RFR: 8253064: monitor list simplifications and getting rid of TSM [v4]

[Coleen Phillimore]

On Tue, 10 Nov 2020 21:25:51 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> src/hotspot/share/runtime/objectMonitor.cpp line 551:
>> 
>>> 549:       if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
>>> 550:         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
>>> 551:         add_to_contentions(-1);
>> 
>> contentions is essentially a refcount, isn't it.  Can you fix the comment to include this at line 360 since that's not the only purpose of this count.
>> 
>>   // Keep track of contention for JVM/TI and M&M queries.
>>   add_to_contentions(1);
>
> No it is not a ref_count. We got rid of the accurate ref_count field
> because maintaining it was too slow.
> 
> contentions just tells you how many threads have blocked on the
> slow-path trying to enter the monitor. The fast-paths never touch
> contentions. The comment on line 360 is accurate.

But contentions is used for more than informing JVMTI, it's used to test whether the monitor is_busy on the slow path.  That's why I wanted the comment to say something like your last sentence, since I spent time trying to understand why the various calls to add_to_contentions(-1) in deflate_monitor earlier.

>> src/hotspot/share/runtime/objectMonitor.hpp line 171:
>> 
>>> 169:   volatile int _SpinDuration;
>>> 170: 
>>> 171:   jint  _contentions;               // Number of active contentions in enter(). It is used by is_busy()
>> 
>> Future RFE - can we replace jint with int32_t or even int or some C++ types.  We're trying not to have Java types leak into runtime code since this doesn't directly interface with Java.
>
> It can be a future RFE, but it won't be at the top of my list of
> things to do. There may already be an RFE for that.

No, I assume it's not high priority.  I'll file an RFE because someday I want these to be cleaned up as a personal nit.

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