Direct promotion to old gen

Tony Printezis Antonios.Printezis at sun.com
Mon Aug 3 10:50:59 PDT 2009


Sujit,

Sujit.Das at cognizant.com wrote:
> Hi All,
>  
> I am trying to capture statistics to denote direct promotion from 
> young gen to old gen.
>  
> As per my understanding, minor collection will empty eden space and 
> put surviving objects into 'To' survivor space. Objects that have 
> survived more than Max Tenuring Threshold value will get promoted.
That's correct.
> Example: If max Tenuring Threshold is 4, then in age 4 size that gets 
> printed in verbose GC output will be the size that should get promoted 
> in old gen as part of next minor collection.
I'm not quite sure what you're saying here. If MTT is 4, when objects 
reach age 4 they get promoted. You can print the per-age information 
with -XX:+PrintTenuringDistribution.
> Anything more than previous age4 size is result of direct promotion 
> from eden space to old gen.
It is possible that objects with a younger age than MTT (4, in this 
case, to be promoted). If, during GC, the survivor space overflows then 
we have to promote everything else (not only objects from the eden, but 
also objects from the from survivor space with younger ages).

BTW, in our terminology, "direct" allocation is actually allocations 
that directly into the old gen and bypass the young gen alltogether 
(large objects typically).
> Please confirm my understanding / approach. If above understanding is 
> correct, I plan to get plot of
> (Promo from current minor collection - Age4 from previous minor 
> collection)
>  
> The values will help me to determine whether I need to tune my eden 
> and survivor spaces.
Please, also look at this talk I gave with Charlie Hunt at this year's 
JavaOne on how to tune the young gen:

http://developers.sun.com/learning/javaoneonline/j1sessn.jsp?sessn=TS-4887&yr=2009&track=javase

Tony
> Your help much appreciated.
>  
> Thanks,
> Sujit
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