FAQ
You do not need to create a new RAMDirectory - just write to the
existing one, and then reopen() the IndexReader using it.

This will prevent lots of big objects being created. This may be the
source of your problem.

Even if the Segment is closed, the ThreadLocal will no longer be
referenced, but there will still be a reference to the
SegmentTermEnum (which will be cleared when the thread dies, or "most
likely" when new thread locals on that thread a created, so here is a
potential problem.

Thread 1 does a search, creates a thread local that references the
RAMDir (A).
Thread 2 does a search, creates a thread local that references the
RAMDir (A).

All readers, are closed on RAMDir (A).

A new RAMDir (B) is opened.

There may still be references in the thread local maps to RAMDir A
(since no new thread local have been created yet).

So you may get OOM depending on the size of the RAMDir (since you
would need room for more than 1). If you extend this out with lots
of threads that don't run very often, you can see how you could
easily run out of memory. "I think" that ThreadLocal should use a
ReferenceQueue so stale object slots can be reclaimed as soon as the
key is dereferenced - but that is an issue for SUN.

This is why you don't want to create new RAMDirs.

A good rule of thumb - don't keep references to large objects in
ThreadLocal (especially indirectly). If needed, use a "key", and
then read the cache using a the "key".
This would be something for the Lucene folks to change.
On Sep 10, 2008, at 10:44 AM, Chris Lu wrote:

I am really want to find out where I am doing wrong, if that's the
case.

Yes. I have made certain that I closed all Readers/Searchers, and
verified that through memory profiler.

Yes. I am creating new RAMDirectory. But that's the problem. I need
to update the content. Sure, if no content update and everything
the same, of course no OOM.

Yes. No guarantee of the thread schedule. But that's the problem.
If Lucene is using ThreadLocal to cache lots of things by the
Thread as the key, and no idea when it'll be released. Of course
ThreadLocal is not Lucene's problem...

Chris

On Wed, Sep 10, 2008 at 8:34 AM, robert engels
wrote:
It is basic Java. Threads are not guaranteed to run on any sort of
schedule. If you create lots of large objects in one thread,
releasing them in another, there is a good chance you will get an
OOM (since the releasing thread may not run before the OOM
occurs)... This is not Lucene specific by any means.

It is a misunderstanding on your part about how GC works.

I assume you must at some point be creating new RAMDirectories -
otherwise the memory would never really increase, since the
IndexReader/enums/etc are not very large...

When you create a new RAMDirectories, you need to BE CERTAIN !!!
that the other IndexReaders/Searchers using the old RAMDirectory
are ALL CLOSED, otherwise their memory will still be in use, which
leads to your OOM...

On Sep 10, 2008, at 10:16 AM, Chris Lu wrote:

I do not believe I am making any mistake. Actually I just got an
email from another user, complaining about the same thing. And I
am having the same usage pattern.

After the reader is opened, the RAMDirectory is shared by several
objects.
There is one instance of RAMDirectory in the memory, and it is
holding lots of memory, which is expected.

If I close the reader in the same thread that has opened it, the
RAMDirectory is gone from the memory.
If I close the reader in other threads, the RAMDirectory is left
in the memory, referenced along the tree I draw in the first email.

I do not think the usage is wrong. Period.

-------------------------------------
Hi,

i found a forum post from you here [1] where you mention that you
have a memory leak using the lucene ram directory. I'd like to ask
you
if you already have resolved the problem and how you did it or maybe
you know where i can read about the solution. We are using
RAMDirectory too and figured out, that over time the memory
consumption raises and raises until the system breaks down but only
when we performing much index updates. if we only create the index
and
don't do nothing except searching it, it work fine.

maybe you can give me a hint or a link,
greetz,
-------------------------------------

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

On Wed, Sep 10, 2008 at 7:12 AM, robert engels
wrote:
Sorry, but I am fairly certain you are mistaken.

If you only have a single IndexReader, the RAMDirectory will be
shared in all cases.

The only memory growth is any buffer space allocated by an
IndexInput (used in many places and cached).

Normally the IndexInput created by a RAMDirectory do not have any
buffer allocated, since the underlying store is already in memory.

You have some other problem in your code...
On Sep 10, 2008, at 1:10 AM, Chris Lu wrote:

Actually, even I only use one IndexReader, some resources are
cached via the ThreadLocal cache, and can not be released unless
all threads do the close action.

SegmentTermEnum itself is small, but it holds RAMDirectory along
the path, which is big.

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

On Tue, Sep 9, 2008 at 10:43 PM, robert engels
wrote:
You do not need a pool of IndexReaders...

It does not matter what class it is, what matters is the class
that ultimately holds the reference.

If the IndexReader is never closed, the SegmentReader(s) is never
closed, so the thread local in TermInfosReader is not cleared
(because the thread never dies). So you will get one
SegmentTermEnum, per thread * per segment.

The SegmentTermEnum is not a large object, so even if you had 100
threads, and 100 segments, for 10k instances, seems hard to
believe that is the source of your memory issue.

The SegmentTermEnum is cached by thread since it needs to
enumerate the terms, not having a per thread cache, would lead to
lots of random access when multiple threads read the index - very
slow.

You need to keep in mind, what if every thread was executing a
search simultaneously - you would still have 100x100
SegmentTermEnum instances anyway ! The only way to prevent that
would be to create and destroy the SegmentTermEnum on each call
(opening and seeking to the proper spot) - which would be SLOW
SLOW SLOW.
On Sep 10, 2008, at 12:19 AM, Chris Lu wrote:

I have tried to create an IndexReader pool and dynamically
create searcher. But the memory leak is the same. It's not
related to the Searcher class specifically, but the
SegmentTermEnum in TermInfosReader.

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

On Tue, Sep 9, 2008 at 10:14 PM, robert engels
wrote:
A searcher uses an IndexReader - the IndexReader is slow to
open, not a Searcher. And searchers can share an IndexReader.

You want to create a single shared (across all threads/users)
IndexReader (usually), and create an Searcher as needed and
dispose. It is VERY CHEAP to create the Searcher.

I am fairly certain the javadoc on Searcher is incorrect. The
warning "For performance reasons it is recommended to open only
one IndexSearcher and use it for all of your searches" is not
true in the case where an IndexReader is passed to the ctor.

Any caching should USUALLY be performed at the IndexReader level.

You are most likely using the "path" ctor, and that is the
source of your problems, as multiple IndexReader instances are
being created, and thus the memory use.

On Sep 9, 2008, at 11:44 PM, Chris Lu wrote:

On J2EE environment, usually there is a searcher pool with
several searchers open.
The speed to opening a large index for every user is not
acceptable.

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

On Tue, Sep 9, 2008 at 9:03 PM, robert engels
wrote:
You need to close the searcher within the thread that is using
it, in order to have it cleaned up quickly... usually right
after you display the page of results.

If you are keeping multiple searcher refs across multiple
threads for paging/whatever, you have not coded it correctly.

Imagine 10,000 users - storing a searcher for each one is not
going to work...
On Sep 9, 2008, at 10:21 PM, Chris Lu wrote:

Right, in a sense I can not release it from another thread.
But that's the problem.

It's a J2EE environment, all threads are kind of equal. It's
simply not possible to iterate through all threads to close
the searcher, thus releasing the ThreadLocal cache.
Unless Lucene is not recommended for J2EE environment, this
has to be fixed.

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!


On Tue, Sep 9, 2008 at 8:14 PM, robert engels
wrote:
Your code is not correct. You cannot release it on another
thread - the first thread may creating hundreds/thousands of
instances before the other thread ever runs...
On Sep 9, 2008, at 10:10 PM, Chris Lu wrote:

If I release it on the thread that's creating the searcher,
by setting searcher=null, everything is fine, the memory is
released very cleanly.
My load test was to repeatedly create a searcher on a
RAMDirectory and release it on another thread. The test will
quickly go to OOM after several runs. I set the heap size to
be 1024M, and the RAMDirectory is of size 250M. Using some
profiling tool, the used size simply stepped up pretty
obviously by 250M.

I think we should not rely on something that's a "maybe"
behavior, especially for a general purpose library.

Since it's a multi-threaded env, the thread that's creating
the entries in the LRU cache may not go away quickly(actually
most, if not all, application servers will try to reuse
threads), so the LRU cache, which uses thread as the key, can
not be released, so the SegmentTermEnum which is in the same
class can not be released.

And yes, I close the RAMDirectory, and the fileMap is
released. I verified that through the profiler by directly
checking the values in the snapshot.

Pretty sure the reference tree wasn't like this using code
before this commit, because after close the searcher in
another thread, the RAMDirectory totally disappeared from the
memory snapshot.

--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

On Tue, Sep 9, 2008 at 5:03 PM, Michael McCandless
wrote:

Chris Lu wrote:

The problem should be similar to what's talked about on this
discussion.
http://lucene.markmail.org/message/keosgz2c2yjc7qre?
q=ThreadLocal

The "rough" conclusion of that thread is that, technically,
this isn't a memory leak but rather a "delayed freeing"
problem. Ie, it may take longer, possibly much longer, than
you want for the memory to be freed.


There is a memory leak for Lucene search from Lucene-1195.
(svn r659602, May23,2008)

This patch brings in a ThreadLocal cache to TermInfosReader.

One thing that confuses me: TermInfosReader was already using
a ThreadLocal to cache the SegmentTermEnum instance. What
was added in this commit (for LUCENE-1195) was an LRU cache
storing Term -> TermInfo instances. But it seems like it's
the SegmentTermEnum instance that you're tracing below.


It's usually recommended to keep the reader open, and reuse
it when
possible. In a common J2EE application, the http requests are
usually
handled by different threads. But since the cache is
ThreadLocal, the cache
are not really usable by other threads. What's worse, the
cache can not be
cleared by another thread!

This leak is not so obvious usually. But my case is using
RAMDirectory,
having several hundred megabytes. So one un-released resource
is obvious to
me.

Here is the reference tree:
org.apache.lucene.store.RAMDirectory
- directory of org.apache.lucene.store.RAMFile
- file of org.apache.lucene.store.RAMInputStream
- base of org.apache.lucene.index.CompoundFileReader
$CSIndexInput
- input of org.apache.lucene.index.SegmentTermEnum
- value of java.lang.ThreadLocal
$ThreadLocalMap$Entry

So you have a RAMDir that has several hundred MB stored in
it, that you're done with yet through this path Lucene is
keeping it alive?

Did you close the RAMDir? (which will null its fileMap and
should also free your memory).

Also, that reference tree doesn't show the ThreadResources
class that was added in that commit -- are you sure this
reference tree wasn't before the commit?

Mike

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--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!





--
Chris Lu
-------------------------
Instant Scalable Full-Text Search On Any Database/Application
site: http://www.dbsight.net
demo: http://search.dbsight.com
Lucene Database Search in 3 minutes: http://wiki.dbsight.com/
index.php?title=Create_Lucene_Database_Search_in_3_minutes
DBSight customer, a shopping comparison site, (anonymous per
request) got 2.6 Million Euro funding!

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