RFR: 8332506: SIGFPE In ObjectSynchronizer::is_async_deflation_needed() [v3]

[David Holmes]

On Mon, 6 Jan 2025 18:48:31 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

>> src/hotspot/share/runtime/synchronizer.cpp line 1307:
>> 
>>> 1305:                                ", monitor_usage=" SIZE_FORMAT ", threshold=%d",
>>> 1306:                                monitors_used, ceiling, monitor_usage, MonitorUsedDeflationThreshold);
>>> 1307:     return is_above_threshold;
>> 
>> This can now return false if the monitor usage has fallen below the threshold again - is that what should be returned??
>
> Rather than thinking that _"...the monitor usage has fallen below the threshold again"_ you should be thinking _"the ceiling value wasn't increased enough"_. So, to answer your question - yes that is what should be returned.
> 
> This has to do with the actual math behind calculating `new_ceiling` which I describe [here](https://github.com/openjdk/jdk/pull/22815#discussion_r1890907858). Even though it's definitely non-intuitive and maybe a bit disturbing, it does work in the end. So fixing that can be postponed to a later PR.

Yeah well, sorry but I don't get the new math or the old math and we now seem to have a change in behaviour, the affect of which I can't gauge.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22815#discussion_r1904938552