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24
25#ifndef SHARE_RUNTIME_BIASEDLOCKING_HPP
26#define SHARE_RUNTIME_BIASEDLOCKING_HPP
27
28#include "runtime/handles.hpp"
29#include "utilities/growableArray.hpp"
30
31// This class describes operations to implement Store-Free Biased
32// Locking. The high-level properties of the scheme are similar to
33// IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased
34// locking mechanisms. The principal difference is in the handling of
35// recursive locking which is how this technique achieves a more
36// efficient fast path than these other schemes.
37//
38// The basic observation is that in HotSpot's current fast locking
39// scheme, recursive locking (in the fast path) causes no update to
40// the object header. The recursion is described simply by stack
41// records containing a specific value (NULL). Only the last unlock by
42// a given thread causes an update to the object header.
43//
44// This observation, coupled with the fact that HotSpot only compiles
45// methods for which monitor matching is obeyed (and which therefore
46// can not throw IllegalMonitorStateException), implies that we can
47// completely eliminate modifications to the object header for
48// recursive locking in compiled code, and perform similar recursion
49// checks and throwing of IllegalMonitorStateException in the
50// interpreter with little or no impact on the performance of the fast
51// path.
52//
53// The basic algorithm is as follows (note, see below for more details
54// and information). A pattern in the low three bits is reserved in
55// the object header to indicate whether biasing of a given object's
56// lock is currently being done or is allowed at all. If the bias
57// pattern is present, the contents of the rest of the header are
58// either the JavaThread* of the thread to which the lock is biased,
59// or NULL, indicating that the lock is "anonymously biased". The
60// first thread which locks an anonymously biased object biases the
61// lock toward that thread. If another thread subsequently attempts to
62// lock the same object, the bias is revoked.
63//
64// Because there are no updates to the object header at all during
65// recursive locking while the lock is biased, the biased lock entry
66// code is simply a test of the object header's value. If this test
67// succeeds, the lock has been acquired by the thread. If this test
68// fails, a bit test is done to see whether the bias bit is still
69// set. If not, we fall back to HotSpot's original CAS-based locking
70// scheme. If it is set, we attempt to CAS in a bias toward this
71// thread. The latter operation is expected to be the rarest operation
72// performed on these locks. We optimistically expect the biased lock
73// entry to hit most of the time, and want the CAS-based fallthrough
74// to occur quickly in the situations where the bias has been revoked.
75//
76// Revocation of the lock's bias is fairly straightforward. We want to
77// restore the object's header and stack-based BasicObjectLocks and
78// BasicLocks to the state they would have been in had the object been
79// locked by HotSpot's usual fast locking scheme. To do this, we bring
80// the system to a safepoint and walk the stack of the thread toward
81// which the lock is biased. We find all of the lock records on the
82// stack corresponding to this object, in particular the first /
83// "highest" record. We fill in the highest lock record with the
84// object's displaced header (which is a well-known value given that
85// we don't maintain an identity hash nor age bits for the object
86// while it's in the biased state) and all other lock records with 0,
87// the value for recursive locks. When the safepoint is released, the
88// formerly-biased thread and all other threads revert back to
89// HotSpot's CAS-based locking.
90//
91// This scheme can not handle transfers of biases of single objects
92// from thread to thread efficiently, but it can handle bulk transfers
93// of such biases, which is a usage pattern showing up in some
94// applications and benchmarks. We implement "bulk rebias" and "bulk
95// revoke" operations using a "bias epoch" on a per-data-type basis.
96// If too many bias revocations are occurring for a particular data
97// type, the bias epoch for the data type is incremented at a
98// safepoint, effectively meaning that all previous biases are
99// invalid. The fast path locking case checks for an invalid epoch in
100// the object header and attempts to rebias the object with a CAS if
101// found, avoiding safepoints or bulk heap sweeps (the latter which
102// was used in a prior version of this algorithm and did not scale
103// well). If too many bias revocations persist, biasing is completely
104// disabled for the data type by resetting the prototype header to the
105// unbiased markOop. The fast-path locking code checks to see whether
106// the instance's bias pattern differs from the prototype header's and
107// causes the bias to be revoked without reaching a safepoint or,
108// again, a bulk heap sweep.
109
110// Biased locking counters
111class BiasedLockingCounters {
112 private:
113 int _total_entry_count;
114 int _biased_lock_entry_count;
115 int _anonymously_biased_lock_entry_count;
116 int _rebiased_lock_entry_count;
117 int _revoked_lock_entry_count;
118 int _fast_path_entry_count;
119 int _slow_path_entry_count;
120
121 public:
122 BiasedLockingCounters() :
123 _total_entry_count(0),
124 _biased_lock_entry_count(0),
125 _anonymously_biased_lock_entry_count(0),
126 _rebiased_lock_entry_count(0),
127 _revoked_lock_entry_count(0),
128 _fast_path_entry_count(0),
129 _slow_path_entry_count(0) {}
130
131 int slow_path_entry_count() const; // Compute this field if necessary
132
133 int* total_entry_count_addr() { return &_total_entry_count; }
134 int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; }
135 int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; }
136 int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; }
137 int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; }
138 int* fast_path_entry_count_addr() { return &_fast_path_entry_count; }
139 int* slow_path_entry_count_addr() { return &_slow_path_entry_count; }
140
141 bool nonzero() { return _total_entry_count > 0; }
142
143 void print_on(outputStream* st) const;
144 void print() const;
145};
146
147
148class BiasedLocking : AllStatic {
149private:
150 static BiasedLockingCounters _counters;
151
152public:
153 static int* total_entry_count_addr();
154 static int* biased_lock_entry_count_addr();
155 static int* anonymously_biased_lock_entry_count_addr();
156 static int* rebiased_lock_entry_count_addr();
157 static int* revoked_lock_entry_count_addr();
158 static int* fast_path_entry_count_addr();
159 static int* slow_path_entry_count_addr();
160
161 enum Condition {
162 NOT_BIASED = 1,
163 BIAS_REVOKED = 2,
164 BIAS_REVOKED_AND_REBIASED = 3
165 };
166
167 // This initialization routine should only be called once and
168 // schedules a PeriodicTask to turn on biased locking a few seconds
169 // into the VM run to avoid startup time regressions
170 static void init();
171
172 // This provides a global switch for leaving biased locking disabled
173 // for the first part of a run and enabling it later
174 static bool enabled();
175
176 // This should be called by JavaThreads to revoke the bias of an object
177 static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS);
178
179 // These do not allow rebiasing; they are used by deoptimization to
180 // ensure that monitors on the stack can be migrated
181 static void revoke(GrowableArray<Handle>* objs);
182 static void revoke_at_safepoint(Handle obj);
183 static void revoke_at_safepoint(GrowableArray<Handle>* objs);
184
185 static void print_counters() { _counters.print(); }
186 static BiasedLockingCounters* counters() { return &_counters; }
187
188 // These routines are GC-related and should not be called by end
189 // users. GCs which do not do preservation of mark words do not need
190 // to call these routines.
191 static void preserve_marks();
192 static void restore_marks();
193};
194
195#endif // SHARE_RUNTIME_BIASEDLOCKING_HPP
196