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Hi Lim,

First, the bad news: Assuming the classic JVM, GC normally runs concurrently in an asynchronous thread. That means other work goes on during a collection. In a "stop the world" collection, nothing else happens until the collection is done. That means the app/web server appears unresponsive during the collection cycle. If you had to find out via a verbose log, either no one uses the applications(s) - unlikely, given the typical causes - or they are just accustomed to poor performance.

It's easy to give the general causes, but can be a real bear to figure out specifics.

- A burst of massive memory requests

- One or more memory leaks that overwhelm available memory over time

- Available CPU can't complete gc or can't complete in a timely manner. This last could actually be some combination of CPU and page swapping, where the JVM is effectively thrashing. .

As you can probably see, the causes can vary from scability isues to program problems to application architecture. More bad news is that while monitoring can help, unless it's a build up over time ( leak ), you probably won't be able to get a dump or snapshot when you need it most.


Joe Sam

Joe Sam Shirah - http://www.conceptgo.com
conceptGO - Consulting/Development/Outsourcing
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----- Original Message ----- From: "Lim Hock-Chai" <Lim.Hock-Chai@xxxxxxxxxxxxxxx>
To: "Java Programming on and around the iSeries / AS400" <java400-l@xxxxxxxxxxxx>
Sent: Tuesday, March 03, 2009 4:07 PM
Subject: starting collection, reason stop the world collection


We turned on the *VERBOSEGC option on our web server and we see the
following message in the log :"starting collection, reason stop the
world collection". Does anybody know why the GC is stopping the world
when doing garbage collection? Is there a way to prevent this? Is this
control by the application or is this a standard JVM runtime function?

thanks
--


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