1 /*
2 * Copyright (c) 2014, 2015, 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_G1STRINGDEDUPQUEUE_HPP
26 #define SHARE_VM_GC_G1_G1STRINGDEDUPQUEUE_HPP
27
28 #include "memory/allocation.hpp"
29 #include "oops/oop.hpp"
30 #include "utilities/stack.hpp"
31
32 class G1StringDedupUnlinkOrOopsDoClosure;
33
34 //
35 // The deduplication queue acts as the communication channel between the stop-the-world
36 // mark/evacuation phase and the concurrent deduplication phase. Deduplication candidates
37 // found during mark/evacuation are placed on this queue for later processing in the
38 // deduplication thread. A queue entry is an oop pointing to a String object (as opposed
39 // to entries in the deduplication hashtable which points to character arrays).
40 //
41 // While users of the queue treat it as a single queue, it is implemented as a set of
42 // queues, one queue per GC worker thread, to allow lock-free and cache-friendly enqueue
43 // operations by the GC workers.
44 //
45 // The oops in the queue are treated as weak pointers, meaning the objects they point to
46 // can become unreachable and pruned (cleared) before being popped by the deduplication
47 // thread.
48 //
49 // Pushing to the queue is thread safe (this relies on each thread using a unique worker
50 // id), but only allowed during a safepoint. Popping from the queue is NOT thread safe
51 // and can only be done by the deduplication thread outside a safepoint.
52 //
53 // The StringDedupQueue_lock is only used for blocking and waking up the deduplication
54 // thread in case the queue is empty or becomes non-empty, respectively. This lock does
55 // not otherwise protect the queue content.
56 //
57 class G1StringDedupQueue : public CHeapObj<mtGC> {
58 private:
59 typedef Stack<oop, mtGC> G1StringDedupWorkerQueue;
60
61 static G1StringDedupQueue* _queue;
62 static const size_t _max_size;
63 static const size_t _max_cache_size;
64
65 G1StringDedupWorkerQueue* _queues;
66 size_t _nqueues;
67 size_t _cursor;
68 bool _cancel;
69 volatile bool _empty;
70
71 // Statistics counter, only used for logging.
72 uintx _dropped;
73
74 G1StringDedupQueue();
75 ~G1StringDedupQueue();
76
77 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, size_t queue);
78
79 public:
80 static void create();
81
82 // Blocks and waits for the queue to become non-empty.
83 static void wait();
84
85 // Wakes up any thread blocked waiting for the queue to become non-empty.
86 static void cancel_wait();
87
88 // Pushes a deduplication candidate onto a specific GC worker queue.
89 static void push(uint worker_id, oop java_string);
90
91 // Pops a deduplication candidate from any queue, returns NULL if
92 // all queues are empty.
93 static oop pop();
94
95 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl);
96
97 static void print_statistics(outputStream* st);
98 static void verify();
99 };
100
101 #endif // SHARE_VM_GC_G1_G1STRINGDEDUPQUEUE_HPP