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From Bernhard Messer <>
Subject Re: ThreadLocal leak (was Re: Leaking org.apache.lucene.index.* objects)
Date Mon, 18 Dec 2006 17:25:47 GMT

i figured out a similar problem when running a very heavy loaded search 
application in a servlet container. The reasone using ThreadLocals was 
to get rid of synchronized method calls e.g in TermVectorsReader which 
would break down the overall search performance. Currently i do not see 
an easy solution to fix both, the synchronization and ThreadLocal problem.


Otis Gospodnetic wrote:
> Moving to java-dev, I think this belongs here.
> I've been looking at this problem some more today and reading about ThreadLocals.  It's
easy to misuse them and end up with memory leaks, apparently... and I think we may have this
problem here.
> The problem here is that ThreadLocals are tied to Threads, and I think the assumption
in TermInfosReader and SegmentReader is that (search) Threads are short-lived: they come in,
scan the index, do the search, return and die.  In this scenario, their ThreadLocals go to
heaven with them, too, and memory is freed up.
> But when Threads are long-lived, as they are in thread pools (e.g. those in servlet containers),
those ThreadLocals stay alive even after a single search request is done.  Moreover, the Thread
is reused, and the new TermInfosReader and SegmentReader put some new values in that ThreadLocal
on top of the old values (I think) from the previous search request.  Because the Thread still
has references to ThreadLocals and the values in them, the values never get GCed.
> I tried making ThreadLocals in TIR and SR static, I tried wrapping values saved in TLs
in WeakReference, I've tried using WeakHashMap like in Robert Engel's FixedThreadLocal class
from LUCENE-436, but nothing helped.  I thought about adding a public static method to TIR
and SR, so one could call it at the end of a search request (think servlet filter) and clear
the TL for the current thread, but that would require making TIR and SR public and I'm not
100% sure if it would work, plus that exposes the implementation details too much.
> I don't have a solution yet.
> But do we *really* need ThreadLocal in TIR and SR?  The only thing that TL is doing there
is acting as a per-thread storage of some cloned value (in TIR we clone SegmentTermEnum and
in SR we clone TermVectorsReader).  Why can't we just store those cloned values in instance
variables?  Isn't whoever is calling TIR and SR going to be calling the same instance of TIR
and SR anyway, and thus get access to those cloned values?
> I'm really amazed that we haven't heard any reports about this before.  I am not sure
why my application started showing this leak only about 3 weeks ago.  It is getting more pounded
on than before, so maybe that made the leak more obvious.  My guess is that more common Lucene
usage is with a single index or a small number of them, and with short-lived threads, where
this problem isn't easily visible.  In my case I deal with a few tens of thousands of indices
and several parallel search threads that live forever in the thread pool.
> Any thoughts about this or possible suggestions for a fix?
> Thanks,
> Otis
> ----- Original Message ----
> From: Otis Gospodnetic <>
> To:
> Sent: Friday, December 15, 2006 12:28:29 PM
> Subject: Leaking org.apache.lucene.index.* objects
> Hi,
> About 2-3 weeks ago I emailed about a memory leak in my application.  I then found some
problems in my code (I wasn't closing IndexSearchers explicitly) and took care of those. 
Now I see my app is still leaking memory - jconsole clearly shows the "Tenured Gen" memory
pool getting filled up until I hit the OOM, but I can't seem to pin-point the source.
> I found that a bunch or o.a.l.index.* objects are not getting GCed, even though they
should.  For example:
> $ jmap -histo:live 7825 | grep apache.lucene.index | head -20 | sort -k2 -nr
> num   #instances    #bytes  class name
> --------------------------------------
>   4:   1764840    98831040  org.apache.lucene.index.CompoundFileReader$CSIndexInput
>   5:   2119215    67814880  org.apache.lucene.index.TermInfo
>   7:   1112459    35598688  org.apache.lucene.index.SegmentReader$Norm
>   9:   2132311    34116976  org.apache.lucene.index.Term
>  12:   1117897    26829528  org.apache.lucene.index.FieldInfo
>  13:    225340    18027200  org.apache.lucene.index.SegmentTermEnum
>  15:    589727    14153448  org.apache.lucene.index.TermBuffer
>  21:     86033     8718504  [Lorg.apache.lucene.index.TermInfo;
>  20:     86033     8718504  [Lorg.apache.lucene.index.Term;
>  23:     86120     7578560  org.apache.lucene.index.SegmentReader
>  26:     90501     5068056
>  27:     86120     4822720  org.apache.lucene.index.TermInfosReader
>  33:     86130     3445200  org.apache.lucene.index.SegmentInfo
>  36:     87355     2795360$Descriptor
>  38:     86120     2755840  org.apache.lucene.index.FieldsReader
>  39:     86050     2753600  org.apache.lucene.index.CompoundFileReader
>  42:     46903     2251344  org.apache.lucene.index.SegmentInfos
>  43:     93778     2250672$Entry
>  45:     93778     1500448$CreationPlaceholder
>  47:     86510     1384160  org.apache.lucene.index.FieldInfos
> I'm running my app in search-only mode - no adds or deletes.
> The counts of these objects just keeps going up, even though I am explicitly closing
the IndexSearcher.  I can see that file descriptors _are_ freed up after searcher.close(),
because lsof no longer shows them, but the above objects just linger and accumulate, even
when I force GC via jconsole or via the profiler.
> I thought maybe various *Readers are not getting close()d, but I've double-checked all
*Readers above, and they all seem to close their IndexInput references.  The static nested
class CompoundFileReader.CSIndexInput has a close() without any implementation.  At first
I thought that was an omission, but adding a close of the inner IndexInput there resulted
in a search-time error.  I've added the lovely print debugging to various close() methods
and see those methods being called.  I've added finalize() with some print debugging to SegmentReader,
TermInfosReader, SegmentTermEnum, FieldsReader, and CompoundFileReader.  All but CFReader
get finalized after a while.
> My application is running as a webapp and has thousands of separate indices.  This means
it's very multi-threaded and the servlet container has a pool of threads that handle requests,
and each request may be for a different index.  I cache IndexSearchers for a while, and purge/close
them every N minutes if they have been idle more than M minutes.
> It occurred to me last night that things like TermInfosReader and SegmentReader are using
ThreadLocal, and since threads are used in a thread pool, and thus shared with requests handling
searches against different indices, it's not clear to me what happens with object instances
that are put in those ThreadLocals in such scenario.  Aren't things going to step on each
others' toes?
> TIR has close() and SR has doClose(), so I put <TL inst>.set(null) there.  This
immediately got rid of those instances of CompoundFileReader.CSIndexInput in my dev environment!!!!
> But in my dev environment I tested my additions by slamming my app against a *single*
index.  I took my modified Lucene to production, and quickly saw all those o.a.l.index.* objects
accumulate again.  I also see a lot of ThreadLocal's kids:
>  16:    419387    13420384  java.lang.ThreadLocal$ThreadLocalMap$Entry
> I *think* that points out to some issues with how that ThreadLocal is used there, in
a multi-threaded, multi-index environments.
> I'm running JDK 6, and while this problem sounds a bit like LUCENE-436, I'm not yet sure
if it's the same thing.  
> Because my IndexSearchers (and thus all those o.a.l.index.* objects) are long-lived,
and threads are shared and reused for searching of other indices, those close() and doClose()
methods are not called at the end of the request life-cycle, so at the end of the request
those TL instances will *still* have something in them.  When their thread is later reused
for searching of another index, new data will be put in them, but the old data will never
be cleaned out!  No?
> It seems a bit odd, but with this ThreadLocals, shouldn't a multi-threaded, multi-index
webapp really have to "clean" those ThreadLocal instances either before or at the end of the
> I'm running out of ideas, and was wondering if anyone has any thoughts about what could
still be holding references to the above classes.  I have some 20-30MB memory snapshots (via
YourKit) and heap dumps (via jmap), if anyone is interested.
> Thanks,
> Otis
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