[LUCENE-3841] CloseableThreadLocal does not work well with Tomcat thread pooling

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.5
• Fix Version/s: 3.6, 4.0-ALPHA
• Component/s: core/other

Environment

Lucene/Tika/Snowball running in a Tomcat web application

• Lucene Fields: New
• Assignee: Michael McCandless
• Reporter: Matthew Bellew
• Created: 02/Mar/12 23:56
• Updated: 28/Aug/22 13:10
• Resolved: 14/Mar/12 15:43

Description

We tracked down a large memory leak (effectively a leak anyway) caused
by how Analyzer users CloseableThreadLocal.
CloseableThreadLocal.hardRefs holds references to Thread objects as
keys. The problem is that it only frees these references in the set()
method, and SnowballAnalyzer will only call set() when it is used by a
NEW thread.

The problem scenario is as follows:

The server experiences a spike in usage (say by robots or whatever)
and many threads are created and referenced by
CloseableThreadLocal.hardRefs. The server quiesces and lets many of
these threads expire normally. Now we have a smaller, but adequate
thread pool. So CloseableThreadLocal.set() may not be called by
SnowBallAnalyzer (via Analyzer) for a long time. The purge code is
never called, and these threads along with their thread local storage
(lucene related or not) is never cleaned up.

I think calling the purge code in both get() and set() would have
avoided this problem, but is potentially expensive. Perhaps using
WeakHashMap instead of HashMap may also have helped. WeakHashMap
purges on get() and set(). So this might be an efficient way to
clean up threads in get(), while set() might do the more expensive
Map.keySet() iteration.

Our current work around is to not share SnowBallAnalyzer instances
among HTTP searcher threads. We open and close one on every request.

Thanks,
Matt