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17764
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1 /*
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2 * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 *
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23 */
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24
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25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP
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26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP
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27
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28 #include "memory/allocation.hpp"
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29 #include "oops/oop.hpp"
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30 #include "utilities/stack.hpp"
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31
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32 class G1StringDedupUnlinkOrOopsDoClosure;
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33
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34 //
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35 // The deduplication queue acts as the communication channel between the stop-the-world
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36 // mark/evacuation phase and the concurrent deduplication phase. Deduplication candidates
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37 // found during mark/evacuation are placed on this queue for later processing in the
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38 // deduplication thread. A queue entry is an oop pointing to a String object (as opposed
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39 // to entries in the deduplication hashtable which points to character arrays).
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40 //
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41 // While users of the queue treat it as a single queue, it is implemented as a set of
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42 // queues, one queue per GC worker thread, to allow lock-free and cache-friendly enqueue
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43 // operations by the GC workers.
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44 //
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45 // The oops in the queue are treated as weak pointers, meaning the objects they point to
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46 // can become unreachable and pruned (cleared) before being popped by the deduplication
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47 // thread.
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48 //
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49 // Pushing to the queue is thread safe (this relies on each thread using a unique worker
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50 // id), but only allowed during a safepoint. Popping from the queue is NOT thread safe
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51 // and can only be done by the deduplication thread outside a safepoint.
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52 //
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53 // The StringDedupQueue_lock is only used for blocking and waking up the deduplication
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54 // thread in case the queue is empty or becomes non-empty, respectively. This lock does
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55 // not otherwise protect the queue content.
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56 //
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57 class G1StringDedupQueue : public CHeapObj<mtGC> {
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58 private:
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59 typedef Stack<oop, mtGC> G1StringDedupWorkerQueue;
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60
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61 static G1StringDedupQueue* _queue;
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62 static const size_t _max_size;
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63 static const size_t _max_cache_size;
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64
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65 G1StringDedupWorkerQueue* _queues;
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66 size_t _nqueues;
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67 size_t _cursor;
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68 volatile bool _empty;
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69
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70 // Statistics counter, only used for logging.
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71 uintx _dropped;
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72
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73 G1StringDedupQueue();
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74 ~G1StringDedupQueue();
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75
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76 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, size_t queue);
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77
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78 public:
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79 static void create();
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80
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81 // Blocks and waits for the queue to become non-empty.
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82 static void wait();
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83
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84 // Pushes a deduplication candidate onto a specific GC worker queue.
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85 static void push(uint worker_id, oop java_string);
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86
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87 // Pops a deduplication candidate from any queue, returns NULL if
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88 // all queues are empty.
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89 static oop pop();
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90
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91 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl);
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92
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93 static void print_statistics(outputStream* st);
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94 static void verify();
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95 };
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96
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97 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPQUEUE_HPP
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