RFR: 8315884: New Object to ObjectMonitor mapping [v9]

[Coleen Phillimore]

On Thu, 18 Jul 2024 11:30:27 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Axel Boldt-Christmas has updated the pull request incrementally with 10 additional commits since the last revision:
>> 
>>  - Remove try_read
>>  - Add explicit to single parameter constructors
>>  - Remove superfluous access specifier
>>  - Remove unused include
>>  - Update assert message OMCache::set_monitor
>>  - Fix indentation
>>  - Remove outdated comment LightweightSynchronizer::exit
>>  - Remove logStream include
>>  - Remove strange comment
>>  - Fix javaThread include
>
> src/hotspot/share/runtime/lightweightSynchronizer.cpp line 77:
> 
>> 75:   using ConcurrentTable = ConcurrentHashTable<Config, MEMFLAGS::mtObjectMonitor>;
>> 76: 
>> 77:   ConcurrentTable* _table;
> 
> So you have a class ObjectMonitorWorld, which references the ConcurrentTable, which, internally also has its actual table. This is 3 dereferences to get to the actual table, if I counted correctly. I'd try to eliminate the outermost ObjectMonitorWorld class, or at least make it a global flat structure instead of a reference to a heap-allocated object. I think, because this is a structure that is global and would exist throughout the lifetime of the Java program anyway, it might be worth figuring out how to do the actual ConcurrentHashTable flat in the global structure, too.

This is a really good suggestion and might help a lot with the performance problems that we see with the table with heavily contended locking.  I think we should change this in a follow-on patch (which I'll work on).

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PR Review Comment: https://git.openjdk.org/jdk/pull/20067#discussion_r1688053792