Re: Question about memory size and performance

Carfield Yim wrote:
> According to this paper -
> http://www.cs.umass.edu/~emery/pubs/gcvsmalloc.pdf , it claim that :
>
> "If their applications will be deployed on systems with at least three
> times as much RAM as needed, then garbage collection should provide
> reasonable performance. However, if the deployed systems will have
> less RAM, or if their applications will have to compete with other
> processes for memory, then practitioners should expect garbage
> collection to exact a substantial performance cost."
>
> Just wonder if anybody have comment about this claim?

I sure wonder too. First I thought that "boy, this must be totally
wonky", but reading the paper, it got more interesting.

Is GC really this bad compared to explicit malloc/free?

> Say my
> applicatioon typical requirement 1024m heap size to run. My computer
> need to have 3072m to have reasonable performance???

This 3-5 times the RAM is based on the _minimum_ the application needs
to actually complete. 3 times that minimum amount will get you going
good, according to the paper.

But yes: Someone that are proper JVM/GC wizards, please comment on this
paper! (Semi-proper wizards and even medium knowledgeable java coders
are also invited to read and comment on it!)


Also, one of the guys that wrote that paper, Matthew Hertz, have also
written another paper on a subject I liked a lot: GC on VM, as in
Virtual Memory. I've found that Java sucks when it comes to "managing"
the memory on a OS with VM, which is pretty much all OSes that matter.
Since most GCs occasionally will have to do a full collect, ALL pages
that the JVM have allocated over time will have to be fetched from disk,
"thrashing" the VM's efforts of sticking unused pages to disk.
   I believe I might be seeing an effect of this after a virus scan have
run on my box during the night: most applications are somewhat sluggish
to restore, except Eclipse, which often are curiously quick to wake up.
   This is obviously not such a big deal on single-purpose systems, but
for "Desktop Java" to rule, something should probably be done - a Java
program cannot go around assuming that it is the only application
running on a box. (This also goes for the huge memory footprint any
sized java GUI application is making - multiplying the GC/VM problem).
This problem should also be present for other GCed languages, like .Net.

However, our guy has an answer: Make the GC interact with the kernel -
that is, make the kernel have access points to let the processes known
which pages of theirs are in and out of memory. This seems such an
obvious idea that I wonder why all OS'es haven't got the feature
implemented already.

Matthew Hertz:
   http://www-cs.canisius.edu/~hertzm/
The "Bookmarking Collector":
   http://www-cs.canisius.edu/~hertzm/bc.html

Kind regards,
Endre.

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