RFR: 8253064: monitor list simplifications and getting rid of TSM [v2]

[David Holmes]

On Sat, 7 Nov 2020 17:55:34 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> src/hotspot/share/runtime/synchronizer.hpp line 173:
>> 
>>> 171: 
>>> 172:   static MonitorList   _in_use_list;
>>> 173:   static jint          _in_use_list_ceiling;
>> 
>> Can you add some commentary on what this ceiling is as I could not understand its role just by looking at the code. Thanks.
>
> How about this:
>   static MonitorList   _in_use_list;
>   // The ratio of the current _in_use_list count to the ceiling is used
>   // to determine if we are above MonitorUsedDeflationThreshold and need
>   // to do an async monitor deflation cycle. The ceiling is increased by
>   // AvgMonitorsPerThreadEstimate when a thread is added to the system
>   // and is decreased by AvgMonitorsPerThreadEstimate when a thread is
>   // removed from the system.
>   // Note: If the _in_use_list max exceeds the ceiling, then
>   // monitors_used_above_threshold() will use the in_use_list max instead
>   // of the thread count derived ceiling because we have used more
>   // ObjectMonitors than the estimated average.
>   static jint          _in_use_list_ceiling;

Thanks for the comment. So instead of checking the threshhold on each OM allocation we use this averaging technique to estimate the number of monitors in use? Can you explain how this came about rather than the simple/obvious check at allocation time. Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/642