Default GC/ empty survivor space

Question

I have a web application running on Solaris 8 using JDK 1.5.0_11 on a 64-bit OS. I am uncertain as to what GC algorithm I am using by default; my machine is a server-class machine, as defined by Sun, with 4 processors and 4G of RAM. I have read things that say that the concurrent mark sweep collector is used by default on this machine, as well as others that say that this option MUST be specified to be used. Which is it?
On this same machine, I have observed that if I specify these parameters
-server
-d64
-Xms2304m
-Xmx2304m
-verbose:gc
-Xloggc:<path to file>
-XX:+HeapDumpOnOutOfMemoryError
-XX:+CMSClassUnloadingEnabled
-XX:+CMSPermGenSweepingEnabled
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamps
according to Jconsole, the "old" generation is empty (never filled). If I indeed do specify -XX:+UseConcMarkSweep, Jconsole shows the survivor space as never being filled. This would suggest that unless I specify -XX:+UseConcMarkSweepGC, the concurrent collector won't be used. Is that correct?
Why is it that either the old generation or the survivor space is never filled? What does that indicate? Is/are there other parameters that I should be setting?
Thanks!

Answer 1

Hi,
CMS is never selected as the default collector. You have to explicitly specify -XX:+UseConcMarkSweepGC to use it.
The default collector for server class machines is the parallel collector:
The following changes take effect with J2SE 5.0.
1. On server-class machines running the server VM, the garbage collector (GC) has changed from the previous serial collector (-XX:+UseSerialGC) to a parallel collector (-XX:+UseParallelGC). You can override this default by using the -XX:+UseSerialGC command-line option to the java command.
[http://java.sun.com/j2se/1.5.0/docs/guide/vm/gc-ergonomics.html]
Not seing your old generation being filled is quite unusual. Are you sure you are running your application long enough? All your objects are always allocated in eden. At each young garbage collection cycle, the surviving objects are copied between the survivor spaces. If they have been copied MaxTenuringThreshold times (can be specified explicitly, the default is somehow object to ergonomics), they tenure into the old generation. If you just run your application for a short time, you might never see this happen.
With CMS, the survivor spaces should be used at each young gc cycle for the objects surviving the gc, unless you specify -XX:MaxTenuringThreshold=0.
Also see http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html for more details.
Nick.