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From "Chris Lu" <chris...@gmail.com>
Subject Re: ThreadLocal causing memory leak with J2EE applications
Date Wed, 10 Sep 2008 03:21:15 GMT
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 <rengels@ix.netcom.com> 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 <
> 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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