Young Generation Heap Layout

[Tao Mao]

Use this link to clearly see the layout (ignore perm-gen part, it's gone)
http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html#par_gc.generations

Thanks.
Tao

On 11/28/13 3:20 PM, Peter B. Kessler wrote:
> I don't know of a reason that you couldn't reorder the spaces within 
> the young generation.  They are arranged that way for a reason similar 
> to the one you are thinking of, but with respect to the heap as a 
> whole.  Long ago the heap used to be
>
>     Eden | From | To | Old
>
> probably because that's the obvious way to do it and no one thought 
> there were advantages to be had from different layouts.  But then we 
> wanted to resize the old generation, and the observation was that 
> after a young generation collection the eden is almost always empty 
> (always in some collectors), and after a compacting collection of the 
> old generation the high end of the old generation is empty, so if we 
> laid the heap out
>
>     Old | Eden | From | To
>
> then we could move the boundary between Old and Eden.
>
> Are you thinking of doing something similar between from (empty after 
> collections) and eden?  Then you could move that boundary every other 
> young generation collection.  That is, when From is adjacent to Eden.  
> But that happens even with the spaces laid out as they are, with To 
> just above Eden.  Now I'm curious as to what you are thinking about.
>
> It's convenient (read, performant) to have a single address comparison 
> to tell you whether an oop is in the young generation or the old 
> generation, and within the young generation whether it is in Eden or 
> one of the survivor spaces.  It doesn't seem like you'd make those 
> less efficient.
>
>             ... peter
>
> On 2013/11/28 14:02, Blair Archibald wrote:
>> Hi,
>>
>> I'm experimenting with new methods of performing heap resizing (like
>> Ergonomics/Adaptive size policy but with different 
>> principles/techniques).
>>
>> I'm working mainly with the ParallelScavenge GC/Heap and was 
>> wondering if
>> there is a particular reason that the young generation needs to be in 
>> the
>> form:
>>
>> Eden | To | From
>>
>> I would like to do some (fairly aggressive) Eden resizing so it would be
>> beneficial to have a young generation as follows:
>>
>> To | From | Eden
>>
>> This provides the possibility of scaling the generation to provide free
>> space at the end of eden. When I tried this however I got various issues
>> with CompressedOOPs and when running without CompressedOOPs I get 
>> strange
>> segfaults.
>>
>> Are there any particular areas where this space layout invariant 
>> needs to
>> be enforced? Would it ever be possible to have a layout with eden at the
>> end?
>>
>> Many thanks,
>> Blair
>>