What does matter is whether you are using more virtual memory than you
have physical memory. The second you get in a spot where you have to GC
and go through the entire object tree, you need to load in every page.
If you have space for N pages in memory, and have objects allocated on
N+1 pages in virtual memory, you are going to do a ton of paging, and
baiscally make your machine unusable. My understanding is that one of
the important reasons for having a Max on the allocation availble to a
VM from the beginning was to make sure the heap never used more than
physical memory.
Zones sound like an interesting strategy, but I'm not sure they help you
much with wanting make the default memory option "unlimited".
Generational is good at reducing the number of full GCs you do, but does
not necassarily eliminate them.
The zones strategy you suggest may work well with apps that have a lot
of class loaders and allocate somewhat evenly across them, but I think
it may cause a lot of overhead. Would your approach be generational?
Would you need Write Barriers for both references from other generations
as well as other Class Loaders?
If you were to have a Web Application, would you basically need a write
barrier for every string you allocate, since the String Class is loaded
in a parent class loader? If so, this may cause more overhead than you
would want for the stated benefit.
--Will
Alex Blewitt wrote:
On 30/07/06, will pugh <[EMAIL PROTECTED]> wrote:
Isn't Full GCs a big problem? If have a bunch of pages in virtual
memory, and need to do a full heap walk. Won't you basically have an
exercise in page faults?
Realistically, doesn't that happen for any full GC though, regardless
of how they're laid out? Even if the JVM requests a single large
contiguous space, it will still be treated as many pages by the OS,
and you'll get a similar amount of page faults. It doesn't matter what
virtual address space they're mapped into.
One of the suggestions I put forward elsewhere was having several
zones (one per classloader); the advantage would then be you could GC
each zone separately, and potentially at different intervals.
Alex.
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