RFR: JDK-8249258 java/util/StringJoiner/StringJoinerTest.java failed due to OOM (JDK 15)

[Thomas Schatzl]

Hi,

On 15.07.20 15:35, Jim Laskey wrote:
> Try this:
> 
> - You have a 4G heap.
> - You allocate some stuff, say 1 byte.
> - You allocate a 2G object - so there is only 2G - 1, left. Not enough space for another 2G object.
> - But you do allocate 1 byte.
> - You have 1 byte, 2G and 1 byte.
> - You free the original 2G object.
> - But something allocates 1 byte in it's old space.
> - Now there is no range that that can accommodate a 2G object. OOM.

   just for clarification: it's a bit more elaborate than that to get 
into this situation as G1 will try a full collection that will compact 
these 1 byte "stuff" into a contiguous range of memory.

G1 at this time will never move "large" (>= heap region size / 2; at 4g 
heap region size is 2M iirc) objects though. I.e. with a suitably placed 
single "large" object in that 4g heap (like covering the middle center 
regions of the heap, i.e. regions #1023 and #1024) you can make 
allocation of that 2g object impossible.

Thanks,
   Thomas