Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 2005-2006 Sun Microsystems, Inc.  All Rights Reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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  * 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.

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  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

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 // 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();

 };