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From "Shapira, Yoav" <>
Subject RE: Tomcat classloads
Date Mon, 02 Dec 2002 13:44:00 GMT

>I am wondering if there is any "short-cut" into seeing how much memory
>webapp in tomcat is taking at runtime ?
>I know it's possible to wrap the classloader object for
>and make it count the numbers of objects instanciated, aproxiate the
>taken by each object by counting the fields each object has, and store
>week-reference object storage (such as week hashmap), and count down
>object gets garbagecollected.

This is basically what profilers do ;)

>The idea is some kind of a "profiling feature" for webapplications and
>think a nice feature to have.

It would be a nice feature to have, I agree.

>Does anyone have thoughts or ideas on this subject ?

Like most really nice features, this is a pain to implement.
Hierarchical memory usage is very very difficult to quantify.  For
example, you can see that there are n String objects in the heap, and
you can approximate (but being exact is very difficult here, and I do
think it comes to a point where we have to be exact) how much memory
they take up collectively.  You can go a step further using the JVMPI
and get allocation backtraces, e.g. x of these n strings were allocated
by this method.  And you can then some those up with by webapp using
your approach, to get some idea of the memory usage by a webapp.

So this solution, which is not trivial to implement will be:
- Unlikely to give accurate results, only approximations of varying
precision (according to data type and other variables).
- Likely to significantly decrease performance of the running webapp,
enough so that it couldn't be used in production.

I'm not saying it's a bad idea: I'm just sharing my thoughts as
requested ;)  It'd a be a cool feature to have and I think we
collectively need to do more in terms of providing profiling and
benchmarking tools, options, and data to our users.  But it's not an
easy domain to work in ;)

Yoav Shapira
Millennium ChemInformatics

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