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From "Chris Lu" <chris...@gmail.com>
Subject Re: ThreadLocal causing memory leak with J2EE applications
Date Wed, 10 Sep 2008 03:10:43 GMT
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 <
lucene@mikemccandless.com> 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!

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