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.
   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_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP
  26 #define SHARE_VM_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP
  27 
  28 #include "memory/allocation.hpp"
  29 #include "oops/oop.hpp"
  30 
  31 class 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 
  53 class StringDedupQueue : public CHeapObj<mtGC> {
  54 private:
  55   static StringDedupQueue*   _queue;
  56   static volatile size_t     _claimed_index;
  57 
  58 public:
  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 
  84 protected:
  85   static StringDedupQueue* const queue();
  86 
  87   // Queue interface.
  88 
  89   // Blocks and waits for the queue to become non-empty.
  90   virtual void queue_wait() = 0;
  91 
  92   // Wakes up any thread blocked waiting for the queue to become non-empty.
  93   virtual void queue_cancel_wait() = 0;
  94 
  95   // Pushes a deduplication candidate onto a specific GC worker queue.
  96   virtual void queue_push(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 queue_pop() = 0;
 101 
 102   virtual void queue_unlink_or_oops_do(StringDedupUnlinkOrOopsDoClosure* cl, size_t queue) = 0;
 103 
 104   virtual void queue_print_statistics() = 0;
 105   virtual void queue_verify() = 0;
 106 
 107   virtual size_t num_queues() const = 0;
 108 
 109   static size_t claim();
 110 };
 111 
 112 #endif // SHARE_VM_GC_SHARED_STRINGDEDUP_STRINGDEDUPQUEUE_HPP