RFR: 8324649: Shenandoah: refactor implementation of free set [v39]

[Roman Kennke]

On Thu, 28 Mar 2024 16:06:58 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:
> 
>   Remove debugging instrumentation

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 45:

> 43: // break abstraction rules, because efficient implementation requires assumptions about superclass internals that
> 44: // might be violatee through future software maintenance.
> 45: class ShenandoahSimpleBitMap {

I think this class should go into its own set of files. It would certainly help readability of ShFreeSet which is now somewhat dominated by ShSimpleBitMap.

src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp line 915:

> 913: public:
> 914:   ShenandoahPostCompactClosure() : _heap(ShenandoahHeap::heap()), _live(0) {
> 915:     _heap->free_set()->clear();

Why is that ok?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/17561#discussion_r1558187740
PR Review Comment: https://git.openjdk.org/jdk/pull/17561#discussion_r1558185718