Thu, 21 Oct 2010 17:29:24 -0700
6992998: CMSWaitDuration=0 causes hangs with +ExplicitGCInvokesConcurrent
Summary: Closed a timing hole during which concurrent full gc requests can be missed. The hole can increase the latency of the response to a full gc request by up to the value of CMSWaitDuration. If CMSWaitDuration=0 is, as currently, interpreted as an unbounded wait, suitable in certain tuning scenarios, the application can potentially hang. Made two obscure tunables, including CMSWaitDuration, manageable.
Reviewed-by: jcoomes, tonyp
1 /*
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25 class ConcurrentMarkSweepGeneration;
26 class CMSCollector;
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();
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;
43 ConcurrentMarkSweepThread* _next;
45 static bool _should_terminate;
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 };
55 static int _CMS_flag;
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();
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
69 // Tracing messages, enabled by CMSTraceThreadState.
70 static inline void trace_state(const char* desc);
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
76 // debugging
77 void verify_ok_to_terminate() const PRODUCT_RETURN;
79 public:
80 // Constructor
81 ConcurrentMarkSweepThread(CMSCollector* collector);
83 static void makeSurrogateLockerThread(TRAPS);
84 static SurrogateLockerThread* slt() { return _slt; }
86 // Tester
87 bool is_ConcurrentGC_thread() const { return true; }
89 static void threads_do(ThreadClosure* tc);
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); }
97 // Returns the CMS Thread
98 static ConcurrentMarkSweepThread* cmst() { return _cmst; }
99 static CMSCollector* collector() { return _collector; }
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; }
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 }
122 // Wait on CMS lock until the next synchronous GC
123 // or given timeout, whichever is earlier. A timeout value
124 // of 0 indicates that there is no upper bound on the wait time.
125 // A concurrent full gc request terminates the wait.
126 void wait_on_cms_lock(long t_millis);
128 // The CMS thread will yield during the work portion of its cycle
129 // only when requested to. Both synchronous and asychronous requests
130 // are provided:
131 // (1) A synchronous request is used for young gen collections and
132 // for direct allocations. The requesting thread increments
133 // _pending_yields at the beginning of an operation, and decrements
134 // _pending_yields when that operation is completed.
135 // In turn, the CMS thread yields when _pending_yields is positive,
136 // and continues to yield until the value reverts to 0.
137 // (2) An asynchronous request, on the other hand, is used by iCMS
138 // for the stop_icms() operation. A single yield satisfies all of
139 // the outstanding asynch yield requests, of which there may
140 // occasionally be several in close succession. To accomplish
141 // this, an asynch-requesting thread atomically increments both
142 // _pending_yields and _pending_decrements. An asynchr requesting
143 // thread does not wait and "acknowledge" completion of an operation
144 // and deregister the request, like the synchronous version described
145 // above does. In turn, after yielding, the CMS thread decrements both
146 // _pending_yields and _pending_decrements by the value seen in
147 // _pending_decrements before the decrement.
148 // NOTE: The above scheme is isomorphic to having two request counters,
149 // one for async requests and one for sync requests, and for the CMS thread
150 // to check the sum of the two counters to decide whether it should yield
151 // and to clear only the async counter when it yields. However, it turns out
152 // to be more efficient for CMS code to just check a single counter
153 // _pending_yields that holds the sum (of both sync and async requests), and
154 // a second counter _pending_decrements that only holds the async requests,
155 // for greater efficiency, since in a typical CMS run, there are many more
156 // pontential (i.e. static) yield points than there are actual
157 // (i.e. dynamic) yields because of requests, which are few and far between.
158 //
159 // Note that, while "_pending_yields >= _pending_decrements" is an invariant,
160 // we cannot easily test that invariant, since the counters are manipulated via
161 // atomic instructions without explicit locking and we cannot read
162 // the two counters atomically together: one suggestion is to
163 // use (for example) 16-bit counters so as to be able to read the
164 // two counters atomically even on 32-bit platforms. Notice that
165 // the second assert in acknowledge_yield_request() below does indeed
166 // check a form of the above invariant, albeit indirectly.
168 static void increment_pending_yields() {
169 Atomic::inc(&_pending_yields);
170 assert(_pending_yields >= 0, "can't be negative");
171 }
172 static void decrement_pending_yields() {
173 Atomic::dec(&_pending_yields);
174 assert(_pending_yields >= 0, "can't be negative");
175 }
176 static void asynchronous_yield_request() {
177 assert(CMSIncrementalMode, "Currently only used w/iCMS");
178 increment_pending_yields();
179 Atomic::inc(&_pending_decrements);
180 assert(_pending_decrements >= 0, "can't be negative");
181 }
182 static void acknowledge_yield_request() {
183 jint decrement = _pending_decrements;
184 if (decrement > 0) {
185 assert(CMSIncrementalMode, "Currently only used w/iCMS");
186 // Order important to preserve: _pending_yields >= _pending_decrements
187 Atomic::add(-decrement, &_pending_decrements);
188 Atomic::add(-decrement, &_pending_yields);
189 assert(_pending_decrements >= 0, "can't be negative");
190 assert(_pending_yields >= 0, "can't be negative");
191 }
192 }
193 static bool should_yield() { return _pending_yields > 0; }
195 // CMS incremental mode.
196 static void start_icms(); // notify thread to start a quantum of work
197 static void stop_icms(); // request thread to stop working
198 void icms_wait(); // if asked to stop, wait until notified to start
200 // Incremental mode is enabled globally by the flag CMSIncrementalMode. It
201 // must also be enabled/disabled dynamically to allow foreground collections.
202 static inline void enable_icms() { _icms_enabled = true; }
203 static inline void disable_icms() { _icms_enabled = false; }
204 static inline void set_icms_enabled(bool val) { _icms_enabled = val; }
205 static inline bool icms_enabled() { return _icms_enabled; }
206 };
208 inline void ConcurrentMarkSweepThread::trace_state(const char* desc) {
209 if (CMSTraceThreadState) {
210 char buf[128];
211 TimeStamp& ts = gclog_or_tty->time_stamp();
212 if (!ts.is_updated()) {
213 ts.update();
214 }
215 jio_snprintf(buf, sizeof(buf), " [%.3f: CMSThread %s] ",
216 ts.seconds(), desc);
217 buf[sizeof(buf) - 1] = '\0';
218 gclog_or_tty->print(buf);
219 }
220 }
222 // For scoped increment/decrement of (synchronous) yield requests
223 class CMSSynchronousYieldRequest: public StackObj {
224 public:
225 CMSSynchronousYieldRequest() {
226 ConcurrentMarkSweepThread::increment_pending_yields();
227 }
228 ~CMSSynchronousYieldRequest() {
229 ConcurrentMarkSweepThread::decrement_pending_yields();
230 }
231 };
233 // Used to emit a warning in case of unexpectedly excessive
234 // looping (in "apparently endless loops") in CMS code.
235 class CMSLoopCountWarn: public StackObj {
236 private:
237 const char* _src;
238 const char* _msg;
239 const intx _threshold;
240 intx _ticks;
242 public:
243 inline CMSLoopCountWarn(const char* src, const char* msg,
244 const intx threshold) :
245 _src(src), _msg(msg), _threshold(threshold), _ticks(0) { }
247 inline void tick() {
248 _ticks++;
249 if (CMSLoopWarn && _ticks % _threshold == 0) {
250 warning("%s has looped %d times %s", _src, _ticks, _msg);
251 }
252 }
253 };