to
45// can become unreachable and pruned (cleared) before being popped by the deduplication
46// thread.
47//
48// Pushing to the queue is thread safe (this relies on each thread using a unique worker
49// id). Popping from the queue is NOT thread safe and can only be done by the deduplication
50// thread outside a safepoint.
51//
52
53class StringDedupQueue : public CHeapObj<mtGC> {
54private:
55 static StringDedupQueue* _queue;
56 static volatile size_t _claimed_index;
57
58public:
59 template <typename Q>
60 static void create();
61
62 // Blocks and waits for the queue to become non-empty.
63 static inline void wait();
64
65 // Wakes up any thread blocked waiting for the queue to become non-empty.
66 static inline void cancel_wait();
67
68 // Pushes a deduplication candidate onto a specific GC worker queue.
69 static inline void push(uint worker_id, oop java_string);
70
71 // Pops a deduplication candidate from any queue, returns NULL if
72 // all queues are empty.
73 static inline oop pop();
74
75 static void unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl);
76
77 static void print_statistics();
78 static void verify();
79
80 // GC support
81 static void gc_prologue();
82 static void gc_epilogue();
83
84protected:
85 static StringDedupQueue* const queue();
86
87 // Queue interface.
88
89 // Blocks and waits for the queue to become non-empty.
90 virtual void wait_impl() = 0;
91
92 // Wakes up any thread blocked waiting for the queue to become non-empty.
93 virtual void cancel_wait_impl() = 0;
94
95 // Pushes a deduplication candidate onto a specific GC worker queue.
96 virtual void push_impl(uint worker_id, oop java_string) = 0;
97
98 // Pops a deduplication candidate from any queue, returns NULL if
99 // all queues are empty.
100 virtual oop pop_impl() = 0;
101
102 virtual void unlink_or_oops_do_impl(StringDedupUnlinkOrOopsDoClosure* cl, size_t queue) = 0;
103
104 virtual void print_statistics_impl() = 0;
105 virtual void verify_impl() = 0;
106
107 virtual size_t num_queues() const = 0;
108
109 static size_t claim();
110};
111
112#endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP
113