hotspot/src/share/vm/services/lowMemoryDetector.cpp - platform/libcore - Git at Google

 /*
  * Copyright 2003-2007 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.
  *
  */

 # include "incls/_precompiled.incl"
 # include "incls/_lowMemoryDetector.cpp.incl"

 LowMemoryDetectorThread* LowMemoryDetector::_detector_thread = NULL;
 volatile bool LowMemoryDetector::_enabled_for_collected_pools = false;
 volatile jint LowMemoryDetector::_disabled_count = 0;

 void LowMemoryDetector::initialize() {
   EXCEPTION_MARK;

   instanceKlassHandle klass (THREAD,  SystemDictionary::thread_klass());
   instanceHandle thread_oop = klass->allocate_instance_handle(CHECK);

   const char thread_name[] = "Low Memory Detector";
   Handle string = java_lang_String::create_from_str(thread_name, CHECK);

   // Initialize thread_oop to put it into the system threadGroup
   Handle thread_group (THREAD, Universe::system_thread_group());
   JavaValue result(T_VOID);
   JavaCalls::call_special(&result, thread_oop,
                           klass,
                           vmSymbolHandles::object_initializer_name(),
                           vmSymbolHandles::threadgroup_string_void_signature(),
                           thread_group,
                           string,
                           CHECK);

   {
     MutexLocker mu(Threads_lock);
     _detector_thread = new LowMemoryDetectorThread(&low_memory_detector_thread_entry);

     // At this point it may be possible that no osthread was created for the
     // JavaThread due to lack of memory. We would have to throw an exception
     // in that case. However, since this must work and we do not allow
     // exceptions anyway, check and abort if this fails.
     if (_detector_thread == NULL || _detector_thread->osthread() == NULL) {
       vm_exit_during_initialization("java.lang.OutOfMemoryError",
                                     "unable to create new native thread");
     }

     java_lang_Thread::set_thread(thread_oop(), _detector_thread);
     java_lang_Thread::set_priority(thread_oop(), NearMaxPriority);
     java_lang_Thread::set_daemon(thread_oop());
     _detector_thread->set_threadObj(thread_oop());

     Threads::add(_detector_thread);
     Thread::start(_detector_thread);
   }
 }

 bool LowMemoryDetector::has_pending_requests() {
   assert(LowMemory_lock->owned_by_self(), "Must own LowMemory_lock");
   bool has_requests = false;
   int num_memory_pools = MemoryService::num_memory_pools();
   for (int i = 0; i < num_memory_pools; i++) {
     MemoryPool* pool = MemoryService::get_memory_pool(i);
     SensorInfo* sensor = pool->usage_sensor();
     if (sensor != NULL) {
       has_requests = has_requests || sensor->has_pending_requests();
     }

     SensorInfo* gc_sensor = pool->gc_usage_sensor();
     if (gc_sensor != NULL) {
       has_requests = has_requests || gc_sensor->has_pending_requests();
     }
   }
   return has_requests;
 }

 void LowMemoryDetector::low_memory_detector_thread_entry(JavaThread* jt, TRAPS) {
   while (true) {
     bool   sensors_changed = false;

     {
       // _no_safepoint_check_flag is used here as LowMemory_lock is a
       // special lock and the VMThread may acquire this lock at safepoint.
       // Need state transition ThreadBlockInVM so that this thread
       // will be handled by safepoint correctly when this thread is
       // notified at a safepoint.

       // This ThreadBlockInVM object is not also considered to be
       // suspend-equivalent because LowMemoryDetector threads are
       // not visible to external suspension.

       ThreadBlockInVM tbivm(jt);

       MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
       while (!(sensors_changed = has_pending_requests())) {
         // wait until one of the sensors has pending requests
         LowMemory_lock->wait(Mutex::_no_safepoint_check_flag);
       }
     }

     {
       ResourceMark rm(THREAD);
       HandleMark hm(THREAD);

       // No need to hold LowMemory_lock to call out to Java
       int num_memory_pools = MemoryService::num_memory_pools();
       for (int i = 0; i < num_memory_pools; i++) {
         MemoryPool* pool = MemoryService::get_memory_pool(i);
         SensorInfo* sensor = pool->usage_sensor();
         SensorInfo* gc_sensor = pool->gc_usage_sensor();
         if (sensor != NULL && sensor->has_pending_requests()) {
           sensor->process_pending_requests(CHECK);
         }
         if (gc_sensor != NULL && gc_sensor->has_pending_requests()) {
           gc_sensor->process_pending_requests(CHECK);
         }
       }
     }
   }
 }

 // This method could be called from any Java threads
 // and also VMThread.
 void LowMemoryDetector::detect_low_memory() {
   MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

   bool has_pending_requests = false;
   int num_memory_pools = MemoryService::num_memory_pools();
   for (int i = 0; i < num_memory_pools; i++) {
     MemoryPool* pool = MemoryService::get_memory_pool(i);
     SensorInfo* sensor = pool->usage_sensor();
     if (sensor != NULL &&
         pool->usage_threshold()->is_high_threshold_supported() &&
         pool->usage_threshold()->high_threshold() != 0) {
       MemoryUsage usage = pool->get_memory_usage();
       sensor->set_gauge_sensor_level(usage,
                                      pool->usage_threshold());
       has_pending_requests = has_pending_requests || sensor->has_pending_requests();
     }
   }

   if (has_pending_requests) {
     LowMemory_lock->notify_all();
   }
 }

 // This method could be called from any Java threads
 // and also VMThread.
 void LowMemoryDetector::detect_low_memory(MemoryPool* pool) {
   SensorInfo* sensor = pool->usage_sensor();
   if (sensor == NULL ||
       !pool->usage_threshold()->is_high_threshold_supported() ||
       pool->usage_threshold()->high_threshold() == 0) {
     return;
   }

   {
     MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

     MemoryUsage usage = pool->get_memory_usage();
     sensor->set_gauge_sensor_level(usage,
                                    pool->usage_threshold());
     if (sensor->has_pending_requests()) {
       // notify sensor state update
       LowMemory_lock->notify_all();
     }
   }
 }

 // Only called by VMThread at GC time
 void LowMemoryDetector::detect_after_gc_memory(MemoryPool* pool) {
   SensorInfo* sensor = pool->gc_usage_sensor();
   if (sensor == NULL ||
       !pool->gc_usage_threshold()->is_high_threshold_supported() ||
       pool->gc_usage_threshold()->high_threshold() == 0) {
     return;
   }

   {
     MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

     MemoryUsage usage = pool->get_last_collection_usage();
     sensor->set_counter_sensor_level(usage, pool->gc_usage_threshold());

     if (sensor->has_pending_requests()) {
       // notify sensor state update
       LowMemory_lock->notify_all();
     }
   }
 }

 // recompute enabled flag
 void LowMemoryDetector::recompute_enabled_for_collected_pools() {
   bool enabled = false;
   int num_memory_pools = MemoryService::num_memory_pools();
   for (int i=0; i<num_memory_pools; i++) {
     MemoryPool* pool = MemoryService::get_memory_pool(i);
     if (pool->is_collected_pool() && is_enabled(pool)) {
       enabled = true;
       break;
     }
   }
   _enabled_for_collected_pools = enabled;
 }

 SensorInfo::SensorInfo() {
   _sensor_obj = NULL;
   _sensor_on = false;
   _sensor_count = 0;
   _pending_trigger_count = 0;
   _pending_clear_count = 0;
 }

 // When this method is used, the memory usage is monitored
 // as a gauge attribute.  Sensor notifications (trigger or
 // clear) is only emitted at the first time it crosses
 // a threshold.
 //
 // High and low thresholds are designed to provide a
 // hysteresis mechanism to avoid repeated triggering
 // of notifications when the attribute value makes small oscillations
 // around the high or low threshold value.
 //
 // The sensor will be triggered if:
 //  (1) the usage is crossing above the high threshold and
 //      the sensor is currently off and no pending
 //      trigger requests; or
 //  (2) the usage is crossing above the high threshold and
 //      the sensor will be off (i.e. sensor is currently on
 //      and has pending clear requests).
 //
 // Subsequent crossings of the high threshold value do not cause
 // any triggers unless the usage becomes less than the low threshold.
 //
 // The sensor will be cleared if:
 //  (1) the usage is crossing below the low threshold and
 //      the sensor is currently on and no pending
 //      clear requests; or
 //  (2) the usage is crossing below the low threshold and
 //      the sensor will be on (i.e. sensor is currently off
 //      and has pending trigger requests).
 //
 // Subsequent crossings of the low threshold value do not cause
 // any clears unless the usage becomes greater than or equal
 // to the high threshold.
 //
 // If the current level is between high and low threhsold, no change.
 //
 void SensorInfo::set_gauge_sensor_level(MemoryUsage usage, ThresholdSupport* high_low_threshold) {
   assert(high_low_threshold->is_high_threshold_supported(), "just checking");

   bool is_over_high = high_low_threshold->is_high_threshold_crossed(usage);
   bool is_below_low = high_low_threshold->is_low_threshold_crossed(usage);

   assert(!(is_over_high && is_below_low), "Can't be both true");

   if (is_over_high &&
         ((!_sensor_on && _pending_trigger_count == 0) ||
          _pending_clear_count > 0)) {
     // low memory detected and need to increment the trigger pending count
     // if the sensor is off or will be off due to _pending_clear_ > 0
     // Request to trigger the sensor
     _pending_trigger_count++;
     _usage = usage;

     if (_pending_clear_count > 0) {
       // non-zero pending clear requests indicates that there are
       // pending requests to clear this sensor.
       // This trigger request needs to clear this clear count
       // since the resulting sensor flag should be on.
       _pending_clear_count = 0;
     }
   } else if (is_below_low &&
                ((_sensor_on && _pending_clear_count == 0) ||
                 (_pending_trigger_count > 0 && _pending_clear_count == 0))) {
     // memory usage returns below the threshold
     // Request to clear the sensor if the sensor is on or will be on due to
     // _pending_trigger_count > 0 and also no clear request
     _pending_clear_count++;
   }
 }

 // When this method is used, the memory usage is monitored as a
 // simple counter attribute.  The sensor will be triggered
 // whenever the usage is crossing the threshold to keep track
 // of the number of times the VM detects such a condition occurs.
 //
 // High and low thresholds are designed to provide a
 // hysteresis mechanism to avoid repeated triggering
 // of notifications when the attribute value makes small oscillations
 // around the high or low threshold value.
 //
 // The sensor will be triggered if:
 //   - the usage is crossing above the high threshold regardless
 //     of the current sensor state.
 //
 // The sensor will be cleared if:
 //  (1) the usage is crossing below the low threshold and
 //      the sensor is currently on; or
 //  (2) the usage is crossing below the low threshold and
 //      the sensor will be on (i.e. sensor is currently off
 //      and has pending trigger requests).
 void SensorInfo::set_counter_sensor_level(MemoryUsage usage, ThresholdSupport* counter_threshold) {
   assert(counter_threshold->is_high_threshold_supported(), "just checking");

   bool is_over_high = counter_threshold->is_high_threshold_crossed(usage);
   bool is_below_low = counter_threshold->is_low_threshold_crossed(usage);

   assert(!(is_over_high && is_below_low), "Can't be both true");

   if (is_over_high) {
     _pending_trigger_count++;
     _usage = usage;
     _pending_clear_count = 0;
   } else if (is_below_low && (_sensor_on || _pending_trigger_count > 0)) {
     _pending_clear_count++;
   }
 }

 void SensorInfo::oops_do(OopClosure* f) {
   f->do_oop((oop*) &_sensor_obj);
 }

 void SensorInfo::process_pending_requests(TRAPS) {
   if (!has_pending_requests()) {
     return;
   }

   int pending_count = pending_trigger_count();
   if (pending_clear_count() > 0) {
     clear(pending_count, CHECK);
   } else {
     trigger(pending_count, CHECK);
   }

 }

 void SensorInfo::trigger(int count, TRAPS) {
   assert(count <= _pending_trigger_count, "just checking");

   if (_sensor_obj != NULL) {
     klassOop k = Management::sun_management_Sensor_klass(CHECK);
     instanceKlassHandle sensorKlass (THREAD, k);
     Handle sensor_h(THREAD, _sensor_obj);
     Handle usage_h = MemoryService::create_MemoryUsage_obj(_usage, CHECK);

     JavaValue result(T_VOID);
     JavaCallArguments args(sensor_h);
     args.push_int((int) count);
     args.push_oop(usage_h);

     JavaCalls::call_virtual(&result,
                             sensorKlass,
                             vmSymbolHandles::trigger_name(),
                             vmSymbolHandles::trigger_method_signature(),
                             &args,
                             CHECK);
   }

   {
     // Holds LowMemory_lock and update the sensor state
     MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
     _sensor_on = true;
     _sensor_count += count;
     _pending_trigger_count = _pending_trigger_count - count;
   }
 }

 void SensorInfo::clear(int count, TRAPS) {
   if (_sensor_obj != NULL) {
     klassOop k = Management::sun_management_Sensor_klass(CHECK);
     instanceKlassHandle sensorKlass (THREAD, k);
     Handle sensor(THREAD, _sensor_obj);

     JavaValue result(T_VOID);
     JavaCallArguments args(sensor);
     args.push_int((int) count);
     JavaCalls::call_virtual(&result,
                             sensorKlass,
                             vmSymbolHandles::clear_name(),
                             vmSymbolHandles::int_void_signature(),
                             &args,
                             CHECK);
   }

   {
     // Holds LowMemory_lock and update the sensor state
     MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
     _sensor_on = false;
     _pending_clear_count = 0;
     _pending_trigger_count = _pending_trigger_count - count;
   }
 }

 //--------------------------------------------------------------
 // Non-product code

 #ifndef PRODUCT
 void SensorInfo::print() {
   tty->print_cr("%s count = %ld pending_triggers = %ld pending_clears = %ld",
                 (_sensor_on ? "on" : "off"),
                 _sensor_count, _pending_trigger_count, _pending_clear_count);
 }

 #endif // PRODUCT
	/*
	* Copyright 2003-2007 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.
	*
	*/

	# include "incls/_precompiled.incl"
	# include "incls/_lowMemoryDetector.cpp.incl"

	LowMemoryDetectorThread* LowMemoryDetector::_detector_thread = NULL;
	volatile bool LowMemoryDetector::_enabled_for_collected_pools = false;
	volatile jint LowMemoryDetector::_disabled_count = 0;

	void LowMemoryDetector::initialize() {
	EXCEPTION_MARK;

	instanceKlassHandle klass (THREAD, SystemDictionary::thread_klass());
	instanceHandle thread_oop = klass->allocate_instance_handle(CHECK);

	const char thread_name[] = "Low Memory Detector";
	Handle string = java_lang_String::create_from_str(thread_name, CHECK);

	// Initialize thread_oop to put it into the system threadGroup
	Handle thread_group (THREAD, Universe::system_thread_group());
	JavaValue result(T_VOID);
	JavaCalls::call_special(&result, thread_oop,
	klass,
	vmSymbolHandles::object_initializer_name(),
	vmSymbolHandles::threadgroup_string_void_signature(),
	thread_group,
	string,
	CHECK);

	{
	MutexLocker mu(Threads_lock);
	_detector_thread = new LowMemoryDetectorThread(&low_memory_detector_thread_entry);

	// At this point it may be possible that no osthread was created for the
	// JavaThread due to lack of memory. We would have to throw an exception
	// in that case. However, since this must work and we do not allow
	// exceptions anyway, check and abort if this fails.
	if (_detector_thread == NULL \|\| _detector_thread->osthread() == NULL) {
	vm_exit_during_initialization("java.lang.OutOfMemoryError",
	"unable to create new native thread");
	}

	java_lang_Thread::set_thread(thread_oop(), _detector_thread);
	java_lang_Thread::set_priority(thread_oop(), NearMaxPriority);
	java_lang_Thread::set_daemon(thread_oop());
	_detector_thread->set_threadObj(thread_oop());

	Threads::add(_detector_thread);
	Thread::start(_detector_thread);
	}
	}

	bool LowMemoryDetector::has_pending_requests() {
	assert(LowMemory_lock->owned_by_self(), "Must own LowMemory_lock");
	bool has_requests = false;
	int num_memory_pools = MemoryService::num_memory_pools();
	for (int i = 0; i < num_memory_pools; i++) {
	MemoryPool* pool = MemoryService::get_memory_pool(i);
	SensorInfo* sensor = pool->usage_sensor();
	if (sensor != NULL) {
	has_requests = has_requests \|\| sensor->has_pending_requests();
	}

	SensorInfo* gc_sensor = pool->gc_usage_sensor();
	if (gc_sensor != NULL) {
	has_requests = has_requests \|\| gc_sensor->has_pending_requests();
	}
	}
	return has_requests;
	}

	void LowMemoryDetector::low_memory_detector_thread_entry(JavaThread* jt, TRAPS) {
	while (true) {
	bool sensors_changed = false;

	{
	// _no_safepoint_check_flag is used here as LowMemory_lock is a
	// special lock and the VMThread may acquire this lock at safepoint.
	// Need state transition ThreadBlockInVM so that this thread
	// will be handled by safepoint correctly when this thread is
	// notified at a safepoint.

	// This ThreadBlockInVM object is not also considered to be
	// suspend-equivalent because LowMemoryDetector threads are
	// not visible to external suspension.

	ThreadBlockInVM tbivm(jt);

	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
	while (!(sensors_changed = has_pending_requests())) {
	// wait until one of the sensors has pending requests
	LowMemory_lock->wait(Mutex::_no_safepoint_check_flag);
	}
	}

	{
	ResourceMark rm(THREAD);
	HandleMark hm(THREAD);

	// No need to hold LowMemory_lock to call out to Java
	int num_memory_pools = MemoryService::num_memory_pools();
	for (int i = 0; i < num_memory_pools; i++) {
	MemoryPool* pool = MemoryService::get_memory_pool(i);
	SensorInfo* sensor = pool->usage_sensor();
	SensorInfo* gc_sensor = pool->gc_usage_sensor();
	if (sensor != NULL && sensor->has_pending_requests()) {
	sensor->process_pending_requests(CHECK);
	}
	if (gc_sensor != NULL && gc_sensor->has_pending_requests()) {
	gc_sensor->process_pending_requests(CHECK);
	}
	}
	}
	}
	}

	// This method could be called from any Java threads
	// and also VMThread.
	void LowMemoryDetector::detect_low_memory() {
	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

	bool has_pending_requests = false;
	int num_memory_pools = MemoryService::num_memory_pools();
	for (int i = 0; i < num_memory_pools; i++) {
	MemoryPool* pool = MemoryService::get_memory_pool(i);
	SensorInfo* sensor = pool->usage_sensor();
	if (sensor != NULL &&
	pool->usage_threshold()->is_high_threshold_supported() &&
	pool->usage_threshold()->high_threshold() != 0) {
	MemoryUsage usage = pool->get_memory_usage();
	sensor->set_gauge_sensor_level(usage,
	pool->usage_threshold());
	has_pending_requests = has_pending_requests \|\| sensor->has_pending_requests();
	}
	}

	if (has_pending_requests) {
	LowMemory_lock->notify_all();
	}
	}

	// This method could be called from any Java threads
	// and also VMThread.
	void LowMemoryDetector::detect_low_memory(MemoryPool* pool) {
	SensorInfo* sensor = pool->usage_sensor();
	if (sensor == NULL \|\|
	!pool->usage_threshold()->is_high_threshold_supported() \|\|
	pool->usage_threshold()->high_threshold() == 0) {
	return;
	}

	{
	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

	MemoryUsage usage = pool->get_memory_usage();
	sensor->set_gauge_sensor_level(usage,
	pool->usage_threshold());
	if (sensor->has_pending_requests()) {
	// notify sensor state update
	LowMemory_lock->notify_all();
	}
	}
	}

	// Only called by VMThread at GC time
	void LowMemoryDetector::detect_after_gc_memory(MemoryPool* pool) {
	SensorInfo* sensor = pool->gc_usage_sensor();
	if (sensor == NULL \|\|
	!pool->gc_usage_threshold()->is_high_threshold_supported() \|\|
	pool->gc_usage_threshold()->high_threshold() == 0) {
	return;
	}

	{
	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);

	MemoryUsage usage = pool->get_last_collection_usage();
	sensor->set_counter_sensor_level(usage, pool->gc_usage_threshold());

	if (sensor->has_pending_requests()) {
	// notify sensor state update
	LowMemory_lock->notify_all();
	}
	}
	}

	// recompute enabled flag
	void LowMemoryDetector::recompute_enabled_for_collected_pools() {
	bool enabled = false;
	int num_memory_pools = MemoryService::num_memory_pools();
	for (int i=0; i<num_memory_pools; i++) {
	MemoryPool* pool = MemoryService::get_memory_pool(i);
	if (pool->is_collected_pool() && is_enabled(pool)) {
	enabled = true;
	break;
	}
	}
	_enabled_for_collected_pools = enabled;
	}

	SensorInfo::SensorInfo() {
	_sensor_obj = NULL;
	_sensor_on = false;
	_sensor_count = 0;
	_pending_trigger_count = 0;
	_pending_clear_count = 0;
	}

	// When this method is used, the memory usage is monitored
	// as a gauge attribute. Sensor notifications (trigger or
	// clear) is only emitted at the first time it crosses
	// a threshold.
	//
	// High and low thresholds are designed to provide a
	// hysteresis mechanism to avoid repeated triggering
	// of notifications when the attribute value makes small oscillations
	// around the high or low threshold value.
	//
	// The sensor will be triggered if:
	// (1) the usage is crossing above the high threshold and
	// the sensor is currently off and no pending
	// trigger requests; or
	// (2) the usage is crossing above the high threshold and
	// the sensor will be off (i.e. sensor is currently on
	// and has pending clear requests).
	//
	// Subsequent crossings of the high threshold value do not cause
	// any triggers unless the usage becomes less than the low threshold.
	//
	// The sensor will be cleared if:
	// (1) the usage is crossing below the low threshold and
	// the sensor is currently on and no pending
	// clear requests; or
	// (2) the usage is crossing below the low threshold and
	// the sensor will be on (i.e. sensor is currently off
	// and has pending trigger requests).
	//
	// Subsequent crossings of the low threshold value do not cause
	// any clears unless the usage becomes greater than or equal
	// to the high threshold.
	//
	// If the current level is between high and low threhsold, no change.
	//
	void SensorInfo::set_gauge_sensor_level(MemoryUsage usage, ThresholdSupport* high_low_threshold) {
	assert(high_low_threshold->is_high_threshold_supported(), "just checking");

	bool is_over_high = high_low_threshold->is_high_threshold_crossed(usage);
	bool is_below_low = high_low_threshold->is_low_threshold_crossed(usage);

	assert(!(is_over_high && is_below_low), "Can't be both true");

	if (is_over_high &&
	((!_sensor_on && _pending_trigger_count == 0) \|\|
	_pending_clear_count > 0)) {
	// low memory detected and need to increment the trigger pending count
	// if the sensor is off or will be off due to _pending_clear_ > 0
	// Request to trigger the sensor
	_pending_trigger_count++;
	_usage = usage;

	if (_pending_clear_count > 0) {
	// non-zero pending clear requests indicates that there are
	// pending requests to clear this sensor.
	// This trigger request needs to clear this clear count
	// since the resulting sensor flag should be on.
	_pending_clear_count = 0;
	}
	} else if (is_below_low &&
	((_sensor_on && _pending_clear_count == 0) \|\|
	(_pending_trigger_count > 0 && _pending_clear_count == 0))) {
	// memory usage returns below the threshold
	// Request to clear the sensor if the sensor is on or will be on due to
	// _pending_trigger_count > 0 and also no clear request
	_pending_clear_count++;
	}
	}

	// When this method is used, the memory usage is monitored as a
	// simple counter attribute. The sensor will be triggered
	// whenever the usage is crossing the threshold to keep track
	// of the number of times the VM detects such a condition occurs.
	//
	// High and low thresholds are designed to provide a
	// hysteresis mechanism to avoid repeated triggering
	// of notifications when the attribute value makes small oscillations
	// around the high or low threshold value.
	//
	// The sensor will be triggered if:
	// - the usage is crossing above the high threshold regardless
	// of the current sensor state.
	//
	// The sensor will be cleared if:
	// (1) the usage is crossing below the low threshold and
	// the sensor is currently on; or
	// (2) the usage is crossing below the low threshold and
	// the sensor will be on (i.e. sensor is currently off
	// and has pending trigger requests).
	void SensorInfo::set_counter_sensor_level(MemoryUsage usage, ThresholdSupport* counter_threshold) {
	assert(counter_threshold->is_high_threshold_supported(), "just checking");

	bool is_over_high = counter_threshold->is_high_threshold_crossed(usage);
	bool is_below_low = counter_threshold->is_low_threshold_crossed(usage);

	assert(!(is_over_high && is_below_low), "Can't be both true");

	if (is_over_high) {
	_pending_trigger_count++;
	_usage = usage;
	_pending_clear_count = 0;
	} else if (is_below_low && (_sensor_on \|\| _pending_trigger_count > 0)) {
	_pending_clear_count++;
	}
	}

	void SensorInfo::oops_do(OopClosure* f) {
	f->do_oop((oop*) &_sensor_obj);
	}

	void SensorInfo::process_pending_requests(TRAPS) {
	if (!has_pending_requests()) {
	return;
	}

	int pending_count = pending_trigger_count();
	if (pending_clear_count() > 0) {
	clear(pending_count, CHECK);
	} else {
	trigger(pending_count, CHECK);
	}

	}

	void SensorInfo::trigger(int count, TRAPS) {
	assert(count <= _pending_trigger_count, "just checking");

	if (_sensor_obj != NULL) {
	klassOop k = Management::sun_management_Sensor_klass(CHECK);
	instanceKlassHandle sensorKlass (THREAD, k);
	Handle sensor_h(THREAD, _sensor_obj);
	Handle usage_h = MemoryService::create_MemoryUsage_obj(_usage, CHECK);

	JavaValue result(T_VOID);
	JavaCallArguments args(sensor_h);
	args.push_int((int) count);
	args.push_oop(usage_h);

	JavaCalls::call_virtual(&result,
	sensorKlass,
	vmSymbolHandles::trigger_name(),
	vmSymbolHandles::trigger_method_signature(),
	&args,
	CHECK);
	}

	{
	// Holds LowMemory_lock and update the sensor state
	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
	_sensor_on = true;
	_sensor_count += count;
	_pending_trigger_count = _pending_trigger_count - count;
	}
	}

	void SensorInfo::clear(int count, TRAPS) {
	if (_sensor_obj != NULL) {
	klassOop k = Management::sun_management_Sensor_klass(CHECK);
	instanceKlassHandle sensorKlass (THREAD, k);
	Handle sensor(THREAD, _sensor_obj);

	JavaValue result(T_VOID);
	JavaCallArguments args(sensor);
	args.push_int((int) count);
	JavaCalls::call_virtual(&result,
	sensorKlass,
	vmSymbolHandles::clear_name(),
	vmSymbolHandles::int_void_signature(),
	&args,
	CHECK);
	}

	{
	// Holds LowMemory_lock and update the sensor state
	MutexLockerEx ml(LowMemory_lock, Mutex::_no_safepoint_check_flag);
	_sensor_on = false;
	_pending_clear_count = 0;
	_pending_trigger_count = _pending_trigger_count - count;
	}
	}

	//--------------------------------------------------------------
	// Non-product code

	#ifndef PRODUCT
	void SensorInfo::print() {
	tty->print_cr("%s count = %ld pending_triggers = %ld pending_clears = %ld",
	(_sensor_on ? "on" : "off"),
	_sensor_count, _pending_trigger_count, _pending_clear_count);
	}

	#endif // PRODUCT