The testcase is still not correct - at least with regards to Lucene. Review the ThreadLocal and ThreadLocalMap code again, you will see that references to the ThreadLocal are kept using weak references, in a slot in an array, and entries are reclaimed() (i.e. the slot cleared) PERIODICALLY as new entries are ADDED or RETRIEVED. The array will never decrease in size, but it is unlikely to grow to be very large.
See TheadLocalMap.cleanSomeSlots() and where/when it is called. **** The reason that your test fails is that each entry in the table maintains a HARD reference to the value, and since the entries are only reclaimed PERIODICALLY (for performance reasons - see cleanSomeSlots()), you do have the possibility to run out of memory (since the ThreadLocal values you are storing are EXTREMELY LARGE). In Lucene, the values used in the ThreadLocal are 1/100000 the size, and thus should cause no problems. -----Original Message----- From: Andy Hind [mailto:[EMAIL PROTECTED] Sent: Thursday, March 23, 2006 12:17 PM To: java-dev@lucene.apache.org; [EMAIL PROTECTED] Subject: RE: [jira] Updated: (LUCENE-529) TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException Well, unfortunately, it is your test case that is not equivalent. OK you make 10 times as many objects that are 1/10 the size. But the thread local map is the same size and likely to end up holding the same number of stale entries, so yes the memory footprint is 10 times smaller and it works. If you increase the size of the thread local table by x10 it will fail again. This will not be an exact memory size match due to the underlying algorithm for cleaning stuff out looking at log2(current.no.of thread locals) + ?*log2(table.size) and being dependent on GC, so I have no idea what fraction of stale entries will remain for any size. I suggest you look at the source code for ThreadLocal and how stale entries are removed.... look for "GC" and you will see where the "value" is set to null to help the GC. I should have proposed using SoftReferences not WeakReferences. Apologies. Soft References also work fine. The finalize method is indeed pointless but makes no difference to the result. The point I was making is that it is pointless in lucene. Yes, creating threads is expensive - my point - somewhat tongue-in-cheek is at least they would be clean :-) You can add an assert to make sure the weak/soft reference exists after the Object is created. It does. You *may* be unlucky and have GC take place in between and it go for weak/soft references. I used 1.5.0_04 and checked the source code for ThreadLocal is 1.33 which is the same version as in 1.5.0_06 and the issue was initially found using 1.5.0_06. The issue is for real. You can blame ThreadLocal but it does what it says on the tin. Regards Andy -----Original Message----- From: Robert Engels [mailto:[EMAIL PROTECTED] Sent: 23 March 2006 16:05 To: java-dev@lucene.apache.org; [EMAIL PROTECTED] Subject: RE: [jira] Updated: (LUCENE-529) TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException The only other thing that may be causing your problem is the use of finalize(). This can interfere with the GC ability to GC objects. I am now sure why the finalize() is used in the Lucene ThreadLocal handling. It doesn't seem necessary to me. -----Original Message----- From: Robert Engels [mailto:[EMAIL PROTECTED] Sent: Thursday, March 23, 2006 10:00 AM To: java-dev@lucene.apache.org Subject: RE: [jira] Updated: (LUCENE-529) TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException Importance: High Your testcase is invalid. Reduce the size by 10, increase the repeat by 10, (SAME amount of memory use), and it works fine. The reason it works in the one case is that you use new WeakReference(new Arrary()), - since the array cannot be referenced, it is immediately GC'd. You should have noticed since there were no finalization messages printed. You can demonstrate this clearly by adding an else to the if in the finalize() to print out that the object was indeed finalized. ThreadLocal's work and are GC'd correctly. There is something else wrong in your system. I ran the test using 1.5.0._06. -----Original Message----- From: Andy Hind (JIRA) [mailto:[EMAIL PROTECTED] Sent: Thursday, March 23, 2006 7:07 AM To: java-dev@lucene.apache.org Subject: [jira] Updated: (LUCENE-529) TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException [ http://issues.apache.org/jira/browse/LUCENE-529?page=all ] Andy Hind updated LUCENE-529: ----------------------------- Attachment: ThreadLocalTest.java Attached is a test which you can use to see how ThreadLocals are left around. Getting an out of memory exception depends on a number things....it is set up to fail for 64M Now I understand what is going on, there are a few alternatives: 1) set null on close - fine for single thread use - probably leaves (n-1)*segments*2 things hanging around for n threaded use 2) Use a weak reference and leave it up to GC to get rid of the referent when it is not being used 3) Manage the things youself by object id and thread id - and clean up on object close() I would go with option 1) and 2) although it may mean things get GCed before a call to close() when not used. The fix I initially suggested is in production, and has been stress tested with a couple of hundred users continually pounding the app, but not for multithreaded use of IndexReaders. Each time does a couple of simple searches with no clever reuse of index readers (which is on the todo list) I do not see how setting the thread local to null on close() has any negative impact. You are not going to use the cached information again?? Before the fix: 10-100 threads - 1G JVM - OOM in a few hours After: 10-100 threads 256M JVM - days with a flat memory footprint I am not sure why the thread local table is so big for us, but that is not really the issue. It could just be building lots of IndexReaders (with thread locals hanging - probably making 10/instance ) and gc not kicking in so this table grows and can hold a lot of stale entries. I may get time to investigate further > TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException > ------------------------------------------------------------------------ ------------------------------- > > Key: LUCENE-529 > URL: http://issues.apache.org/jira/browse/LUCENE-529 > Project: Lucene - Java > Type: Bug > Components: Index > Versions: 1.9 > Environment: Lucene 1.4.3 with 1.5.0_04 JVM or newer......will aplpy to 1.9 code > Reporter: Andy Hind > Attachments: ThreadLocalTest.java > > TermInfosReader uses an instance level ThreadLocal for enumerators. > This is a transient/odd memory leak in lucene 1.4.3-1.9 and applies to current JVMs, > not just an old JVM issue as described in the finalizer of the 1.9 code. > There is also an instance level thread local in SegmentReader....which will have the same issue. > There may be other uses which also need to be fixed. > I don't understand the intended use for these variables.....however > Each ThreadLocal has its own hashcode used for look up, see the ThreadLocal source code. Each instance of TermInfosReader will be creating an instance of the thread local. All this does is create an instance variable on each thread when it accesses the thread local. Setting it to null in the finaliser will set it to null on one thread, the finalizer thread, where it has never been created. There is no point to this :-( > I assume there is a good concurrency reason why an instance variable can not be used... > I have not used multi-threaded searching, but I have used a lot of threads each making searchers and searching. > 1.4.3 has a clear memory leak caused by this thread local. This use case above is definitely solved by setting the thread local to null in the close(). This at least has a chance of being on the correct thread :-) > I know reusing Searchers would help but that is my choice and I will get to that later .... > Now you wnat to know why.... > Thread locals are stored in a table of entries. Each entry is *weak reference* to the key (Here the TermInfosReader instance) and a *simple reference* to the thread local value. When the instance is GCed its key becomes null. > This is now a stale entry in the table. > Stale entries are cleared up in an ad hoc way and until they are cleared up the value will not be garbage collected. > Until the instance is GCed it is a valid key and its presence may cause the table to expand. > See the ThreadLocal code. > So if you have lots of threads, all creating thread locals rapidly, you can get each thread holding a large table of thread locals which all contain many stale entries and preventing some objects from being garbage collected. > The limited GC of the thread local table is not enough to save you from running out of memory. > Summary: > ======== > - remove finalizer() > - set the thread local to null in close() > - values will be available for gc -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
