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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: Tomcat 6.0.29 using more and more RAM until it collapses?
Date Sat, 13 Nov 2010 16:02:55 GMT
> From: Brian [mailto:bbprefix-mail@yahoo.com] 
> Subject: RE: Tomcat 6.0.29 using more and more RAM until it collapses?

> the Eden Space is barely used (10MB right now). The survivor 
> space is even less used (1MB right now).

An object is normally created in Eden and will migrate to survivor if still reachable during
the next minor GC.  In most applications, the vast majority of objects become unreachable
very quickly, and never make it to a survivor space.

> But the Tenured Gen space has 120MB right now! In fact, when 
> my JVM start to eat houndreds of MB, most of that goes to the
> Tenured Gen.

Once the survivor space fills up, long-lived objects migrate to tenured, where they stay until
the application discards them.  Very large objects may be initially allocated in tenured if
they won't fit in Eden.  Anything you see in tenured has either been around for quite some
time or exceeds the Eden allocation threshold.  The dead objects in tenured space won't be
cleaned out until a major GC occurs; the JVM tries to minimize the number of those since they
take quite a bit more time than a minor GC.  You can force a major GC with the JConsole button.

> The perm gen is using 22MB right now, which is not a lot so I 
> guess it is normal.

A one-time look at the size of the PermGen isn't interesting; you need to see whether it increases
over time, especially after restarting a webapp.  If it does increase after a restart, that
means the old instance of the webapp is still hanging around.

 - Chuck


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