What are the "new" defaults for CMS??

[Jesper Wilhelmsson]

Fino,

The RFE you link to has a link to the changeset in question. It has a fairly 
descriptive comment:

6896099: Integrate CMS heap ergo with default heap sizing ergo
6627787: CMS: JVM refuses to start up with -Xms16m -Xmx16m
7000125: CMS: Anti-monotone young gen sizing with respect to maximum whole 
heap size specification
7027529: CMS: retire CMSUseOldDefaults flag

Summary: Simplify CMS heap sizing code, relying on ergonomic initial sizing 
consistent with other collectors for the most part, controlling only young gen 
sizing to rein in pause times. Make CMS young gen sizing default statically 
cpu-dependant. Remove inconsistencies wrt generation sizing and policy code, 
allowing for the fixing for 6627787 and 7000125. For 7027529, retire the flag 
CMSUseOldDefaults which had been introduced as a bridge from JDK 5 to JDK 6 a 
number of years ago.


For even more details there is a diff provided. It shows what the comment 
says, that CMSUseOldDefaults has been removed and that the default maximum 
size of young gen in CMS that used to 16MB all over, now depends on CPU: x86, 
PPC: 64MB ; SPARC, Zero, ARM: 16MB

Hope this helps.
/Jesper


On 2012-09-28 12:28, Gmail wrote:
> I believe I asked this a few weeks ago but did not get an answer, so here goes
> again.
> In the Java7 release notes, it says:
>
> *Area*: HotSpot
> *Standard/Platform*: JDK 7
> *Synopsis*: The default out-of-the-box heap size and shape parameters for the
> concurrent mark sweep collector (CMS) have been modified. The new settings
> take advantage of faster platforms that have been introduced since JDK 6 was
> released. Like the other collectors in HotSpot, CMS will now use the available
> physical memory on the platform to size its heap, while attempting to shape
> that heap to keep pause times associated with minor collections "reasonable".
> The specific shape of the heap may be platform-depenent in other ways as well.
> Users can override all or some of these default settings by explicitly sizing
> or shaping the heap, AKA "heap tuning", to suit their specific needs.
> For more information on the default settings for this and other garbage
> collectors, please see the Heap Tuning Guide for JDK 6
> <http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html>.
> *RFE*: 6896099 <http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6896099>
>
> But the exact changes are not spelled out. Can I get some specifics please?
>
> Thx
> Fino
>
> Sent from Fino's iPhone 4S
>
>
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