jdk8-mips64-public/hotspot: src/share/vm/runtime/biasedLocking.hpp@595c0f60d50d (annotated)

src/share/vm/runtime/biasedLocking.hpp@595c0f60d50d (annotated)

src/share/vm/runtime/biasedLocking.hpp

Tue, 18 Mar 2014 19:07:22 +0100

author: pliden
date: Tue, 18 Mar 2014 19:07:22 +0100
changeset 6413: 595c0f60d50d
parent 2314: f95d63e2154a
child 6876: 710a3c8b516e
permissions: -rw-r--r--

8029075: String deduplication in G1
Summary: Implementation of JEP 192, http://openjdk.java.net/jeps/192
Reviewed-by: brutisso, tschatzl, coleenp

 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  * or visit www.oracle.com if you need additional information or have any
  * questions.
  *
  */
 #ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP
 #define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP
 #include "runtime/handles.hpp"
 #include "utilities/growableArray.hpp"
 // 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();
 };
 #endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/runtime/biasedLocking.hpp@595c0f60d50d (annotated)

src/share/vm/runtime/biasedLocking.hpp