Java6 vs Java5

Jon Masamitsu Jon.Masamitsu at Sun.COM
Fri Mar 27 11:43:09 PDT 2009


Shaun Hennessy wrote On 03/27/09 11:20,:

>A few quick questions I'm hoping someone can help me with...
>(I'm running Java6u12)
>
>1) Is there any plans for the new option 
>CMSInitiatingPermOccupancyFraction (CR 6634032)
>to put into Java6?  (The fixed in listed as hs13(b01) - I'm not sure 
>what this is - Java7?)
>  
>

This change will make it to a 6 update.  Probably 6u14.

>2) Was there a change in the default CMS initiating occupancy from Java5 
>to Java6 (68% to 92%)
>when using CMS?
>  
>

Unless explicitly set on the command line the initiating occupancy is
calculated
from other parameters (MinHeapFreeRatio and MinHeapFreeRatio) and
I don't think those latter changed between 5 and 6.  Other CMS
parameters changed.

http://java.sun.com/javase/6/docs/technotes/guides/vm/cms-6.html


>3a) If using the Throughput GC -- our old-gen will be triggered when 
>it's full right? There's no need
>to have it go earlier as we're going to stop-the-world....   Nothing 
>would be different java5 vs java6?
>  
>

Yes, with the throughput collector (and indeed with all the stop the world
(aka STW) collectors), a collection is started when a generation is full.

>3b) - If I am going to the throughput collector, is there anyway to 
>force the application to keep
>a certain amount of the heap free (ie -Xms=Xms=256M --) I always want to 
>get say 100M free
>so once the free heap drop to that level trigger a GC?  (Basically I 
>want a CmsInitiatingOccupancyFraction
>but for the throughput collector)
>  
>

There is no JVM option that will allow you to request that
a certain amount be kept free with the STW collectors.

>
>  
>

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