As I'm sure you know this is something of a large can of worms... All I can say is that from my experience of this modern JVM's are getting much better at working well with the default gc options (ie jdk 5.0 and above). We use jrockit for our systems, and just use a simple:
-server -Xms256m -Xmx1024m Our server are dual processor boxes with 6Gig ram and all runs well for us. From what I understand both current jrockit and sun gc strategies are to delay gc work until as late as possible (jrockit seems to leave it later than sun tbh). This tends to mean that memory usage never really seems to drops much, our servers run with jrockit having grabbed around 1.9Gig memory all the time, but then seem to run happily with that for long periods of time (ie weeks at a time). Both jrockit and sun default to adaptive gc systems for "large" systems, which ought to get to the right approach in due course. Having said that experience suggests that if the application churns memory very fast (tomcat doesn't really qualify for that for us, but some of our other apps do), using a two generational concurrent gc approach is best, especially for jrockit. The other thing to note is that a call to System.gc() does very little in some jvm's, I think its increasingly taken as a hint, and most gc's almost ignore it. A real full gc is expensive and can cause heavy pauses, gc systems tend to avoid it like a plague. Hope this is of some help, although I'd be interested to hear others thoughts on this. Mark On Wednesday 29 March 2006 19:17, John Powers wrote: > I am trying to tune an instance of tomcat running a single application. > And have a couple different questions. > > > > I have read lots of faqs and webpages on the various types of garbage > collectors. I get from most of these that it takes lots of tuning > particular to each machine/server/application. > > > > My main question is why the memory allocated keeps climbing. If I > have a fairly consistent number of users, then as some sessions are > freed up, others are made. Wouldn't the amount of allocated level off > at some point? When a full GC is started, why does it never go back down > to a low number? At night, lets say when there are no users, why > wouldn't a GC drop the memory down to what is used by just the bare > minimum? > > > > Most faqs and webpages say to allocate as much memory as possible. How > can you use the aggressiveHeap tag if tomcat is also trying to specify > Mmx and Mms settings? Does anyone have an example memory settings for > a tomcat running on a 2-4 chip machine with around 2G of ram? > > > > I understand for a JSP application that we need to keep our permHeap > size large. What do you guys normally do for that? 128M? 256M? 512M? > > > > Is it common that a tomcat needs restarting nightly to free up memory? > In all my other apps I've had with tomcat, I've never needed to do that. > I've been trying to use the profiler yourkit, but am uncertain what is a > valid object in memory and what is being held on unnecessarily. > > > > Thanks for any help you can provide. > > > > > > > ________________________________________________________________________ > This email has been scanned for all known viruses by the MessageLabs > SkyScan service. ________________________________________________________________________ This email has been scanned for all known viruses by the MessageLabs SkyScan service. --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
