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hotspot/src/share/vm/runtime/biasedLocking.hpp

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+/*
+ * 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();
+};