1 /*
   2  * Copyright (c) 2018, 2020, 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 "memory/padded.hpp"
  30 #include "runtime/mutex.hpp"
  31 
  32 class TaskQueueSetSuper;
  33 class TerminatorTerminator;
  34 class Thread;
  35 
  36 /*
  37  * Provides a task termination protocol.
  38  *
  39  * This is an enhanced implementation of Google's OWST work stealing task termination
  40  * protocol (OWST stands for Optimized Work Stealing Threads).
  41  *
  42  * It is described in the paper:
  43  * "Wessam Hassanein. 2016. Understanding and improving JVM GC work
  44  * stealing at the data center scale. In Proceedings of the 2016 ACM
  45  * SIGPLAN International Symposium on Memory Management (ISMM 2016). ACM,
  46  * New York, NY, USA, 46-54. DOI: https://doi.org/10.1145/2926697.2926706"
  47  *
  48  * Instead of a dedicated spin-master, our implementation will let spin-master relinquish
  49  * the role before it goes to sleep/wait, allowing newly arrived threads to compete for the role.
  50  * The intention of above enhancement is to reduce spin-master's latency on detecting new tasks
  51  * for stealing and termination condition.
  52  */
  53 class TaskTerminator : public CHeapObj<mtGC> {
  54   struct DelayContext {
  55     uint _yield_count;
  56     // Number of hard spin loops done since last yield
  57     uint _hard_spin_count;
  58     // Number of iterations in the hard spin loop.
  59     uint _hard_spin_limit;
  60 
  61     DelayContext();
  62   };
  63 
  64   uint _n_threads;
  65   TaskQueueSetSuper* _queue_set;
  66 
  67   DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);
  68   volatile uint _offered_termination;
  69   DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile uint));
  70 
  71   void assert_queue_set_empty() const NOT_DEBUG_RETURN;
  72 
  73   Monitor _blocker;
  74   Thread* _spin_master;
  75 
  76   // Prepare for return from offer_termination. Gives up the spin master token
  77   // and wakes up up to tasks threads waiting on _blocker (the default value
  78   // means to wake up everyone).
  79   void prepare_for_return(Thread* this_thread, size_t tasks = SIZE_MAX);
  80 
  81   // If we should exit current termination protocol
  82   bool exit_termination(size_t tasks, TerminatorTerminator* terminator);
  83 
  84   size_t tasks_in_queue_set() const;
  85 
  86   // Perform one iteration of spin-master work.
  87   bool do_delay_step(DelayContext& delay_context);
  88 
  89   NONCOPYABLE(TaskTerminator);
  90 
  91 public:
  92   TaskTerminator(uint n_threads, TaskQueueSetSuper* queue_set);
  93   ~TaskTerminator();
  94 
  95   // The current thread has no work, and is ready to terminate if everyone
  96   // else is.  If returns "true", all threads are terminated.  If returns
  97   // "false", available work has been observed in one of the task queues,
  98   // so the global task is not complete.
  99   bool offer_termination() {
 100     return offer_termination(NULL);
 101   }
 102 
 103   // As above, but it also terminates if the should_exit_termination()
 104   // method of the terminator parameter returns true. If terminator is
 105   // NULL, then it is ignored.
 106   bool offer_termination(TerminatorTerminator* terminator);
 107 
 108   // Reset the terminator, so that it may be reused again.
 109   // The caller is responsible for ensuring that this is done
 110   // in an MT-safe manner, once the previous round of use of
 111   // the terminator is finished.
 112   void reset_for_reuse();
 113   // Same as above but the number of parallel threads is set to the
 114   // given number.
 115   void reset_for_reuse(uint n_threads);
 116 };
 117 
 118 #endif // SHARE_GC_SHARED_TASKTERMINATOR_HPP