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From Christophe Warland <christophe.warl...@s1.com>
Subject Tiles excessive memory usage
Date Mon, 10 Mar 2003 21:47:09 GMT
At my company, we have been recently forced to patch Tiles to solve some
major memory problems where Tiles was eating a lot of memory for no apparent
reasons. We wish to share our findings with you. And we will be happy to
send our code change to Cedric if he wishes so. 
 
Here are some quick numbers about our J2EE runtime after complete bootup
(appserver and EAR file are up, deployed and ready to serve HTTP requests):
 - With original Tiles code: 79 MB of RAM is used
 - After our custom code change: 28 MB of RAM is used
 
Note that the Tiles version that we use is an old one (our source zip shows
09/10/2001). However, a quick analysis of the more recent Tiles source found
in Struts 1.1 RC1 shows that most the code where the defect lies is still in
there. But we haven't actually been able to confirm the problem at runtime
since our app is not compatible with the latest Struts and Tiles
development.
 
Here are a few excerpt of our internal analysis so that you can understand
the issue.
 
--
 
Our application does not actually make use of the Tiles template mechanism
but instead builds on top of Tiles' i18n concept to offer localized Web
pages based on an individual's language, country, personality and channel.
We make extensive use of Tiles Component definitions and inheritance in XML
files, and 667 out of the 668 definitions used by our app inherit from
another one.
 
With the help of a Java profiler, we showed that 64% of the 79 MB of memory
was used by HashMap entries created by Tiles.  This corresponds to 50.5 MB
of RAM. After close investigation, it turns out that the very useful Tiles'
component inheritance has been mis-implemented. 
 
Our application has only one definition that doesn't extend another one: the
root "pageDefinition", which contains common information such as
"pageCopyrightLink" for example. Other Component Definitions that extend
"pageDefinition", such as "disbursementCB", do not need to repeat this
information. When queried for the "pageCopyrightLink" value, the
"disbursementCB" component will delegate the processing to its parent
component, in this case "pageDefinition".
 
Unfortunately, this handy conceptual delegation is actually not implemented
in a similar way in the Tiles source code.
 
In our example, when the "disbursementCB" component is instantiated in
memory, the Tiles source code does not pass it a reference to the parent
"pageDefiniton" component. Instead, Tiles forces the new "disbursementCB"
component to make a deep copy of all values defined in its parent(s). This
means that, ultimately, our application ends up with 668 copies of the
"pageCopyrightLink" value in memory, instead of one. 
 
After modification of the Tiles source code so that true delegation is
actually happening in memory at runtime, new heap allocation statistics
showed that the problem with excessive usage of HashMap$Entry has been
completely solved. The most in-use object in the JVM is now of the type
"[C", a common and normal trend in typical Java applications.
 
The total amount of memory was also down to 28 MB. This is a pleasant 51 MB
gain over the previous result.

--
 
Finally, without being too demanding, we would appreciate if a patched
version of Tiles could be made available for for both the upcoming Struts
1.1 release and the current stabe Strust 1.0.2 one.
 
 
Best Regards,
 
 
Christophe Warland

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