Does memory usage really increase when upgrading to WebSphere Application Server V6.1 on Solaris ?


In reference to IBM Flash(Alert) that has been posted by IBM does not provide the
technical details and does not discuss about why one would observe this
growth in memory if not used in way they have been using it with J2SE
1.4.   In nutshell, this is not a growth in memory.  It
is just the technology changes from J2SE 1.4.2 to 1.5.  In J2SE
5.0, we all know that a lot of things has been
changed.

Some of the defaults(configuration) that were used with J2SE 1.4.2
before has been changed in 1.5.  As a result, the Java Virtual
Machine (JVM) if started on the same hardware or server will have
different settings with 1.5 JVM which is tuned mostly for the server
class application as opposed to 1.4.2 which is tuned for client class
uses.  When you start your JVM without supplying any JVM options
(defaults) with 1.4.2 and 1.5, you will find lot of difference in the
JVM settings in the two JVM's. We need to understand a bit about this
as to why this is changed.  To understand it well, we have to look
at the changes in J2SE 1.5 that has introduced following new feature
which is relevant in this case:


I am sure if you are interesetd you will follow the link and get the
details but here is a summary whats happening in here. When 1.5 JVM is
started on a particular classe of machine, it adjusts the JVM
parameters based on resources on the machine e.g. memory and cpu. It is
not just the heap size settings (e.g. Xms and Xmx), there are other
options as well which will have different settings. If your machine is
deteced to be the server class machine, it is going to start the server
JVM otherwise it will start the client  JVM. While in J2SE 1.4.2,
client JVM has always been the case unless you specify the "-server"
excplicitly in your settings. Then, the Garbage Collector is also
different on the class of machine you have.


The IBM Flash tuning reccomendation is just to override the Garbage
Collector Ergornomics and Server-Class machine detection feature of the
J2SE 1.5 and bring it to the same options level as 1.4.2 JVM. It makes
sense here as now we will be comparing the two JVM at the same level
and you will find that the 1.5 JVM will have the lesser footprint. You
may find people using the term like de-tuning for this but actually you
may by this time will understand that this is actually not de-tuning
but bringing the JVM 1.5 to 1.4.2 level and doing an apple to apple
comparision. Not many production system that drive really heavy
workloads will use in these type of constrained environment and will
favour the ergonomics feature of the 1.5 JVM.

If you are interested in more details about the Sun JVM tuning, there
are plenty of resources to help you such as Java SE Performance is great place to start.

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