RFR: 8324649: Shenandoah: replace implementation of free set [v59]

[Y. Srinivas Ramakrishna]

On Mon, 13 May 2024 21:31:40 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Several objectives:
>> 1. Reduce humongous allocation failures by segregating regular regions from humongous regions
>> 2. Do not retire regions just because an allocation failed within the region if the memory remaining within the region is large enough to represent a LAB
>> 3. Track range of empty regions in addition to range of available regions in order to expedite humongous allocations
>> 4. Treat collector reserves as available for Mutator allocations after evacuation completes
>> 5. Improve encapsulation so as to enable an OldCollector reserve for future integration of generational Shenandoah
>> 
>> We have compared performance of existing FreeSet implementation with the proposed PR over a broad set of performance workloads and see that the impact is mostly neutral.
>> 
>> Comparing 105235.0 metrics from control, 220638.0 from experiment.
>> Compare: 0.589s
>>                           Most impacted benchmarks |                              Most impacted metrics
>> -------------------------------------------------------------------------------------------------------
>>                                  Shenandoah/jython |                                          cwr_total
>> 
>> 
>>                                 Only in experiment |                                    Only in control
>> -------------------------------------------------------------------------------------------------------
>>                  crypto.signverify/trigger_failure |                        crypto.rsa/cmr_thread_roots
>>                 extremem-large-31g/adjust_pointers |       scimark.sparse.small/concurrent_thread_roots
>>             extremem-large-31g/calculate_addresses |              xml.transform/concurrent_thread_roots
>>       crypto.signverify/class_unloading_rendezvous |                    mpegaudio/concurrent_weak_roots
>>                                   serial/cmr_total |                        crypto.rsa/ctr_thread_roots
>> 
>> Shenandoah
>> -------------------------------------------------------------------------------------------------------
>> +5.64% jython/cwr_total p=0.00037
>>   Control:      1.928ms (+/-272.40us)        170
>>   Test:         2.037ms (+/-322.73us)        344
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix copyright notice on test file

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 109:

> 107:   // Set the Mutator intervals, usage, and capacity according to arguments.  Reset the Collector intervals, used, capacity
> 108:   // to represent empty Collector free set.
> 109:   void establish_intervals(ssize_t mutator_leftmost, ssize_t mutator_rightmost,

Why not clarify in name via `establish_mutator_intervals` ? (I noted there isn't a corresponding method for "collector intervals", although there are corresponding fields for its left and rightmosts etc.) A documentation comment somewhere indicating how/when those get established would be useful. I see them being incrementally updated in the incremental manipulation of set memberships, but wanted to get a sense of the reasons for this asymmetry (and for that reason to be documented when we revisit this code many moons in the future).

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PR Review Comment: https://git.openjdk.org/jdk/pull/17561#discussion_r1599261601