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From Thilo Goetz <twgo...@gmx.de>
Subject Re: [jira] Created: (UIMA-1068) Use of the JCas cache should be configurable
Date Thu, 12 Jun 2008 06:45:47 GMT
Marshall Schor wrote:
> Thilo Goetz wrote:
>> Marshall Schor wrote:
>>> Thilo Goetz (JIRA) wrote:
>>> Some applications may break if they require == between instances of 
>>> the same JCas object.  Other of course won't care.  So - it's good 
>>> for this to be configurable.
>> Any annotator that works with this assumption is broken IMO.
>> Why would anybody make such an assumption?  
> One use case: With JCas it is possible to add fields to the cover class 
> (thus, you could add a hashmap object, for instance); this is described 
> in the documentation for JCas.  Those field values are only preserved 
> for different iterations if the JCas instance is kept.
> -Marshall

I have nothing to add to what I said earlier.  The JCas cache is an
"optimization" (or not), and it shouldn't be relied upon for program
correctness - nor should our documentation suggest that it should.

Now that the JCas cache is configurable, annotators that rely on the
JCas cache for correctness will no longer work in UIMA instances
configured not to use the cache.

I would like to see benchmarks that show a clear *performance*
advantage from using the JCas cache.  So far all the benchmarks
I've run show that turning off the JCas cache not only reduces
the memory overhead significantly, it's also faster.  YMMV.


>> I don't see anything
>> in our documentation that encourages this.  To the contrary,
>> we say that we don't guarantee object identity for feature
>> structures, and that equals() should be used to compare them.
>>> It might be good, also, to put in "soft references" for this - which 
>>> will be reclaimed if memory gets low.  But this might end up doubling 
>>> the size of the storage used for this (to hold the soft reference)...
>>> -Marshall
>>>> Use of the JCas cache should be configurable
>>>> --------------------------------------------
>>>>                  Key: UIMA-1068
>>>>                  URL: https://issues.apache.org/jira/browse/UIMA-1068
>>>>              Project: UIMA
>>>>           Issue Type: Improvement
>>>>           Components: Core Java Framework
>>>>     Affects Versions: 2.2.2
>>>>             Reporter: Thilo Goetz
>>>>             Assignee: Thilo Goetz
>>>>              Fix For: 2.3
>>>> The JCas caches all CAS objects that are accessed through it.  This 
>>>> means that JCas objects that are no longer used can't be garbage 
>>>> collected.  If only part of the processing chain uses the JCas, or 
>>>> the caching is redundant for some other reason, this produces a 
>>>> severe memory overhead.
>>>> I ran the same experiment I ran for UIMA-1067: doubled the size of 
>>>> Moby Dick and ran the POS tagger from the sandbox.  I used the 
>>>> improved version from UIMA-1067 as base case and simply commented 
>>>> out the line that adds JCas objects to the cache.  This reduced the 
>>>> required heap size from 115MB to 105MB.  It also improved the 
>>>> performance from around 10s for the base case to consistently under 
>>>> 9s for the version without any caching.  I looked at the tagger 
>>>> source code, and saw that it keeps its own list of tokens around.  
>>>> So the savings are just the caching data structure.
>>>> There may be cases where the JCas cache is a performance win, though 
>>>> I'd be curious to see the benchmarks.  So we should not just turn it 
>>>> off, but make it configurable.

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