Re: Forcing System.gc()

From:

Eric Sosman <Eric.Sosman@sun.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 08 Jun 2007 13:13:23 -0400

Message-ID:

<1181322804.455376@news1nwk>

Matt Humphrey wrote On 06/07/07 21:19,:

"Philipp Leitner" <philipp.leitner@gmx.at> wrote in message
news:1181256561.583990.286530@g4g2000hsf.googlegroups.com...
|> I am wondering if I may not be able to improve performance by System.gc()
in
| > some "right" places in my code (wherever those might be!). It seems to
me
| > though, that the gc facilities in Java are so good, that this is
something
| > best left up to the JVM, and NOT mess with putting System.gc() in
anyplace?
|
| Probably. But you can never 'force' the JVM to do garbage collection.
| System.gc() will only 'suggest' that now may be a good time to do GC
| (but under certain circumstances, I think it has to do with system
| load, the JVM might simply ignore the call). Anyway, I think the cost
| for calling System.gc() is not huge, so perhaps I think it might be
| worth a try.
|
| I for myself remember having used System.gc() exactly once so far - in
| one project I was running >200 Unit Tests, and every test case was
| creating an in-memory representation of a WSDL file and compiled XML
| Schemata using XMLBeans. At some point in the test run the JVM just
| stopped with OutOfMemory. Putting a System.gc() at the end of every
| single test case solved the problem just nice.

Can you explain how your request for gc would prevent an OutOfMemory error?
I thought that OutOfMemory occurs only when gc cannot reclaim any more--it
is never necessary to request gc to prevent OutOfMemory.

Might a really memory-hungry finalize() cause such
a thing? I imagine that if finalization uses a lot of
memory, running GC early could schedule deceased objects
for finalization while memory is still plentiful, while
postponing GC until memory is scarce might leave too
little for the finalizers' use. (I hasten to add that
I don't say things *would* play out this way, just that
it seems to me that they might -- I'm entirely ready to
be corrected by someone who knows better.)

A memory-hungry finalizer seems an awfully silly
thing to write -- but that doesn't mean nobody writes
them ...

--
Eric.Sosman@sun.com