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MariaDB recently implemented memory PSI monitoring but failed with that in a curious way and disabled it afterwards by default. The failure is that under memory pressure, they flushed the entire InnoDB buffer pool.

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The issue is that there’s no generally correct behavior. Should a database under memory pressure stay up at all costs even if it becomes unusably slow (by e.g. nuking 99% of the buffer cache)? Or should it crash/failover hard with a likelihood of potential recovery afterwards, even if it technically could have stayed up? Something in between?

There’s no correct-in-general answers to those questions. This is a hard problem due to context dependence; that’s why there are so many knobs.

In this specific case, the correct behaviour would be to drop a part of the buffer pool until the memory pressure is gone. The context-dependent question is how much and how fast to drop. The current implementation drops to a single configurable level but I suspect it could have implemented better heuristics.

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