1 /*
   2  * Copyright (c) 2018, 2019, Red Hat, Inc. All rights reserved.
   3  * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
   4  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   5  *
   6  * This code is free software; you can redistribute it and/or modify it
   7  * under the terms of the GNU General Public License version 2 only, as
   8  * published by the Free Software Foundation.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  *
  24  */
  25 #ifndef SHARE_GC_SHARED_TASKTERMINATOR_HPP
  26 #define SHARE_GC_SHARED_TASKTERMINATOR_HPP
  27 
  28 #include "memory/allocation.hpp"
  29 #include "runtime/mutex.hpp"
  30 #include "runtime/thread.hpp"
  31 
  32 class TaskQueueSetSuper;
  33 class TerminatorTerminator;
  34 
  35 /*
  36  * Provides a task termination protocol.
  37  *
  38  * This is an enhanced implementation of Google's OWST work stealing task termination
  39  * protocol (OWST stands for Optimized Work Stealing Threads).
  40  *
  41  * It is described in the paper:
  42  * "Wessam Hassanein. 2016. Understanding and improving JVM GC work
  43  * stealing at the data center scale. In Proceedings of the 2016 ACM
  44  * SIGPLAN International Symposium on Memory Management (ISMM 2016). ACM,
  45  * New York, NY, USA, 46-54. DOI: https://doi.org/10.1145/2926697.2926706"
  46  *
  47  * Instead of a dedicated spin-master, our implementation will let spin-master relinquish
  48  * the role before it goes to sleep/wait, allowing newly arrived threads to compete for the role.
  49  * The intention of above enhancement is to reduce spin-master's latency on detecting new tasks
  50  * for stealing and termination condition.
  51  */
  52 class TaskTerminator : public CHeapObj<mtGC> {
  53   uint _n_threads;
  54   TaskQueueSetSuper* _queue_set;
  55 
  56   DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
  57   volatile uint _offered_termination;
  58   DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile uint));
  59 
  60   void assert_queue_set_empty() const NOT_DEBUG_RETURN;
  61 
  62   void yield();
  63 
  64   Monitor*    _blocker;
  65   Thread*     _spin_master;
  66 
  67   // If we should exit current termination protocol
  68   bool exit_termination(size_t tasks, TerminatorTerminator* terminator);
  69 
  70   size_t tasks_in_queue_set() const;
  71 
  72   // Perform spin-master task.
  73   // Return true if termination condition is detected, otherwise return false
  74   bool do_spin_master_work(TerminatorTerminator* terminator);
  75 
  76   NONCOPYABLE(TaskTerminator);
  77 
  78 public:
  79   TaskTerminator(uint n_threads, TaskQueueSetSuper* queue_set);
  80   ~TaskTerminator();
  81 
  82   // The current thread has no work, and is ready to terminate if everyone
  83   // else is.  If returns "true", all threads are terminated.  If returns
  84   // "false", available work has been observed in one of the task queues,
  85   // so the global task is not complete.
  86   bool offer_termination() {
  87     return offer_termination(NULL);
  88   }
  89 
  90   // As above, but it also terminates if the should_exit_termination()
  91   // method of the terminator parameter returns true. If terminator is
  92   // NULL, then it is ignored.
  93   bool offer_termination(TerminatorTerminator* terminator);
  94 
  95   // Reset the terminator, so that it may be reused again.
  96   // The caller is responsible for ensuring that this is done
  97   // in an MT-safe manner, once the previous round of use of
  98   // the terminator is finished.
  99   void reset_for_reuse();
 100   // Same as above but the number of parallel threads is set to the
 101   // given number.
 102   void reset_for_reuse(uint n_threads);
 103 };
 104 
 105 #endif // SHARE_GC_SHARED_TASKTERMINATOR_HPP