RFR: 8047998: -XX:MaxNewSize is unnecessarily set to NewSize if NewSize is too low

[Guoxiong Li]

On Tue, 9 May 2023 03:22:52 GMT, Guoxiong Li <gli at openjdk.org> wrote:

> Hi all,
> 
> When `MaxHeapSize` is equal to `InitialHeapSize` and `NewSize` is set in command line,
> currently, the `max_young_size/MaxNewSize` will be set to the value of `NewSize`.
> 
> Considering the document of the `NewSize` (shown below), someone may set the `NewSize` 
> to a very small value and expect the JVM to adjust the value dynamically.
> Then when the `MaxHeapSize` is equal to `InitialHeapSize` (set by user or ergonomics),
> the `MaxNewSize` is set to the value of `NewSize`, which is small unexpectedly.
> 
> 
>   product(size_t, NewSize, ScaleForWordSize(1*M),                           \
>           "Initial new generation size (in bytes)")                         \
>           constraint(NewSizeConstraintFunc,AfterErgo)                       \
> 
> 
> This patch fixes the issue by setting the `MaxNewSize` to `NewSize` only when the `NewSize` 
> is larger than the original `max_young_size/MaxNewSize`.
> 
> The title of JDK-8047998 may need to adjusted.
> 
> Thanks for the review.
> 
> Best Regards,
> -- Guoxiong

> OK, maybe sth along these lines:
> 
> ```c++
> if (MaxHeapSize == InitialHeapSize) {
>   if (FLAG_IS_CMDLINE(NewSize) && FLAG_IS_CMDLINE(MaxNewSize) && NewSize != MaxNewSize) {
>     vm_exit_during_initialization(...);
>   }
>   ...
> }
> ```
> 
> I think exiting here is fine because this is an impossible constraint to satisfy, variable young-gen size in constant whole-heap setup. What do you think?

The condition `MaxHeapSize == InitialHeapSize` is often meet, I don't think it is good to abort the vm. I do worry that such exiting may break some current online systems, which is unacceptable for us (Do nothing is better to break the online systems). Considering the JDK21 is a LTS version, I don't want to do such breaking change.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/13876#issuecomment-1564444723