Re: Question about memory size and performance

So, i havent read the article in question and don't have time right now.

But GC performance in general is *highly* dependant on your memory
access patterns.

Hotspot does very well on memory that is eden-collectible.  There is
virtually no cost.

Long lived memory has a cost but should be VERY long lived in a good
design so it
doesn't extract that cost that often.

Where you get into trouble is if you have a lot of objects that escape
the eden, either through
lifetime or through things the defeat the eden like begin referenced by
Reference Objects, but
then frequently need collection.

As always, the profiler is your friend.

JK

Endre Stølsvik wrote:
> 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.
>
> ===================================
> This list is hosted by DevelopMentor�  http://www.develop.com
>
> View archives and manage your subscription(s) at
> http://discuss.develop.com


--
=======================================================================
Journalism 101: The difference between news and a press release

News tells you what the reporter thinks is true.

A press release tells you what a company wants *you* to think is true.

Only bad reporters think the two are the same.

===================================
This list is hosted by DevelopMentor®  http://www.develop.com

View archives and manage your subscription(s) at http://discuss.develop.com