On 01/22/2014 03:19 AM, David Holmes wrote:
Hi Peter,On 22/01/2014 12:00 AM, Peter Levart wrote:Hi, David, Kalyan, Summing up the discussion, I propose the following patch for ReferenceHandler:http://cr.openjdk.java.net/~plevart/jdk9-dev/OOMEInReferenceHandler/webrev.01/I can live with it, though it maybe that once Cleaner has been preloaded instanceof can no longer throw OOME. Can't be 100% sure. And there's some duplication/verbosity in the commentary that could be trimmed down :)Any specific reason to use Unsafe to do the preload rather than Class.forName ? Does this force Unsafe to be loaded earlier than it otherwise would?
Good question. In systemDictionary.hpp they are both on the preloaded list in this order:
do_klass(Reference_klass, java_lang_ref_Reference, Pre ) \
...do_klass(misc_Unsafe_klass, sun_misc_Unsafe, Pre ) \
So when Reference is initialized, the Unsafe is already loaded. But I don't know if it is already initialized. This should be studied.
I'll try to find out what is the case and get back to you. Regards, Peter
Thanks, Davidall 10 java/lang/ref tests pass on my PC (including OOMEInReferenceHandler). I kindly ask Kalyan to try to re-run the OOMEInReferenceHandler test with this code and report any failure. Thanks, Peter On 01/21/2014 08:57 AM, David Holmes wrote:On 21/01/2014 4:54 PM, Peter Levart wrote:On 01/21/2014 03:22 AM, David Holmes wrote:Hi Peter, I do not see Cleaner being loaded prior to the main class on either Windows or Linux. Which platform are you on? Did you see it loaded before the main class or as part of executing it?Before. The main class is empty: public class Test { public static void main(String... a) {} } Here's last few lines of -verbose:class: [Loaded java.util.TimeZone from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] [Loaded sun.util.calendar.ZoneInfo from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] [Loaded sun.util.calendar.ZoneInfoFile from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] [Loaded sun.util.calendar.ZoneInfoFile$1 from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] [Loaded java.io.DataInput from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] [Loaded java.io.DataInputStream from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar] *[Loaded sun.misc.Cleaner from /home/peter/Apps64/jdk1.8.0-ea-b121/jre/lib/rt.jar]*Curious. I wonder what the controlling factor is ??I'm on linux, 64bit and using official EA build 121 of JDK 8...But if I try with JDK 7u45, I don't see it. So perhaps it would be goodto trigger Cleaner loading and initialization as part of ReferenceHandler initialization to play things safe.If we do that for Cleaner we may as well do it for InterruptedException too.Also, it is not that I think ReferenceHandler is responsible for reporting OOME, but that it is responsible for reporting that it was unable to perform a clean or enqueue because of OOME.This would be necessary if we skipped a Reference because of OOME, butif we just re-try until we eventually succeed, nothing is lost, nothingto report (but a slow response)...Agreed - just trying to clarify things.Your suggested approach seems okay though I'm not sure why we shouldn't help things along by calling System.gc() ourselves rather than just yielding and hoping things will get cleaned up elsewhere. But for the present purposes your approach will suffice I think.Maybe my understanding is wrong but isn't the fact that OOME is rised aconsequence of that VM has already attempted to clear things up (executing a GC round synchronously) but didn't succeed to make enough free space to satisfy the allocation request? If this is only how some collectors/allocators are implemented and not a general rule, then we should put a System.gc() in place of Thread.yield(). Should we alsocombine that with Thread.yield()? I'm concerned of a possibility that we spin, consume too much CPU (ReferenceHandler thread has MAX priority) so that other threads dont' get enough CPU time to proceed and clean things up (we hope other threads will also get OOME and release things as theirstacks unwind...).You are probably right about the System.gc() - OOME should be thrown after GC fails to create space, so it really needs some other thread to drop live references to allow further space to be reclaimed. But note that Thread.yield() can behave badly on some linux systems too, so spinning is still a possibility - but either way this would only be "really bad" on a uniprocessor system where yield() is unlikely to misbehave. David -----Regards, PeterThanks, David On 20/01/2014 6:42 PM, Peter Levart wrote:On 01/20/2014 09:00 AM, Peter Levart wrote:On 01/20/2014 02:51 AM, David Holmes wrote:Hi Peter, On 17/01/2014 11:24 PM, Peter Levart wrote:On 01/17/2014 02:13 PM, Peter Levart wrote:// Fast path for cleaners boolean isCleaner = false; try { isCleaner = r instanceof Cleaner; } catch (OutofMemoryError oome) { continue; } if (isCleaner) { ((Cleaner)r).clean(); continue; }Hi David, Kalyan, I've caught-up now. Just thinking: is "instanceof Cleaner" throwingOOME as a result of loading the Cleaner class? Wouldn't the above code then throw some error also in ((Cleaner)r) - the checkcast,since Cleaner class would not be successfully initialized?Well, no. The above code would just skip Cleaner processing in this situation. And will never be doing it again after the heap is freed... So it might be good to load and initialize Cleaner class as part of ReferenceHandler initialization to ensure correct operation...Well, yes and no. Let me try once more:Above code will skip Cleaner processing if the 1st time "instanceof Cleaner" is executed, OOME is thrown as a consequence of full heapwhile loading and initializing the Cleaner class.Yes - I was assuming that this would not fail the very first time andso the Cleaner class would already be loaded. Failing to be able toload the Cleaner class was one of the potential issues flagged earlier with this problem. I was actually assuming that Cleaner wouldbe loaded already due to some actual Cleaner subclasses being used,but this does not happen as part of the default initialization. :( The irony being that if the Cleaner class is not loaded then r cannot be an instance of Cleaner and so we would fail to load the classin a case where we didn't need it anyway. What I wanted to focus on here was an OOME from the instanceof itself, but as you say that might trigger classloading of Cleaner (which is not what I was interested in).The 2nd time the "instanceof Cleaner" is executed after such OOME, the same line would throw NoClassDefFoundError as a consequence of referencing a class that failed initialization. Am I right?instanceof is not one of the class initialization triggers, so we should not see an OOME generated due to a class initializationexception and so the class will not be put into the Erroneous stateand so subsequent attempts to use the class will not automatically trigger NoClassdefFoundError.If OOME occurs during actual loading/linking of the class Cleaner itis unclear what would happen on subsequent attempts. OOME is not aLinkageError that must be rethrown on subsequent attempts, and it ispotentially a transient condition, so I would expect a re-load attempt to be allowed. However we are now deep into the details ofthe VM and it may well depend on the exact place from which the OOMEoriginates.The bottom line with the current problem is that there are multiplenon-obvious paths by which the ReferenceHandler can encounter anOOME. In such cases we do not want the ReferenceHandler to terminate - which implies catching the OOME and continuing. However we also donot want to silently skip Cleaner processing or reference queueprocessing - as that would lead to hard to diagnoze bugs. But tryingto report the problem may not be possible due to being out-of-memory. It may be that we need to break things up into multiple try/catch blocks, where each catch does a System.gc() and then reports that theOOME occurred. Of course the reporting must still be in a try/catch for the OOME. Though at some point letting the ReferenceHandler diemay be the only way to "report" a major memory problem. DavidHm... If I give -verbose:class option to run a simple test program: public class Test { public static void main(String... a) {} } I see Cleaner class being loaded before Test class. I don't see by which tread or if it might get loaded after main() starts, but Isuspect that loading of Cleaner is not a problem here. Initializationof Cleaner class is not performed by ReferenceHandler thread as you pointed out. The instanceof does not trigger it and if it returns true then Cleaner has already been initialized. So there must be some other cause for instanceof throwing OOME...What do you say about this variant of ReferenceHandler.run() method:public void run() { for (;;) { Reference r; Cleaner c; synchronized (lock) { r = pending; if (r != null) { // instanceof operator might throw OOME sometimes. Just retry after // yielding - might have better luck next time... try {c = r instanceof Cleaner ? (Cleaner) r :null; } catch (OutOfMemoryError x) { Thread.yield(); continue; } pending = r.discovered; r.discovered = null; } else {// The waiting on the lock may cause an OOMEbecause it may try to allocate// exception objects, so also catch OOME hereto avoid silent exit of the // reference handler thread. // // Explicitly define the order of the two exceptions we catch here // when waiting for the lock. //// We do not want to try to potentially loadthe InterruptedException class// (which would be done if this was its firstuse, and InterruptedException // were checked first) in this situation. //// This may lead to the VM not ever trying toload the InterruptedException // class again. try { try { lock.wait(); } catch (OutOfMemoryError x) { } } catch (InterruptedException x) { } continue; } } // Fast path for cleaners if (c != null) { c.clean(); continue; } ReferenceQueue q = r.queue; if (q != ReferenceQueue.NULL) q.enqueue(r); } } ... it tries to not consume and skip Cleaner instances when OOME is caught. I don't think ReferenceHandler is to make responsible for reporting OOMEs. Full heap is a global condition and ReferenceHandler is the last to accuse for it. Regards, PeterHi David, I think the following variation is even better. It executes Thread.yield() after catching OOME but outside synchronized block so that given CPU slice can be used by GC threads to make progressenqueueing pending References (they are not able to enqueue them whileReferenceHandler is holding the lock): public void run() { for (;;) { Reference r; Cleaner c; try { try { synchronized (lock) { r = pending; if (r != null) { // 'instanceof' might throw OOME sometimes so do this before // unlinking 'r' from the 'pending' chain... c = r instanceof Cleaner ? (Cleaner) r : null; // unlink 'r' from 'pending' chain pending = r.discovered; r.discovered = null; } else { // The waiting on the lock may cause an OOME because it may try to allocate // exception objects. lock.wait(); continue; } } } catch (OutOfMemoryError x) {// Catch OOME from 'r instanceof Cleaner' or'lock.wait()' 1st so that we don't // try to potentially load the InterruptedException class// (which would be done if this was its firstuse, and InterruptedException // were checked first) in this situation. // Give other threads CPU time so they hopefully release some objects and GC // clears some heap. // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above// persistently throws OOME for some time...Thread.yield(); // retry continue; } } catch (InterruptedException x) {// Catch InterruptedException from 'lock.wait()'and retry continue; } // Fast path for cleaners if (c != null) { c.clean(); continue; } ReferenceQueue q = r.queue; if (q != ReferenceQueue.NULL) q.enqueue(r); } } Regards, Peter
