Call for Discussion: New Project: CRaC

[Anton Kozlov]

Hi Ruslan,

On 7/28/21 11:16 AM, Ruslan Synytsky wrote:
> Tech question: what do you think about the need to adjust the heap size
> after restoration from a checkpointed runtime? As I understand, in some
> cases, the restored runtimes may need different heap size compared to the
> initial runtime from which the state was saved. There is a JEP
> https://openjdk.java.net/jeps/8204088 that might be relevant to this
> discussion.


Before saving the state, in GCs where it was easy to implement, we uncommit
unused parts of the heap. In other cases (except ZGC), we re-commit parts of
the heap, so the RSS is still low. The driver was to avoid saving garbage, but
this also makes RSS size equal to the size of the live set of the heap.
Coordination with checkpoint/restore mechanism includes JVM, so there is a
trigger to give up resources that may be unneeded after restoring the state.

Resizing the heap, in general, seems to be a lot of effort. However, if the
implementation for the enhancement would exist, it likely could be reused for
what we need and do.

Thanks,
Anton