Hello Mandy,
Thanks for taking a look at the changes. My comments inlined below: From: Mandy Chung Sent: Wednesday, June 14, 2017 10:18 AM To: Poonam Parhar Cc: hotspot-runtime-...@openjdk.java.net; serviceability-dev Subject: Re: RFR(10): JDK-8178536: OOM ERRORS + SERVICE-THREAD TAKES A PROCESSOR TO 100% On Jun 7, 2017, at 8:53 AM, Poonam Parhar <HYPERLINK "mailto:poonam.ba...@oracle.com"poonam.ba...@oracle.com> wrote: Hello, Could I have reviews for the changes for the Bug HYPERLINK "https://bugs.openjdk.java.net/browse/JDK-8178536"JDK-8178536: OOM ERRORS + SERVICE-THREAD TAKES A PROCESSOR TO 100%. Problem: If there are listeners installed for MemoryMXBean then in the event of an OOME, JVM internal thread 'Service Thread' responsible for delivering notifications to the Java threads itself may encounter an OOM exception and get into a loop of processing its pending requests. This happens if and when the Service Thread executing the native code calls its corresponding java methods and faces an OOM exception, this pending exception makes the thread exit early from SensorInfo∷trigger() function before it can update its pending requests counter (_pending_trigger_count). This pending exception is never cleared and that makes the thread loop in LowMemoryDetector::process_sensor_changes(). Hotspot changes: http://cr.openjdk.java.net/~poonam/8178536/webrev.hotspot/ These changes check for the pending exception and clear it, and make sure that the pending requests counters are kept in sync on both the Java and the VM side. [Poonam] First of all apologies for this weird bold font. Not sure why my email client sent two emails one with this bold font and one without. This fix would drop any notification when it runs out of memory. This approach is reasonable. Even we preallocate the notification object, the listener code may fail due to OOM. There are two places doing the object allocation - VM allocates MemoryUsage and the Sensor::trigger implementation. Another idea is to change Sensor::trigger to take the values of init, used, committed, max instead MemoryUsage so that the VM code can be simplified and the Java code will catch all exception and return a boolean to indicate success/fail. This will also prepare moving the invocation of the user listener code in a Java thread instead of ServiceThread to address the issue about ServiceThread is a hidden thread that can’t be debugged. [Poonam] yes, that could also be another way. But for this bug, I would like to keep the changes in sync with how the exceptions in the Java code called from the VM side of the Service Thread are handled. For example, ServiceThread::service_thread_entry() also calls GCNotifier::sendNotification() and here’s how the pending exception is handled. if(has_gc_notification_event) { GCNotifier::sendNotification(CHECK); } void GCNotifier::sendNotification(TRAPS) { GCNotifier::sendNotificationInternal(THREAD); // Clearing pending exception to avoid premature termination of // the service thread if (HAS_PENDING_EXCEPTION) { CLEAR_PENDING_EXCEPTION; } } When we get to the issue of enabling the debugging of the user code currently being executed on the ServiceThread, we will have to be looking at these other notifications as well and I would prefer that we make those changes together under the same bug and not with this change. Please let me know what you think. Thanks, Poonam JDK changes: http://cr.openjdk.java.net/~poonam/8178536/webrev.jdk/ These changes make the triggerAction() a no-op since we need to call this method if MemoryService::create_MemoryUsage_obj() encounters an OOM exception and we want to avoid further potential OOM exceptions in triggerAction(MemoryUsage). Testcase: I have written a testcase though I won't be integrating it along with the fix because the failure is unreliable; the Service Thread does not always fall into the situation that causes it to loop i.e. it has _pending_trigger_count>0 and hits an OOM in its Java calls. http://cr.openjdk.java.net/~poonam/8178536/webrev.hotspot.testcase/ OK. Mandy
