RFR: 8225631: Consider replacing muxAcquire/Release with PlatformMonitor

[David Holmes]

Hi Kim,

Thanks for the review.

On 16/11/2020 6:11 am, Kim Barrett wrote:
> On Fri, 13 Nov 2020 13:47:42 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:
> 
>>> This RFE was filed a while ago to see if we could get rid of `muxAcquire/release` to reduce overall code complexity. Initially some usages seemed to suffer some slight performance loss when I tried this. However, since then all but one usage have been removed and the final case, the `gInflationLock`'s in `ObjectSynchronizer read_stable_mark` does not suffer any performance degradation. It can also use `PlatformMutex` rather than `PlatformMonitor`.
>>>
>>> On the plus side we remove a large chunk of complex synchronization code, we delete a `ParkEvent` per thread, and an int field per `ParkEvent` - which is good for footprint.
>>>
>>> Testing:
>>> - GH actions
>>> - mach5 tiers 1-3
>>> - selected performance testing (guided by that used for JDK-8253064)
>>>
>>> Thanks,
>>> David
>>
>> src/hotspot/share/runtime/synchronizer.cpp line 801:
>>
>>> 799:         // would detach the list and set the markword to inflated with a single CAS and
>>> 800:         // then for each thread on the list, set the flag and unpark() the thread.
>>> 801:         int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (ObjectSynchronizer::NINFLATIONLOCKS-1);
>>
>> I was trying to understand what this did.  The comment is a lot less helpful than the code, but there are 3 TODO comments above this that I don't think we'll ever TODO (or haven't in the past 20 years) and refers to SafepointSynchronize::do_call_back().  Can you remove these in this change since adjacent?  Also these ideas for future work in this comment really don't look like good ideas to me.
> 
> This is assuming NINFLATIONLOCKS is a power of 2.  Consider adding a static_assert.  (Note: is_power_of_2 was made constexpr by JDK-8247910.)

Added to the initialize() function.

Thanks,
David
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> PR: https://git.openjdk.java.net/jdk/pull/1196
>