RE: [aus-dotnet] A Comparison of .NET and Java GC

[Nick Lothian]

> 
> ".NET GC is Excellent - Better Than Java" -
> http://weblogs.asp.net/pwilson/archive/2004/02/20/77448.aspx
> 
> Ok, first 4 probably not exactly what you're after, but the 
> last one is an
> interesting read. First 4 may give those asking about the GC 
> a bit more info
> on it.

I don't know much about the .NET GC, but I do know the Java GC enough to
know that this guy is confusing garbage collection and resource management.
I guess that's a common enough confusion, but it isn't good to see it put
around as a fact.

He says:

"As for the Dispose method, I learned that this pattern is commonly used and
accepted as necessary in Java, but it was never officially made part of Java
itself, which is one place .NET is actually better since they learned from
Java's experience."

To a certain extend this is true - is you have a resource in Java that you
need to clean up after user you need to use try {...} finally {} blocks.
IDisposable is a nice piece of syntactic sugar that makes that a bit
cleaner. A nice summary of this is at
http://www.neward.net/ted/weblog/index.jsp?date=20040123

However, that has NOTHING to do with garbage collection - in fact it is the
opposite. IDisposable or try {..} finally {} let you clean up resources that
garbage collection do not manage (typical examples are things like managing
pooled connections to something where you want to keep the connection
objects, but close the network connection).

Back to Garbage Collection:

A good overview of the .NET GC is
<http://msdn.microsoft.com/msdnmag/issues/1100/GCI/default.aspx> and
<http://msdn.microsoft.com/msdnmag/issues/1200/GCI2/default.aspx>

There is probably more documentation available on Java GC, simply because it
has been around longer and gone through more revisions. Start with
http://java.sun.com/docs/hotspot/gc/index.html and
http://java.sun.com/docs/hotspot/gc1.4.2/faq.html

I wrote a blog entry on tuning the Java GC with the Java Jvmstat tool, which
includes some more links:
<http://www.mackmo.com/nick/blog/java/?permalink=jvmstat.txt>. Note that the
Java 1.5 VM (or Java 5 VM as it is now know) does more goal orientated self
tuning of its GC algorithm (they call it garbage collector ergonomics). I
haven't looked at this in a great deal of detail, but I'd consider it a good
thing since GC tuning is currently something of a black art. See
http://java.sun.com/j2se/1.5.0/docs/guide/vm/gc-ergonomics.html

If you are really interested in garbage collection I highly recommend
running jvmstat against a Java program. It is quite amazing to see the
generational GC working - you watch as objects get copied from the Eden
space to the survivor pools, and then on to the old and permanent
generations. You can see the difference the various GC algorithms make, too.


Nick

Nick
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