[ 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
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