Semantics of `-Xmn`

[Thomas Schatzl]

Hi,

On 07.12.22 08:38, Stefan Johansson wrote:
> Hi Ramki,
> 
> On 2022-12-06 14:47, Stefan Karlsson wrote:
>> On 2022-12-04 03:38, Ramakrishna, Ramki wrote:
>>> Question: Can `-Xmn` be interpreted as "fix young gen size to the 
>>> specified value, not allowing any resizing"?
>>>
> 
> Kind of, but not exactly for G1. With G1 setting `-Xmn` will create a 
> policy that doesn't recalculate the desired size of the young generation 
> and sets it to the given value. But this is just a "desired" size and it 
> doesn't set a fixed size for the old generation. So if the application 
> keeps a lot of objects alive the old generation might grow "too large" 
> and cause the young generation to be smaller than what is set with `-Xmn`.
>  > Same goes for `-XX:NewSize` in G1 it will not be a hard minimum but set
> the desired minimum to this value.

+1

G1's policy in this situation is fairly crude: it by default tries to 
keep this free region reserve (G1ReservePercent) to accommodate 
surviving objects in the next gc.

Since this is a static measure, this is fairly prone to being wrong in 
either direction, either too large (for large heaps) or too small (for 
small heaps), in the best case using a smaller than expected young gen 
(degrading "nicely") or in the worst case running into Full GCs.

Not much effort has been put into better behavior in this situation.

Thanks,
   Thomas