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