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
- 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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