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annotate src/share/vm/runtime/biasedLocking.hpp @ 18096:ca6d25be853b jdk8u25-b13
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| author | jiangli |
|---|---|
| date | Tue, 12 Aug 2014 17:46:16 -0400 |
| parents | f95d63e2154a |
| children |
| rev | line source |
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| 0 | 1 /* |
| 1972 | 2 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved. |
| 0 | 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 | |
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| 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 * | |
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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20 * or visit www.oracle.com if you need additional information or have any |
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21 * questions. |
| 0 | 22 * |
| 23 */ | |
| 24 | |
| 1972 | 25 #ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP |
| 26 #define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP | |
| 27 | |
| 28 #include "runtime/handles.hpp" | |
| 29 #include "utilities/growableArray.hpp" | |
| 30 | |
| 0 | 31 // This class describes operations to implement Store-Free Biased |
| 32 // Locking. The high-level properties of the scheme are similar to | |
| 33 // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased | |
| 34 // locking mechanisms. The principal difference is in the handling of | |
| 35 // recursive locking which is how this technique achieves a more | |
| 36 // efficient fast path than these other schemes. | |
| 37 // | |
| 38 // The basic observation is that in HotSpot's current fast locking | |
| 39 // scheme, recursive locking (in the fast path) causes no update to | |
| 40 // the object header. The recursion is described simply by stack | |
| 41 // records containing a specific value (NULL). Only the last unlock by | |
| 42 // a given thread causes an update to the object header. | |
| 43 // | |
| 44 // This observation, coupled with the fact that HotSpot only compiles | |
| 45 // methods for which monitor matching is obeyed (and which therefore | |
| 46 // can not throw IllegalMonitorStateException), implies that we can | |
| 47 // completely eliminate modifications to the object header for | |
| 48 // recursive locking in compiled code, and perform similar recursion | |
| 49 // checks and throwing of IllegalMonitorStateException in the | |
| 50 // interpreter with little or no impact on the performance of the fast | |
| 51 // path. | |
| 52 // | |
| 53 // The basic algorithm is as follows (note, see below for more details | |
| 54 // and information). A pattern in the low three bits is reserved in | |
| 55 // the object header to indicate whether biasing of a given object's | |
| 56 // lock is currently being done or is allowed at all. If the bias | |
| 57 // pattern is present, the contents of the rest of the header are | |
| 58 // either the JavaThread* of the thread to which the lock is biased, | |
| 59 // or NULL, indicating that the lock is "anonymously biased". The | |
| 60 // first thread which locks an anonymously biased object biases the | |
| 61 // lock toward that thread. If another thread subsequently attempts to | |
| 62 // lock the same object, the bias is revoked. | |
| 63 // | |
| 64 // Because there are no updates to the object header at all during | |
| 65 // recursive locking while the lock is biased, the biased lock entry | |
| 66 // code is simply a test of the object header's value. If this test | |
| 67 // succeeds, the lock has been acquired by the thread. If this test | |
| 68 // fails, a bit test is done to see whether the bias bit is still | |
| 69 // set. If not, we fall back to HotSpot's original CAS-based locking | |
| 70 // scheme. If it is set, we attempt to CAS in a bias toward this | |
| 71 // thread. The latter operation is expected to be the rarest operation | |
| 72 // performed on these locks. We optimistically expect the biased lock | |
| 73 // entry to hit most of the time, and want the CAS-based fallthrough | |
| 74 // to occur quickly in the situations where the bias has been revoked. | |
| 75 // | |
| 76 // Revocation of the lock's bias is fairly straightforward. We want to | |
| 77 // restore the object's header and stack-based BasicObjectLocks and | |
| 78 // BasicLocks to the state they would have been in had the object been | |
| 79 // locked by HotSpot's usual fast locking scheme. To do this, we bring | |
| 80 // the system to a safepoint and walk the stack of the thread toward | |
| 81 // which the lock is biased. We find all of the lock records on the | |
| 82 // stack corresponding to this object, in particular the first / | |
| 83 // "highest" record. We fill in the highest lock record with the | |
| 84 // object's displaced header (which is a well-known value given that | |
| 85 // we don't maintain an identity hash nor age bits for the object | |
| 86 // while it's in the biased state) and all other lock records with 0, | |
| 87 // the value for recursive locks. When the safepoint is released, the | |
| 88 // formerly-biased thread and all other threads revert back to | |
| 89 // HotSpot's CAS-based locking. | |
| 90 // | |
| 91 // This scheme can not handle transfers of biases of single objects | |
| 92 // from thread to thread efficiently, but it can handle bulk transfers | |
| 93 // of such biases, which is a usage pattern showing up in some | |
| 94 // applications and benchmarks. We implement "bulk rebias" and "bulk | |
| 95 // revoke" operations using a "bias epoch" on a per-data-type basis. | |
| 96 // If too many bias revocations are occurring for a particular data | |
| 97 // type, the bias epoch for the data type is incremented at a | |
| 98 // safepoint, effectively meaning that all previous biases are | |
| 99 // invalid. The fast path locking case checks for an invalid epoch in | |
| 100 // the object header and attempts to rebias the object with a CAS if | |
| 101 // found, avoiding safepoints or bulk heap sweeps (the latter which | |
| 102 // was used in a prior version of this algorithm and did not scale | |
| 103 // well). If too many bias revocations persist, biasing is completely | |
| 104 // disabled for the data type by resetting the prototype header to the | |
| 105 // unbiased markOop. The fast-path locking code checks to see whether | |
| 106 // the instance's bias pattern differs from the prototype header's and | |
| 107 // causes the bias to be revoked without reaching a safepoint or, | |
| 108 // again, a bulk heap sweep. | |
| 109 | |
| 110 // Biased locking counters | |
| 111 class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC { | |
| 112 private: | |
| 113 int _total_entry_count; | |
| 114 int _biased_lock_entry_count; | |
| 115 int _anonymously_biased_lock_entry_count; | |
| 116 int _rebiased_lock_entry_count; | |
| 117 int _revoked_lock_entry_count; | |
| 118 int _fast_path_entry_count; | |
| 119 int _slow_path_entry_count; | |
| 120 | |
| 121 public: | |
| 122 BiasedLockingCounters() : | |
| 123 _total_entry_count(0), | |
| 124 _biased_lock_entry_count(0), | |
| 125 _anonymously_biased_lock_entry_count(0), | |
| 126 _rebiased_lock_entry_count(0), | |
| 127 _revoked_lock_entry_count(0), | |
| 128 _fast_path_entry_count(0), | |
| 129 _slow_path_entry_count(0) {} | |
| 130 | |
| 131 int slow_path_entry_count(); // Compute this field if necessary | |
| 132 | |
| 133 int* total_entry_count_addr() { return &_total_entry_count; } | |
| 134 int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; } | |
| 135 int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; } | |
| 136 int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; } | |
| 137 int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; } | |
| 138 int* fast_path_entry_count_addr() { return &_fast_path_entry_count; } | |
| 139 int* slow_path_entry_count_addr() { return &_slow_path_entry_count; } | |
| 140 | |
| 141 bool nonzero() { return _total_entry_count > 0; } | |
| 142 | |
| 143 void print_on(outputStream* st); | |
| 144 void print() { print_on(tty); } | |
| 145 }; | |
| 146 | |
| 147 | |
| 148 class BiasedLocking : AllStatic { | |
| 149 private: | |
| 150 static BiasedLockingCounters _counters; | |
| 151 | |
| 152 public: | |
| 153 static int* total_entry_count_addr(); | |
| 154 static int* biased_lock_entry_count_addr(); | |
| 155 static int* anonymously_biased_lock_entry_count_addr(); | |
| 156 static int* rebiased_lock_entry_count_addr(); | |
| 157 static int* revoked_lock_entry_count_addr(); | |
| 158 static int* fast_path_entry_count_addr(); | |
| 159 static int* slow_path_entry_count_addr(); | |
| 160 | |
| 161 enum Condition { | |
| 162 NOT_BIASED = 1, | |
| 163 BIAS_REVOKED = 2, | |
| 164 BIAS_REVOKED_AND_REBIASED = 3 | |
| 165 }; | |
| 166 | |
| 167 // This initialization routine should only be called once and | |
| 168 // schedules a PeriodicTask to turn on biased locking a few seconds | |
| 169 // into the VM run to avoid startup time regressions | |
| 170 static void init(); | |
| 171 | |
| 172 // This provides a global switch for leaving biased locking disabled | |
| 173 // for the first part of a run and enabling it later | |
| 174 static bool enabled(); | |
| 175 | |
| 176 // This should be called by JavaThreads to revoke the bias of an object | |
| 177 static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS); | |
| 178 | |
| 179 // These do not allow rebiasing; they are used by deoptimization to | |
| 180 // ensure that monitors on the stack can be migrated | |
| 181 static void revoke(GrowableArray<Handle>* objs); | |
| 182 static void revoke_at_safepoint(Handle obj); | |
| 183 static void revoke_at_safepoint(GrowableArray<Handle>* objs); | |
| 184 | |
| 185 static void print_counters() { _counters.print(); } | |
| 186 static BiasedLockingCounters* counters() { return &_counters; } | |
| 187 | |
| 188 // These routines are GC-related and should not be called by end | |
| 189 // users. GCs which do not do preservation of mark words do not need | |
| 190 // to call these routines. | |
| 191 static void preserve_marks(); | |
| 192 static void restore_marks(); | |
| 193 }; | |
| 1972 | 194 |
| 195 #endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP |