RFR: 8267246: -XX:MaxRAMPercentage=0 is unreasonable for jtreg tests on many-core machines

[Aleksey Shipilev]

On Mon, 17 May 2021 13:24:16 GMT, Jie Fu <jiefu at openjdk.org> wrote:

> Hi all,
> 
> vmTestbase/vm/mlvm/anonloader/stress/oome/metaspace/Test.java fails on our many-core machines due to `-XX:MaxRAMPercentage=0`.
> This is because `MaxRAMPercentage` will be always 0 if JTREG_JOBS > 25 [1].
> 
> It can be reproduced by: `make test TEST="vmTestbase/vm/mlvm/anonloader/stress/oome/metaspace/Test.java" JTREG="JOBS=26"` on almost all machines.
> 
> Setting `-XX:MaxRAMPercentage=0` on many-core machines seems unreasonable.
> It would be better to fix it.
> 
> Thanks.
> Best regards,
> Jie
> 
> 
> [1] https://github.com/openjdk/jdk/blob/master/make/RunTests.gmk#L741

`-XX:MaxRAMPercentage` is actually `double`, so it can accept values below 1. The last time I looked into trouble like this, it was a problem with doing floating-point division in the make files -- not sure if something can be done about that. 

But my point is that the failing test -- is that the only test that fails? -- expects some heap size to accommodate Java allocations until the expected Metaspace OOM happens. In that case, the fix should be in the test itself. Because even if we do +1 to `MaxRAMPercentage`, it might still be not enough.

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PR: https://git.openjdk.java.net/jdk/pull/4062