Mercurial > hg > truffle
diff src/share/vm/runtime/biasedLocking.hpp @ 0:a61af66fc99e jdk7-b24
Initial load
author | duke |
---|---|
date | Sat, 01 Dec 2007 00:00:00 +0000 |
parents | |
children | c18cbe5936b8 |
line wrap: on
line diff
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/src/share/vm/runtime/biasedLocking.hpp Sat Dec 01 00:00:00 2007 +0000 @@ -0,0 +1,187 @@ +/* + * Copyright 2005-2006 Sun Microsystems, Inc. All Rights Reserved. + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. + * + * This code is free software; you can redistribute it and/or modify it + * under the terms of the GNU General Public License version 2 only, as + * published by the Free Software Foundation. + * + * This code is distributed in the hope that it will be useful, but WITHOUT + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License + * version 2 for more details (a copy is included in the LICENSE file that + * accompanied this code). + * + * You should have received a copy of the GNU General Public License version + * 2 along with this work; if not, write to the Free Software Foundation, + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. + * + * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, + * CA 95054 USA or visit www.sun.com if you need additional information or + * have any questions. + * + */ + +// This class describes operations to implement Store-Free Biased +// Locking. The high-level properties of the scheme are similar to +// IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased +// locking mechanisms. The principal difference is in the handling of +// recursive locking which is how this technique achieves a more +// efficient fast path than these other schemes. +// +// The basic observation is that in HotSpot's current fast locking +// scheme, recursive locking (in the fast path) causes no update to +// the object header. The recursion is described simply by stack +// records containing a specific value (NULL). Only the last unlock by +// a given thread causes an update to the object header. +// +// This observation, coupled with the fact that HotSpot only compiles +// methods for which monitor matching is obeyed (and which therefore +// can not throw IllegalMonitorStateException), implies that we can +// completely eliminate modifications to the object header for +// recursive locking in compiled code, and perform similar recursion +// checks and throwing of IllegalMonitorStateException in the +// interpreter with little or no impact on the performance of the fast +// path. +// +// The basic algorithm is as follows (note, see below for more details +// and information). A pattern in the low three bits is reserved in +// the object header to indicate whether biasing of a given object's +// lock is currently being done or is allowed at all. If the bias +// pattern is present, the contents of the rest of the header are +// either the JavaThread* of the thread to which the lock is biased, +// or NULL, indicating that the lock is "anonymously biased". The +// first thread which locks an anonymously biased object biases the +// lock toward that thread. If another thread subsequently attempts to +// lock the same object, the bias is revoked. +// +// Because there are no updates to the object header at all during +// recursive locking while the lock is biased, the biased lock entry +// code is simply a test of the object header's value. If this test +// succeeds, the lock has been acquired by the thread. If this test +// fails, a bit test is done to see whether the bias bit is still +// set. If not, we fall back to HotSpot's original CAS-based locking +// scheme. If it is set, we attempt to CAS in a bias toward this +// thread. The latter operation is expected to be the rarest operation +// performed on these locks. We optimistically expect the biased lock +// entry to hit most of the time, and want the CAS-based fallthrough +// to occur quickly in the situations where the bias has been revoked. +// +// Revocation of the lock's bias is fairly straightforward. We want to +// restore the object's header and stack-based BasicObjectLocks and +// BasicLocks to the state they would have been in had the object been +// locked by HotSpot's usual fast locking scheme. To do this, we bring +// the system to a safepoint and walk the stack of the thread toward +// which the lock is biased. We find all of the lock records on the +// stack corresponding to this object, in particular the first / +// "highest" record. We fill in the highest lock record with the +// object's displaced header (which is a well-known value given that +// we don't maintain an identity hash nor age bits for the object +// while it's in the biased state) and all other lock records with 0, +// the value for recursive locks. When the safepoint is released, the +// formerly-biased thread and all other threads revert back to +// HotSpot's CAS-based locking. +// +// This scheme can not handle transfers of biases of single objects +// from thread to thread efficiently, but it can handle bulk transfers +// of such biases, which is a usage pattern showing up in some +// applications and benchmarks. We implement "bulk rebias" and "bulk +// revoke" operations using a "bias epoch" on a per-data-type basis. +// If too many bias revocations are occurring for a particular data +// type, the bias epoch for the data type is incremented at a +// safepoint, effectively meaning that all previous biases are +// invalid. The fast path locking case checks for an invalid epoch in +// the object header and attempts to rebias the object with a CAS if +// found, avoiding safepoints or bulk heap sweeps (the latter which +// was used in a prior version of this algorithm and did not scale +// well). If too many bias revocations persist, biasing is completely +// disabled for the data type by resetting the prototype header to the +// unbiased markOop. The fast-path locking code checks to see whether +// the instance's bias pattern differs from the prototype header's and +// causes the bias to be revoked without reaching a safepoint or, +// again, a bulk heap sweep. + +// Biased locking counters +class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC { + private: + int _total_entry_count; + int _biased_lock_entry_count; + int _anonymously_biased_lock_entry_count; + int _rebiased_lock_entry_count; + int _revoked_lock_entry_count; + int _fast_path_entry_count; + int _slow_path_entry_count; + + public: + BiasedLockingCounters() : + _total_entry_count(0), + _biased_lock_entry_count(0), + _anonymously_biased_lock_entry_count(0), + _rebiased_lock_entry_count(0), + _revoked_lock_entry_count(0), + _fast_path_entry_count(0), + _slow_path_entry_count(0) {} + + int slow_path_entry_count(); // Compute this field if necessary + + int* total_entry_count_addr() { return &_total_entry_count; } + int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; } + int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; } + int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; } + int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; } + int* fast_path_entry_count_addr() { return &_fast_path_entry_count; } + int* slow_path_entry_count_addr() { return &_slow_path_entry_count; } + + bool nonzero() { return _total_entry_count > 0; } + + void print_on(outputStream* st); + void print() { print_on(tty); } +}; + + +class BiasedLocking : AllStatic { +private: + static BiasedLockingCounters _counters; + +public: + static int* total_entry_count_addr(); + static int* biased_lock_entry_count_addr(); + static int* anonymously_biased_lock_entry_count_addr(); + static int* rebiased_lock_entry_count_addr(); + static int* revoked_lock_entry_count_addr(); + static int* fast_path_entry_count_addr(); + static int* slow_path_entry_count_addr(); + + enum Condition { + NOT_BIASED = 1, + BIAS_REVOKED = 2, + BIAS_REVOKED_AND_REBIASED = 3 + }; + + // This initialization routine should only be called once and + // schedules a PeriodicTask to turn on biased locking a few seconds + // into the VM run to avoid startup time regressions + static void init(); + + // This provides a global switch for leaving biased locking disabled + // for the first part of a run and enabling it later + static bool enabled(); + + // This should be called by JavaThreads to revoke the bias of an object + static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS); + + // These do not allow rebiasing; they are used by deoptimization to + // ensure that monitors on the stack can be migrated + static void revoke(GrowableArray<Handle>* objs); + static void revoke_at_safepoint(Handle obj); + static void revoke_at_safepoint(GrowableArray<Handle>* objs); + + static void print_counters() { _counters.print(); } + static BiasedLockingCounters* counters() { return &_counters; } + + // These routines are GC-related and should not be called by end + // users. GCs which do not do preservation of mark words do not need + // to call these routines. + static void preserve_marks(); + static void restore_marks(); +};