TermInfosReader and other + instance ThreadLocal => transient/odd memory leaks => OutOfMemoryException

• Key: LUCENE-529
• Type: Bug
• Status: Closed
• Resolution: Duplicate
• Priority: Major
• Assignee: Unassigned
• Reporter: Andy Hind
• Component/s: Index
• Affects Version/s: 1.9

Description

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