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 // Define this to enable additional tracing probes.
  33 #undef TRACESPINNING
  34 
  35 class TaskQueueSetSuper;
  36 class TerminatorTerminator;
  37 
  38 /*
  39  * Provides a task termination protocol.
  40  *
  41  * This is an enhanced implementation of Google's OWST work stealing task termination
  42  * protocol (OWST stands for Optimized Work Stealing Threads).
  43  * 
  44  * It is described in the paper:
  45  * "Wessam Hassanein. 2016. Understanding and improving JVM GC work
  46  * stealing at the data center scale. In Proceedings of the 2016 ACM
  47  * SIGPLAN International Symposium on Memory Management (ISMM 2016). ACM,
  48  * New York, NY, USA, 46-54. DOI: https://doi.org/10.1145/2926697.2926706"
  49  *
  50  * Instead of a dedicated spin-master, our implementation will let spin-master relinquish
  51  * the role before it goes to sleep/wait, allowing newly arrived threads to compete for the role.
  52  * The intention of above enhancement is to reduce spin-master's latency on detecting new tasks
  53  * for stealing and termination condition.
  54  */
  55 class TaskTerminator : public CHeapObj<mtGC> {
  56   uint _n_threads;
  57   TaskQueueSetSuper* _queue_set;
  58 
  59   DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
  60   volatile uint _offered_termination;
  61   DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile uint));
  62 
  63 #ifdef ASSERT
  64   bool peek_in_queue_set();
  65 #endif
  66   void yield();
  67 
  68   Monitor*    _blocker;
  69   Thread*     _spin_master;
  70 
  71 #ifdef TRACESPINNING
  72   static uint _total_yields;
  73   static uint _total_spins;
  74   static uint _total_peeks;
  75 #endif
  76 
  77   // If we should exit current termination protocol
  78   bool exit_termination(size_t tasks, TerminatorTerminator* terminator);
  79 
  80   size_t tasks_in_queue_set() const;
  81 
  82   // Perform spin-master task.
  83   // Return true if termination condition is detected, otherwise return false
  84   bool do_spin_master_work(TerminatorTerminator* terminator);
  85 
  86   NONCOPYABLE(TaskTerminator);
  87 
  88 public:
  89   TaskTerminator(uint n_threads, TaskQueueSetSuper* queue_set);
  90   ~TaskTerminator();
  91 
  92   // The current thread has no work, and is ready to terminate if everyone
  93   // else is.  If returns "true", all threads are terminated.  If returns
  94   // "false", available work has been observed in one of the task queues,
  95   // so the global task is not complete.
  96   bool offer_termination() {
  97     return offer_termination(NULL);
  98   }
  99 
 100   // As above, but it also terminates if the should_exit_termination()
 101   // method of the terminator parameter returns true. If terminator is
 102   // NULL, then it is ignored.
 103   bool offer_termination(TerminatorTerminator* terminator);
 104 
 105   // Reset the terminator, so that it may be reused again.
 106   // The caller is responsible for ensuring that this is done
 107   // in an MT-safe manner, once the previous round of use of
 108   // the terminator is finished.
 109   void reset_for_reuse();
 110   // Same as above but the number of parallel threads is set to the
 111   // given number.
 112   void reset_for_reuse(uint n_threads);
 113 
 114 #ifdef TRACESPINNING
 115   static uint total_yields() { return _total_yields; }
 116   static uint total_spins() { return _total_spins; }
 117   static uint total_peeks() { return _total_peeks; }
 118   static void print_termination_counts();
 119 #endif
 120 };
 121 
 122 #endif // SHARE_GC_SHARED_TASKTERMINATOR_HPP