Mon, 09 Mar 2020 12:54:53 +0000
8240295: hs_err elapsed time in seconds is not accurate enough
Reviewed-by: dholmes, sspitsyn
1 /*
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25 #ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP
26 #define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP
28 #include "runtime/handles.hpp"
29 #include "utilities/growableArray.hpp"
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.
110 // Biased locking counters
111 class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC {
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;
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) {}
131 int slow_path_entry_count(); // Compute this field if necessary
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; }
141 bool nonzero() { return _total_entry_count > 0; }
143 void print_on(outputStream* st);
144 void print() { print_on(tty); }
145 };
148 class BiasedLocking : AllStatic {
149 private:
150 static BiasedLockingCounters _counters;
152 public:
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();
161 enum Condition {
162 NOT_BIASED = 1,
163 BIAS_REVOKED = 2,
164 BIAS_REVOKED_AND_REBIASED = 3
165 };
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();
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();
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);
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);
185 static void print_counters() { _counters.print(); }
186 static BiasedLockingCounters* counters() { return &_counters; }
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 };
195 #endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP