Direct promotion to old gen

Peter B. Kessler Peter.Kessler at Sun.COM
Mon Aug 3 11:38:32 PDT 2009


Tony Printezis wrote:
> 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.
>>  
>> ....
>> 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).
> ....

A minor correction:  I'm pretty sure that one object failing to fit in to-space doesn't imply that all further attempts will result in promotion.  E.g., if you have one big object that fails to fit in the to-space, then it will be put in the old-space, but after that smaller objects may well fit in the to-space so we will put them there.  That is, each object is considered individually: if the policy says we should put it in the old generation then we do, but otherwise we try to put it in the to-space.  If it fits, we're happy, if it doesn't fit we put it in the old generation.

The point of the age bits and the tenuring threshold policy is to advise us which relatively older objects should be promoted so that there's room for the relatively younger objects to fit in the to-space.  We need a policy like that because we don't consider the objects in age order.

			... peter
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