1 /*
2 * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 class ConcurrentMarkSweepGeneration;
26 class CMSCollector;
27
28 // The Concurrent Mark Sweep GC Thread (could be several in the future).
29 class ConcurrentMarkSweepThread: public ConcurrentGCThread {
30 friend class VMStructs;
31 friend class ConcurrentMarkSweepGeneration; // XXX should remove friendship
32 friend class CMSCollector;
33 public:
34 virtual void run();
35
36 private:
37 static ConcurrentMarkSweepThread* _cmst;
38 static CMSCollector* _collector;
39 static SurrogateLockerThread* _slt;
40 static SurrogateLockerThread::SLT_msg_type _sltBuffer;
41 static Monitor* _sltMonitor;
42
43 ConcurrentMarkSweepThread* _next;
44
45 static bool _should_terminate;
46
47 enum CMS_flag_type {
48 CMS_nil = NoBits,
49 CMS_cms_wants_token = nth_bit(0),
50 CMS_cms_has_token = nth_bit(1),
51 CMS_vm_wants_token = nth_bit(2),
52 CMS_vm_has_token = nth_bit(3)
53 };
54
55 static int _CMS_flag;
56
57 static bool CMS_flag_is_set(int b) { return (_CMS_flag & b) != 0; }
58 static bool set_CMS_flag(int b) { return (_CMS_flag |= b) != 0; }
59 static bool clear_CMS_flag(int b) { return (_CMS_flag &= ~b) != 0; }
60 void sleepBeforeNextCycle();
61
62 // CMS thread should yield for a young gen collection, direct allocation,
63 // and iCMS activity.
64 static char _pad_1[64 - sizeof(jint)]; // prevent cache-line sharing
65 static volatile jint _pending_yields;
66 static volatile jint _pending_decrements; // decrements to _pending_yields
67 static char _pad_2[64 - sizeof(jint)]; // prevent cache-line sharing
68
69 // Tracing messages, enabled by CMSTraceThreadState.
70 static inline void trace_state(const char* desc);
71
72 static volatile bool _icms_enabled; // iCMS enabled?
73 static volatile bool _should_run; // iCMS may run
74 static volatile bool _should_stop; // iCMS should stop
75
76 // debugging
77 void verify_ok_to_terminate() const PRODUCT_RETURN;
78
79 public:
80 // Constructor
81 ConcurrentMarkSweepThread(CMSCollector* collector);
82
83 static void makeSurrogateLockerThread(TRAPS);
84 static SurrogateLockerThread* slt() { return _slt; }
85
86 // Tester
87 bool is_ConcurrentGC_thread() const { return true; }
88
89 static void threads_do(ThreadClosure* tc);
90
91 // Printing
92 void print_on(outputStream* st) const;
93 void print() const { print_on(tty); }
94 static void print_all_on(outputStream* st);
95 static void print_all() { print_all_on(tty); }
96
97 // Returns the CMS Thread
98 static ConcurrentMarkSweepThread* cmst() { return _cmst; }
99 static CMSCollector* collector() { return _collector; }
100
101 // Create and start the CMS Thread, or stop it on shutdown
102 static ConcurrentMarkSweepThread* start(CMSCollector* collector);
103 static void stop();
104 static bool should_terminate() { return _should_terminate; }
105
106 // Synchronization using CMS token
107 static void synchronize(bool is_cms_thread);
108 static void desynchronize(bool is_cms_thread);
109 static bool vm_thread_has_cms_token() {
110 return CMS_flag_is_set(CMS_vm_has_token);
111 }
112 static bool cms_thread_has_cms_token() {
113 return CMS_flag_is_set(CMS_cms_has_token);
114 }
115 static bool vm_thread_wants_cms_token() {
116 return CMS_flag_is_set(CMS_vm_wants_token);
117 }
118 static bool cms_thread_wants_cms_token() {
119 return CMS_flag_is_set(CMS_cms_wants_token);
120 }
121
122 // Wait on CMS lock until the next synchronous GC
123 // or given timeout, whichever is earlier.
124 void wait_on_cms_lock(long t); // milliseconds
125
126 // The CMS thread will yield during the work portion of its cycle
127 // only when requested to. Both synchronous and asychronous requests
128 // are provided:
129 // (1) A synchronous request is used for young gen collections and
130 // for direct allocations. The requesting thread increments
131 // _pending_yields at the beginning of an operation, and decrements
132 // _pending_yields when that operation is completed.
133 // In turn, the CMS thread yields when _pending_yields is positive,
134 // and continues to yield until the value reverts to 0.
135 // (2) An asynchronous request, on the other hand, is used by iCMS
136 // for the stop_icms() operation. A single yield satisfies all of
137 // the outstanding asynch yield requests, of which there may
138 // occasionally be several in close succession. To accomplish
139 // this, an asynch-requesting thread atomically increments both
140 // _pending_yields and _pending_decrements. An asynchr requesting
141 // thread does not wait and "acknowledge" completion of an operation
142 // and deregister the request, like the synchronous version described
143 // above does. In turn, after yielding, the CMS thread decrements both
144 // _pending_yields and _pending_decrements by the value seen in
145 // _pending_decrements before the decrement.
146 // NOTE: The above scheme is isomorphic to having two request counters,
147 // one for async requests and one for sync requests, and for the CMS thread
148 // to check the sum of the two counters to decide whether it should yield
149 // and to clear only the async counter when it yields. However, it turns out
150 // to be more efficient for CMS code to just check a single counter
151 // _pending_yields that holds the sum (of both sync and async requests), and
152 // a second counter _pending_decrements that only holds the async requests,
153 // for greater efficiency, since in a typical CMS run, there are many more
154 // pontential (i.e. static) yield points than there are actual
155 // (i.e. dynamic) yields because of requests, which are few and far between.
156 //
157 // Note that, while "_pending_yields >= _pending_decrements" is an invariant,
158 // we cannot easily test that invariant, since the counters are manipulated via
159 // atomic instructions without explicit locking and we cannot read
160 // the two counters atomically together: one suggestion is to
161 // use (for example) 16-bit counters so as to be able to read the
162 // two counters atomically even on 32-bit platforms. Notice that
163 // the second assert in acknowledge_yield_request() below does indeed
164 // check a form of the above invariant, albeit indirectly.
165
166 static void increment_pending_yields() {
167 Atomic::inc(&_pending_yields);
168 assert(_pending_yields >= 0, "can't be negative");
169 }
170 static void decrement_pending_yields() {
171 Atomic::dec(&_pending_yields);
172 assert(_pending_yields >= 0, "can't be negative");
173 }
174 static void asynchronous_yield_request() {
175 assert(CMSIncrementalMode, "Currently only used w/iCMS");
176 increment_pending_yields();
177 Atomic::inc(&_pending_decrements);
178 assert(_pending_decrements >= 0, "can't be negative");
179 }
180 static void acknowledge_yield_request() {
181 jint decrement = _pending_decrements;
182 if (decrement > 0) {
183 assert(CMSIncrementalMode, "Currently only used w/iCMS");
184 // Order important to preserve: _pending_yields >= _pending_decrements
185 Atomic::add(-decrement, &_pending_decrements);
186 Atomic::add(-decrement, &_pending_yields);
187 assert(_pending_decrements >= 0, "can't be negative");
188 assert(_pending_yields >= 0, "can't be negative");
189 }
190 }
191 static bool should_yield() { return _pending_yields > 0; }
192
193 // CMS incremental mode.
194 static void start_icms(); // notify thread to start a quantum of work
195 static void stop_icms(); // request thread to stop working
196 void icms_wait(); // if asked to stop, wait until notified to start
197
198 // Incremental mode is enabled globally by the flag CMSIncrementalMode. It
199 // must also be enabled/disabled dynamically to allow foreground collections.
200 static inline void enable_icms() { _icms_enabled = true; }
201 static inline void disable_icms() { _icms_enabled = false; }
202 static inline void set_icms_enabled(bool val) { _icms_enabled = val; }
203 static inline bool icms_enabled() { return _icms_enabled; }
204 };
205
206 inline void ConcurrentMarkSweepThread::trace_state(const char* desc) {
207 if (CMSTraceThreadState) {
208 char buf[128];
209 TimeStamp& ts = gclog_or_tty->time_stamp();
210 if (!ts.is_updated()) {
211 ts.update();
212 }
213 jio_snprintf(buf, sizeof(buf), " [%.3f: CMSThread %s] ",
214 ts.seconds(), desc);
215 buf[sizeof(buf) - 1] = '\0';
216 gclog_or_tty->print(buf);
217 }
218 }
219
220 // For scoped increment/decrement of (synchronous) yield requests
221 class CMSSynchronousYieldRequest: public StackObj {
222 public:
223 CMSSynchronousYieldRequest() {
224 ConcurrentMarkSweepThread::increment_pending_yields();
225 }
226 ~CMSSynchronousYieldRequest() {
227 ConcurrentMarkSweepThread::decrement_pending_yields();
228 }
229 };
230
231 // Used to emit a warning in case of unexpectedly excessive
232 // looping (in "apparently endless loops") in CMS code.
233 class CMSLoopCountWarn: public StackObj {
234 private:
235 const char* _src;
236 const char* _msg;
237 const intx _threshold;
238 intx _ticks;
239
240 public:
241 inline CMSLoopCountWarn(const char* src, const char* msg,
242 const intx threshold) :
243 _src(src), _msg(msg), _threshold(threshold), _ticks(0) { }
244
245 inline void tick() {
246 _ticks++;
247 if (CMSLoopWarn && _ticks % _threshold == 0) {
248 warning("%s has looped %d times %s", _src, _ticks, _msg);
249 }
250 }
251 };