Wed, 17 Sep 2008 19:59:35 +0400
Merge
duke@435 | 1 | /* |
duke@435 | 2 | * Copyright 2005-2006 Sun Microsystems, Inc. All Rights Reserved. |
duke@435 | 3 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
duke@435 | 4 | * |
duke@435 | 5 | * This code is free software; you can redistribute it and/or modify it |
duke@435 | 6 | * under the terms of the GNU General Public License version 2 only, as |
duke@435 | 7 | * published by the Free Software Foundation. |
duke@435 | 8 | * |
duke@435 | 9 | * This code is distributed in the hope that it will be useful, but WITHOUT |
duke@435 | 10 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
duke@435 | 11 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
duke@435 | 12 | * version 2 for more details (a copy is included in the LICENSE file that |
duke@435 | 13 | * accompanied this code). |
duke@435 | 14 | * |
duke@435 | 15 | * You should have received a copy of the GNU General Public License version |
duke@435 | 16 | * 2 along with this work; if not, write to the Free Software Foundation, |
duke@435 | 17 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
duke@435 | 18 | * |
duke@435 | 19 | * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
duke@435 | 20 | * CA 95054 USA or visit www.sun.com if you need additional information or |
duke@435 | 21 | * have any questions. |
duke@435 | 22 | * |
duke@435 | 23 | */ |
duke@435 | 24 | |
duke@435 | 25 | // This class describes operations to implement Store-Free Biased |
duke@435 | 26 | // Locking. The high-level properties of the scheme are similar to |
duke@435 | 27 | // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased |
duke@435 | 28 | // locking mechanisms. The principal difference is in the handling of |
duke@435 | 29 | // recursive locking which is how this technique achieves a more |
duke@435 | 30 | // efficient fast path than these other schemes. |
duke@435 | 31 | // |
duke@435 | 32 | // The basic observation is that in HotSpot's current fast locking |
duke@435 | 33 | // scheme, recursive locking (in the fast path) causes no update to |
duke@435 | 34 | // the object header. The recursion is described simply by stack |
duke@435 | 35 | // records containing a specific value (NULL). Only the last unlock by |
duke@435 | 36 | // a given thread causes an update to the object header. |
duke@435 | 37 | // |
duke@435 | 38 | // This observation, coupled with the fact that HotSpot only compiles |
duke@435 | 39 | // methods for which monitor matching is obeyed (and which therefore |
duke@435 | 40 | // can not throw IllegalMonitorStateException), implies that we can |
duke@435 | 41 | // completely eliminate modifications to the object header for |
duke@435 | 42 | // recursive locking in compiled code, and perform similar recursion |
duke@435 | 43 | // checks and throwing of IllegalMonitorStateException in the |
duke@435 | 44 | // interpreter with little or no impact on the performance of the fast |
duke@435 | 45 | // path. |
duke@435 | 46 | // |
duke@435 | 47 | // The basic algorithm is as follows (note, see below for more details |
duke@435 | 48 | // and information). A pattern in the low three bits is reserved in |
duke@435 | 49 | // the object header to indicate whether biasing of a given object's |
duke@435 | 50 | // lock is currently being done or is allowed at all. If the bias |
duke@435 | 51 | // pattern is present, the contents of the rest of the header are |
duke@435 | 52 | // either the JavaThread* of the thread to which the lock is biased, |
duke@435 | 53 | // or NULL, indicating that the lock is "anonymously biased". The |
duke@435 | 54 | // first thread which locks an anonymously biased object biases the |
duke@435 | 55 | // lock toward that thread. If another thread subsequently attempts to |
duke@435 | 56 | // lock the same object, the bias is revoked. |
duke@435 | 57 | // |
duke@435 | 58 | // Because there are no updates to the object header at all during |
duke@435 | 59 | // recursive locking while the lock is biased, the biased lock entry |
duke@435 | 60 | // code is simply a test of the object header's value. If this test |
duke@435 | 61 | // succeeds, the lock has been acquired by the thread. If this test |
duke@435 | 62 | // fails, a bit test is done to see whether the bias bit is still |
duke@435 | 63 | // set. If not, we fall back to HotSpot's original CAS-based locking |
duke@435 | 64 | // scheme. If it is set, we attempt to CAS in a bias toward this |
duke@435 | 65 | // thread. The latter operation is expected to be the rarest operation |
duke@435 | 66 | // performed on these locks. We optimistically expect the biased lock |
duke@435 | 67 | // entry to hit most of the time, and want the CAS-based fallthrough |
duke@435 | 68 | // to occur quickly in the situations where the bias has been revoked. |
duke@435 | 69 | // |
duke@435 | 70 | // Revocation of the lock's bias is fairly straightforward. We want to |
duke@435 | 71 | // restore the object's header and stack-based BasicObjectLocks and |
duke@435 | 72 | // BasicLocks to the state they would have been in had the object been |
duke@435 | 73 | // locked by HotSpot's usual fast locking scheme. To do this, we bring |
duke@435 | 74 | // the system to a safepoint and walk the stack of the thread toward |
duke@435 | 75 | // which the lock is biased. We find all of the lock records on the |
duke@435 | 76 | // stack corresponding to this object, in particular the first / |
duke@435 | 77 | // "highest" record. We fill in the highest lock record with the |
duke@435 | 78 | // object's displaced header (which is a well-known value given that |
duke@435 | 79 | // we don't maintain an identity hash nor age bits for the object |
duke@435 | 80 | // while it's in the biased state) and all other lock records with 0, |
duke@435 | 81 | // the value for recursive locks. When the safepoint is released, the |
duke@435 | 82 | // formerly-biased thread and all other threads revert back to |
duke@435 | 83 | // HotSpot's CAS-based locking. |
duke@435 | 84 | // |
duke@435 | 85 | // This scheme can not handle transfers of biases of single objects |
duke@435 | 86 | // from thread to thread efficiently, but it can handle bulk transfers |
duke@435 | 87 | // of such biases, which is a usage pattern showing up in some |
duke@435 | 88 | // applications and benchmarks. We implement "bulk rebias" and "bulk |
duke@435 | 89 | // revoke" operations using a "bias epoch" on a per-data-type basis. |
duke@435 | 90 | // If too many bias revocations are occurring for a particular data |
duke@435 | 91 | // type, the bias epoch for the data type is incremented at a |
duke@435 | 92 | // safepoint, effectively meaning that all previous biases are |
duke@435 | 93 | // invalid. The fast path locking case checks for an invalid epoch in |
duke@435 | 94 | // the object header and attempts to rebias the object with a CAS if |
duke@435 | 95 | // found, avoiding safepoints or bulk heap sweeps (the latter which |
duke@435 | 96 | // was used in a prior version of this algorithm and did not scale |
duke@435 | 97 | // well). If too many bias revocations persist, biasing is completely |
duke@435 | 98 | // disabled for the data type by resetting the prototype header to the |
duke@435 | 99 | // unbiased markOop. The fast-path locking code checks to see whether |
duke@435 | 100 | // the instance's bias pattern differs from the prototype header's and |
duke@435 | 101 | // causes the bias to be revoked without reaching a safepoint or, |
duke@435 | 102 | // again, a bulk heap sweep. |
duke@435 | 103 | |
duke@435 | 104 | // Biased locking counters |
duke@435 | 105 | class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC { |
duke@435 | 106 | private: |
duke@435 | 107 | int _total_entry_count; |
duke@435 | 108 | int _biased_lock_entry_count; |
duke@435 | 109 | int _anonymously_biased_lock_entry_count; |
duke@435 | 110 | int _rebiased_lock_entry_count; |
duke@435 | 111 | int _revoked_lock_entry_count; |
duke@435 | 112 | int _fast_path_entry_count; |
duke@435 | 113 | int _slow_path_entry_count; |
duke@435 | 114 | |
duke@435 | 115 | public: |
duke@435 | 116 | BiasedLockingCounters() : |
duke@435 | 117 | _total_entry_count(0), |
duke@435 | 118 | _biased_lock_entry_count(0), |
duke@435 | 119 | _anonymously_biased_lock_entry_count(0), |
duke@435 | 120 | _rebiased_lock_entry_count(0), |
duke@435 | 121 | _revoked_lock_entry_count(0), |
duke@435 | 122 | _fast_path_entry_count(0), |
duke@435 | 123 | _slow_path_entry_count(0) {} |
duke@435 | 124 | |
duke@435 | 125 | int slow_path_entry_count(); // Compute this field if necessary |
duke@435 | 126 | |
duke@435 | 127 | int* total_entry_count_addr() { return &_total_entry_count; } |
duke@435 | 128 | int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; } |
duke@435 | 129 | int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; } |
duke@435 | 130 | int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; } |
duke@435 | 131 | int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; } |
duke@435 | 132 | int* fast_path_entry_count_addr() { return &_fast_path_entry_count; } |
duke@435 | 133 | int* slow_path_entry_count_addr() { return &_slow_path_entry_count; } |
duke@435 | 134 | |
duke@435 | 135 | bool nonzero() { return _total_entry_count > 0; } |
duke@435 | 136 | |
duke@435 | 137 | void print_on(outputStream* st); |
duke@435 | 138 | void print() { print_on(tty); } |
duke@435 | 139 | }; |
duke@435 | 140 | |
duke@435 | 141 | |
duke@435 | 142 | class BiasedLocking : AllStatic { |
duke@435 | 143 | private: |
duke@435 | 144 | static BiasedLockingCounters _counters; |
duke@435 | 145 | |
duke@435 | 146 | public: |
duke@435 | 147 | static int* total_entry_count_addr(); |
duke@435 | 148 | static int* biased_lock_entry_count_addr(); |
duke@435 | 149 | static int* anonymously_biased_lock_entry_count_addr(); |
duke@435 | 150 | static int* rebiased_lock_entry_count_addr(); |
duke@435 | 151 | static int* revoked_lock_entry_count_addr(); |
duke@435 | 152 | static int* fast_path_entry_count_addr(); |
duke@435 | 153 | static int* slow_path_entry_count_addr(); |
duke@435 | 154 | |
duke@435 | 155 | enum Condition { |
duke@435 | 156 | NOT_BIASED = 1, |
duke@435 | 157 | BIAS_REVOKED = 2, |
duke@435 | 158 | BIAS_REVOKED_AND_REBIASED = 3 |
duke@435 | 159 | }; |
duke@435 | 160 | |
duke@435 | 161 | // This initialization routine should only be called once and |
duke@435 | 162 | // schedules a PeriodicTask to turn on biased locking a few seconds |
duke@435 | 163 | // into the VM run to avoid startup time regressions |
duke@435 | 164 | static void init(); |
duke@435 | 165 | |
duke@435 | 166 | // This provides a global switch for leaving biased locking disabled |
duke@435 | 167 | // for the first part of a run and enabling it later |
duke@435 | 168 | static bool enabled(); |
duke@435 | 169 | |
duke@435 | 170 | // This should be called by JavaThreads to revoke the bias of an object |
duke@435 | 171 | static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS); |
duke@435 | 172 | |
duke@435 | 173 | // These do not allow rebiasing; they are used by deoptimization to |
duke@435 | 174 | // ensure that monitors on the stack can be migrated |
duke@435 | 175 | static void revoke(GrowableArray<Handle>* objs); |
duke@435 | 176 | static void revoke_at_safepoint(Handle obj); |
duke@435 | 177 | static void revoke_at_safepoint(GrowableArray<Handle>* objs); |
duke@435 | 178 | |
duke@435 | 179 | static void print_counters() { _counters.print(); } |
duke@435 | 180 | static BiasedLockingCounters* counters() { return &_counters; } |
duke@435 | 181 | |
duke@435 | 182 | // These routines are GC-related and should not be called by end |
duke@435 | 183 | // users. GCs which do not do preservation of mark words do not need |
duke@435 | 184 | // to call these routines. |
duke@435 | 185 | static void preserve_marks(); |
duke@435 | 186 | static void restore_marks(); |
duke@435 | 187 | }; |