On Thu, Apr 2, 2009 at 9:33 AM, Mark Perry <[email protected]> wrote:
> JAVA JVMs like real storage :-) > The Garbage Collection (GC) runs periodically and references ALL JVM > allocated storage. If Linux has paged out any of the JVM storage it must > be brought back in for every GC cycle. This can be annoying if the JVM > is not really using all of its allocated storage! That indeed confirms my assumption that the JVM uses "in-band" controls to manage the heap. Rather unattractive with virtualization because it increases the footprint of the management process a lot (you need to touch the objects to manage them). It's not without reason we have book titles printed on the spine: to reduce overhead of browsing the shelf... > There are 2 parms for the JVM: > > -Xms<size> Set initial Java heap size > -Xmx<size> Set maximum Java heap size > > If you set the first one low (Xms) then the JVM will release unused > memory down to this value, which is useful if you need the page frames > for other tasks. GC will only reference the allocated storage, which > could drop to this minimum value. Popular recommendation however (not mine) is to set them equal so GC does not waste resources moving things in the corner to free a buffer, and then get it back immediately after. I thought you could avoid that by creating some hysteresis with the "increment" settings. It probably depends a lot on the variation in memory requirement during the day. > swappiness is a control on how many *free* page frames linux is going to > try an keep - if necessary by periodically paging some pages out. I'd rather say it steers Linux in *how* to free pages when needed, whether by dropping cached data or bring portions to swap space. > So if you are going to have a higher value for swappiness and run java, > consider the above. Certainly. It creates that 2nd layer of memory management that interferes with the first layer. Having two managers is not better than having just one; it's worse! ;-) Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
