1 /* 2 * Copyright (c) 2000, 2015, 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_MEMORY_SHAREDHEAP_HPP 26 #define SHARE_VM_MEMORY_SHAREDHEAP_HPP 27 28 #include "gc_interface/collectedHeap.hpp" 29 30 // A "SharedHeap" is an implementation of a java heap for HotSpot. This 31 // is an abstract class: there may be many different kinds of heaps. This 32 // class defines the functions that a heap must implement, and contains 33 // infrastructure common to all heaps. 34 35 // Note on use of FlexibleWorkGang's for GC. 36 // There are three places where task completion is determined. 37 // In 38 // 1) ParallelTaskTerminator::offer_termination() where _n_threads 39 // must be set to the correct value so that count of workers that 40 // have offered termination will exactly match the number 41 // working on the task. Tasks such as those derived from GCTask 42 // use ParallelTaskTerminator's. Tasks that want load balancing 43 // by work stealing use this method to gauge completion. 44 // 2) SubTasksDone has a variable _n_threads that is used in 45 // all_tasks_completed() to determine completion. all_tasks_complete() 46 // counts the number of tasks that have been done and then reset 47 // the SubTasksDone so that it can be used again. When the number of 48 // tasks is set to the number of GC workers, then _n_threads must 49 // be set to the number of active GC workers. G1RootProcessor and 50 // GenCollectedHeap have SubTasksDone. 51 // 3) SequentialSubTasksDone has an _n_threads that is used in 52 // a way similar to SubTasksDone and has the same dependency on the 53 // number of active GC workers. CompactibleFreeListSpace and Space 54 // have SequentialSubTasksDone's. 55 // 56 // Examples of using SubTasksDone and SequentialSubTasksDone: 57 // G1RootProcessor and GenCollectedHeap::process_roots() use 58 // SubTasksDone* _process_strong_tasks to claim tasks for workers 59 // 60 // GenCollectedHeap::gen_process_roots() calls 61 // rem_set()->younger_refs_iterate() 62 // to scan the card table and which eventually calls down into 63 // CardTableModRefBS::par_non_clean_card_iterate_work(). This method 64 // uses SequentialSubTasksDone* _pst to claim tasks. 65 // Both SubTasksDone and SequentialSubTasksDone call their method 66 // all_tasks_completed() to count the number of GC workers that have 67 // finished their work. That logic is "when all the workers are 68 // finished the tasks are finished". 69 // 70 // The pattern that appears in the code is to set _n_threads 71 // to a value > 1 before a task that you would like executed in parallel 72 // and then to set it to 0 after that task has completed. A value of 73 // 0 is a "special" value in set_n_threads() which translates to 74 // setting _n_threads to 1. 75 // 76 // Some code uses _n_termination to decide if work should be done in 77 // parallel. The notorious possibly_parallel_oops_do() in threads.cpp 78 // is an example of such code. Look for variable "is_par" for other 79 // examples. 80 // 81 // The active_workers is not reset to 0 after a parallel phase. It's 82 // value may be used in later phases and in one instance at least 83 // (the parallel remark) it has to be used (the parallel remark depends 84 // on the partitioning done in the previous parallel scavenge). 85 86 class SharedHeap : public CollectedHeap { 87 friend class VMStructs; 88 89 protected: 90 // Full initialization is done in a concrete subtype's "initialize" 91 // function. 92 SharedHeap(); 93 94 public: 95 // Note, the below comment needs to be updated to reflect the changes 96 // introduced by JDK-8076225. This should be done as part of JDK-8076289. 97 // 98 //Some collectors will perform "process_strong_roots" in parallel. 99 // Such a call will involve claiming some fine-grained tasks, such as 100 // scanning of threads. To make this process simpler, we provide the 101 // "strong_roots_parity()" method. Collectors that start parallel tasks 102 // whose threads invoke "process_strong_roots" must 103 // call "change_strong_roots_parity" in sequential code starting such a 104 // task. (This also means that a parallel thread may only call 105 // process_strong_roots once.) 106 // 107 // For calls to process_roots by sequential code, the parity is 108 // updated automatically. 109 // 110 // The idea is that objects representing fine-grained tasks, such as 111 // threads, will contain a "parity" field. A task will is claimed in the 112 // current "process_roots" call only if its parity field is the 113 // same as the "strong_roots_parity"; task claiming is accomplished by 114 // updating the parity field to the strong_roots_parity with a CAS. 115 // 116 // If the client meats this spec, then strong_roots_parity() will have 117 // the following properties: 118 // a) to return a different value than was returned before the last 119 // call to change_strong_roots_parity, and 120 // c) to never return a distinguished value (zero) with which such 121 // task-claiming variables may be initialized, to indicate "never 122 // claimed". 123 public: 124 125 // Call these in sequential code around process_roots. 126 // strong_roots_prologue calls change_strong_roots_parity, if 127 // parallel tasks are enabled. 128 class StrongRootsScope : public MarkingCodeBlobClosure::MarkScope { 129 SharedHeap* _sh; 130 131 public: 132 StrongRootsScope(SharedHeap* heap, bool activate = true); 133 ~StrongRootsScope(); 134 }; 135 }; 136 137 #endif // SHARE_VM_MEMORY_SHAREDHEAP_HPP