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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: ThreadLocal in SegmentReader
Date Mon, 07 Jul 2008 11:34:11 GMT

Hmmm, I see... you're right.  ThreadLocal is dangerous.

So how would you recommend fixing it?

One thing we can do, in SegmentReader.close, is to call  
termVectorsLocal.set(null).  We do this eg in FieldsReader.close,  
which uses a ThreadLocal to hold thread-private clones of the  
fieldsStream

However, that only works if the very same thread that had opened the  
reader also calls close... which likely often is the case but in  
general is not guaranteed and we should not expect/require.

How about if we set termVectorsLocal itself to null?  Will GC then "do  
the right thing", ie, recognize (eventually) that this ThreadLocal  
instance is no longer referenced, and clear all Objects for all  
threads that were held in it?

Mike

Roman Puchkovskiy wrote:

>
> Unfortunately, it's not ok sometimes. For instance, when Lucene is  
> loaded by
> a web-application from its WEB-INF/lib and SegmentReader is  
> initialized
> during the application start-up (i.e. it's initialized in the thread  
> which
> will never die), this causes problems with unloading of a  
> classloader of the
> web-application. When web-application is undeployed, there's still a
> ThreadLocal in a thread which is external to webapp classloader, and  
> this
> ThreadLocal contains an object which references its class, and this  
> class
> was loaded through web-app classloader and hence references it... so  
> we have
> a chain of hard links from an alive thread to our classloader. In the
> result, the classloader cannot be unloaded together will all its  
> classes, so
> memory waste is considerable.
>
> I've found a way to work this around by creating a new thread during  
> webapp
> start-up and executing code which eventually initializes Lucene  
> indices from
> this thread, so all spawned ThreadLocals go to this short-lived  
> thread and
> get garbage-collected shortly after the webapp start-up is finished.  
> But
> this does not seem to be a pretty solution. Besides, one has to  
> guess that
> ThreadLocals are the problem to invent such a work-around :)
>
>
> Michael McCandless-2 wrote:
>>
>>
>> Well ... if the thread dies, the value in its ThreadLocal should be
>> GC'd.
>>
>> If the thread does not die (eg thread pool in an app server) then the
>> ThreadLocal value remains, but that value is a shallow clone of the
>> original TermVectorsReader and should not be consuming that much RAM
>> per thread.
>>
>> So I think it's OK?
>>
>> Mike
>>
>> Roman Puchkovskiy wrote:
>>
>>>
>>> Hi.
>>>
>>> There's a ThreadLocal field in SegmentReader (it's called
>>> termVectorsLocal).
>>> Some value is put to it, but it's never cleared.
>>> Is it ok? It looks like sometimes this behavior may lead to leaks.
>>>
>>> This is the same in lucene-2.2.0 and lucene-2.3.2.
>>>
>>> Thanks in advance.
>>> -- 
>>> View this message in context:
>>> http://www.nabble.com/ThreadLocal-in-SegmentReader-tp18306230p18306230.html
>>> Sent from the Lucene - Java Users mailing list archive at  
>>> Nabble.com.
>>>
>>>
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>>
>>
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>>
>>
>
> -- 
> View this message in context: http://www.nabble.com/ThreadLocal-in-SegmentReader-tp18306230p18314310.html
> Sent from the Lucene - Java Users mailing list archive at Nabble.com.
>
>
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