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 <[EMAIL PROTECTED]>
To: java-user@lucene.apache.org
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 org.apache.lucene.store.FSIndexInput
27: 86120 4822720 org.apache.lucene.index.TermInfosReader
33: 86130 3445200 org.apache.lucene.index.SegmentInfo
36: 87355 2795360 org.apache.lucene.store.FSIndexInput$Descriptor
38: 86120 2755840 org.apache.lucene.index.FieldsReader
39: 86050 2753600 org.apache.lucene.index.CompoundFileReader
42: 46903 2251344 org.apache.lucene.index.SegmentInfos
43: 93778 2250672 org.apache.lucene.search.FieldCacheImpl$Entry
45: 93778 1500448
org.apache.lucene.search.FieldCacheImpl$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!!!! Yeeees!
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 request?
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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