lucene-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Adrian Tarau <adrian.ta...@gmail.com>
Subject Re: ThreadLocal in SegmentReader
Date Wed, 09 Jul 2008 20:59:36 GMT
As soon as the object can be collected...which doesn't happen in case 
of  (some) thread pools.

Here is an example : 
http://www.opensubscriber.com/message/ojb-user@db.apache.org/1500135.html 
(it's pretty old but you can find newer examples googling around). There 
are some replies with workarounds.

I'm not saying there is something wrong using ThreadLocal in Lucene, but 
it could cause problems when classloaders have to be released, as 
Michael noticed.

As Jason suggested, a global manager for ThreadLocals could be the solution.
I prefer to use ThreadLocal to pass "context" objects but I didn't use 
them to store long-lived objects. I'm always releasing the object after 
it was used. I'm not sure if is too preventive, but I had some similar 
problems with web applications(redeploy), ThreadLocals  and long-lived 
objects.

robert engels wrote:
> Aside from the pre-1.5 thread local "perceived leak", there are no 
> issues with ThreadLocals if used properly.
>
> There is no need for try/finally blocks, unless you MUST release 
> resources immediately, usually this is not the case, which is why a 
> ThreadLocal is used in the first place.
>
> From the ThreadLocalMap javadoc...
>
>  /**
>      * ThreadLocalMap is a customized hash map suitable only for
>      * maintaining thread local values. No operations are exported
>      * outside of the ThreadLocal class. The class is package private to
>      * allow declaration of fields in class Thread.  To help deal with
>      * very large and long-lived usages, the hash table entries use
>      * WeakReferences for keys. However, since reference queues are not
>      * used, stale entries are guaranteed to be removed only when
>      * the table starts running out of space.
>      */
>
> /**
>          * Heuristically scan some cells looking for stale entries.
>          * This is invoked when either a new element is added, or
>          * another stale one has been expunged. It performs a
>          * logarithmic number of scans, as a balance between no
>          * scanning (fast but retains garbage) and a number of scans
>          * proportional to number of elements, that would find all
>          * garbage but would cause some insertions to take O(n) time.
>          *
>          * @param i a position known NOT to hold a stale entry. The
>          * scan starts at the element after i.
>          *
>          * @param n scan control: <tt>log2(n)</tt> cells are scanned,
>          * unless a stale entry one is found, in which case
>          * <tt>log2(table.length)-1</tt> additional cells are scanned.
>          * When called from insertions, this parameter is the number
>          * of elements, but when from replaceStaleEntry, it is the
>          * table length. (Note: all this could be changed to be either
>          * more or less aggressive by weighting n instead of just
>          * using straight log n. But this version is simple, fast, and
>          * seems to work well.)
>          *
>          * @return true if any stale entries have been removed.
>          */
>
>
> The instance ThreadLocals (and what the refer to) will be GC'd when 
> the containing Object is GC'd.
>
> There IS NO MEMORY LEAK in ThreadLocal. If the ThreadLocal refers to 
> an object that has native resources (e.g. file handles), it may not be 
> released until other thread locals are created by the thread (or the 
> thread terminates).
>
> You can avoid this "delay" by calling remove(), but in most 
> applications it should never be necessary - unless a very strange usage...
>
> On Jul 9, 2008, at 2:37 PM, Adrian Tarau wrote:
>
>> From what I know, storing objects in ThreadLocal is safe as long as 
>> you release the object within a try {} finall {} block or store 
>> objects which are independent of the rest of the code(no 
>> dependencies).Otherwise it can get pretty tricky(memory leaks, 
>> classloader problems) after awhile.
>>
>> It is pretty convenient to pass HTTP request information with a 
>> ThreadLocal in a servlet(but you should cleanup the variable before 
>> leaving the servlet) but I'm not sure how safe it is in this case.
>>
>> robert engels wrote:
>>> Using synchronization is a poor/invalid substitute for thread locals 
>>> in many cases.
>>>
>>> The point of the thread local in these referenced cases is too allow 
>>> streaming reads on a file descriptor. if you use a shared file 
>>> descriptor/buffer you are going to continually invalidate the buffer.
>>>
>>> On Jul 8, 2008, at 5:12 AM, Michael McCandless wrote:
>>>
>>>>
>>>> Well ... SegmentReader uses ThreadLocal to hold a thread-private 
>>>> instance of TermVectorsReader, to avoid synchronizing per-document 
>>>> when loading term vectors.
>>>>
>>>> Clearing this ThreadLocal value per call to SegmentReader's methods 
>>>> that load term vectors would defeat its purpose.
>>>>
>>>> Though, of course, we then synchronize on the underlying file (when 
>>>> using FSDirectory), so perhaps we are really not saving much by 
>>>> using ThreadLocal here.  But we are looking to relax that low level 
>>>> synchronization with LUCENE-753.
>>>>
>>>> Maybe we could make our own ThreadLocal that just uses a HashMap, 
>>>> which we'd have to synchronize on when getting the per-thread 
>>>> instances.  Or, go back to sharing a single TermVectorsReader and 
>>>> synchronize per-document.
>>>>
>>>> Jason has suggested moving to a model where you ask the IndexReader 
>>>> for an object that can return term vectors / stored fields / etc, 
>>>> and then you interact with that many times to retrieve each doc.  
>>>> We could then synchronize only on retrieving that object, and 
>>>> provide a thread-private instance.
>>>>
>>>> It seems like we should move away from using ThreadLocal in Lucene 
>>>> and do "normal" synchronization instead.
>>>>
>>>> Mike
>>>>
>>>> Adrian Tarau wrote:
>>>>
>>>>> Usually ThreadLocal.remove() should be called at the end(in a 
>>>>> finally block), before the current call leaves your code.
>>>>>
>>>>> Ex : if during searching ThreadLocal is used, every search(..) 
>>>>> method should cleanup any ThreadLocal variables, or even deeper in 
>>>>> the implementation. When the call leaves Lucene any used 
>>>>> ThreadLocal should be cleaned up.
>>>>>
>>>>> Michael McCandless wrote:
>>>>>>
>>>>>> ThreadLocal, which we use in several places in Lucene, causes a 
>>>>>> leak in app servers because the classloader never fully 
>>>>>> deallocates Lucene's classes because the ThreadLocal is holding 
>>>>>> strong references.
>>>>>>
>>>>>> Yet, ThreadLocal is very convenient for avoiding synchronization.
>>>>>>
>>>>>> Does anyone have any ideas on how to solve this w/o falling back

>>>>>> to "normal" synchronization?
>>>>>>
>>>>>> Mike
>>>>>>
>>>>>> Begin forwarded message:
>>>>>>
>>>>>>> From: "Yonik Seeley" <yonik@apache.org <mailto:yonik@apache.org>>
>>>>>>> Date: July 7, 2008 3:30:28 PM EDT
>>>>>>> To: java-user@lucene.apache.org <mailto:java-user@lucene.apache.org>
>>>>>>> Subject: Re: ThreadLocal in SegmentReader
>>>>>>> Reply-To: java-user@lucene.apache.org 
>>>>>>> <mailto:java-user@lucene.apache.org>
>>>>>>>
>>>>>>> On Mon, Jul 7, 2008 at 2:43 PM, Michael McCandless
>>>>>>> <lucene@mikemccandless.com <mailto:lucene@mikemccandless.com>>

>>>>>>> wrote:
>>>>>>>> So now I'm confused: the SegmentReader itself should no longer

>>>>>>>> be reachable,
>>>>>>>> assuming you are not holding any references to your IndexReader.
>>>>>>>>
>>>>>>>> Which means the ThreadLocal instance should no longer be
reachable.
>>>>>>>
>>>>>>> It will still be referenced from the Thread(s) ThreadLocalMap
>>>>>>> The key (the ThreadLocal) will be weakly referenced, but the
values
>>>>>>> (now stale) are strongly referenced and won't be actually removed
>>>>>>> until the table is resized (under the Java6 impl at least).
>>>>>>> Nice huh?
>>>>>>>
>>>>>>> -Yonik
>>>>>>>
>>>>>>> ---------------------------------------------------------------------
>>>>>>> To unsubscribe, e-mail: java-user-unsubscribe@lucene.apache.org

>>>>>>> <mailto:java-user-unsubscribe@lucene.apache.org>
>>>>>>> For additional commands, e-mail: 
>>>>>>> java-user-help@lucene.apache.org 
>>>>>>> <mailto:java-user-help@lucene.apache.org>
>>>>>>>
>>>>>>
>>>>>>
>>>>>> ---------------------------------------------------------------------
>>>>>> To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org 
>>>>>> <mailto:java-dev-unsubscribe@lucene.apache.org>
>>>>>> For additional commands, e-mail: java-dev-help@lucene.apache.org

>>>>>> <mailto:java-dev-help@lucene.apache.org>
>>>>>>
>>>>>
>>>>>
>>>>> ---------------------------------------------------------------------
>>>>> To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org 
>>>>> <mailto:java-dev-unsubscribe@lucene.apache.org>
>>>>> For additional commands, e-mail: java-dev-help@lucene.apache.org 
>>>>> <mailto:java-dev-help@lucene.apache.org>
>>>>>
>>>>
>>>>
>>>> ---------------------------------------------------------------------
>>>> To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org 
>>>> <mailto:java-dev-unsubscribe@lucene.apache.org>
>>>> For additional commands, e-mail: java-dev-help@lucene.apache.org 
>>>> <mailto:java-dev-help@lucene.apache.org>
>>>>
>>>
>>>
>>> ---------------------------------------------------------------------
>>> To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org 
>>> <mailto:java-dev-unsubscribe@lucene.apache.org>
>>> For additional commands, e-mail: java-dev-help@lucene.apache.org 
>>> <mailto:java-dev-help@lucene.apache.org>
>>>
>>
>>
>> ---------------------------------------------------------------------
>> To unsubscribe, e-mail: java-dev-unsubscribe@lucene.apache.org 
>> <mailto:java-dev-unsubscribe@lucene.apache.org>
>> For additional commands, e-mail: java-dev-help@lucene.apache.org 
>> <mailto:java-dev-help@lucene.apache.org>
>>
>


Mime
View raw message