RFR: 8342382: Implementation of JEP G1: Improve Application Throughput with a More Efficient Write-Barrier [v11]

[Albert Mingkun Yang]

On Tue, 4 Mar 2025 11:56:56 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that implements (currently Draft) JEP: G1: Improve Application Throughput with a More Efficient Write-Barrier.
>> 
>> The reason for posting this early is that this is a large change, and the JEP process is already taking very long with no end in sight but we would like to have this ready by JDK 25.
>> 
>> ### Current situation
>> 
>> With this change, G1 will reduce the post write barrier to much more resemble Parallel GC's as described in the JEP. The reason is that G1 lacks in throughput compared to Parallel/Serial GC due to larger barrier.
>> 
>> The main reason for the current barrier is how g1 implements concurrent refinement:
>> * g1 tracks dirtied cards using sets (dirty card queue set - dcqs) of buffers (dirty card queues - dcq) containing the location of dirtied cards. Refinement threads pick up their contents to re-refine. The barrier needs to enqueue card locations.
>> * For correctness dirty card updates requires fine-grained synchronization between mutator and refinement threads,
>> * Finally there is generic code to avoid dirtying cards altogether (filters), to avoid executing the synchronization and the enqueuing as much as possible.
>> 
>> These tasks require the current barrier to look as follows for an assignment `x.a = y` in pseudo code:
>> 
>> 
>> // Filtering
>> if (region(@x.a) == region(y)) goto done; // same region check
>> if (y == null) goto done;     // null value check
>> if (card(@x.a) == young_card) goto done;  // write to young gen check
>> StoreLoad;                // synchronize
>> if (card(@x.a) == dirty_card) goto done;
>> 
>> *card(@x.a) = dirty
>> 
>> // Card tracking
>> enqueue(card-address(@x.a)) into thread-local-dcq;
>> if (thread-local-dcq is not full) goto done;
>> 
>> call runtime to move thread-local-dcq into dcqs
>> 
>> done:
>> 
>> 
>> Overall this post-write barrier alone is in the range of 40-50 total instructions, compared to three or four(!) for parallel and serial gc.
>> 
>> The large size of the inlined barrier not only has a large code footprint, but also prevents some compiler optimizations like loop unrolling or inlining.
>> 
>> There are several papers showing that this barrier alone can decrease throughput by 10-20% ([Yang12](https://dl.acm.org/doi/10.1145/2426642.2259004)), which is corroborated by some benchmarks (see links).
>> 
>> The main idea for this change is to not use fine-grained synchronization between refinement and mutator threads, but coarse grained based on atomically switching c...
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   iwalulya review
>     * comments for variables tracking to-collection-set and just dirtied cards after GC/refinement
>     * predicate for determining whether the refinement has been disabled
>     * some other typos/comment improvements
>     * renamed _has_xxx_ref to _has_ref_to_xxx to be more consistent with naming

src/hotspot/share/gc/g1/g1ConcurrentRefine.cpp line 356:

> 354:     bool do_heap_region(G1HeapRegion* r) override {
> 355:       if (!r->is_free()) {
> 356:         // Need to scan all parts of non-free regions, so reset the claim.

Why is the condition "is_free"? I thought we scan only old-or-humongous regions?

src/hotspot/share/gc/g1/g1ConcurrentRefine.hpp line 116:

> 114:     SwapGlobalCT,                // Swap global card table.
> 115:     SwapJavaThreadsCT,           // Swap java thread's card tables.
> 116:     SwapGCThreadsCT,             // Swap GC thread's card tables.

Do GC threads have card-table?

src/hotspot/share/gc/g1/g1ConcurrentRefineThread.cpp line 219:

> 217:       // The young gen revising mechanism reads the predictor and the values set
> 218:       // here. Avoid inconsistencies by locking.
> 219:       MutexLocker x(G1RareEvent_lock, Mutex::_no_safepoint_check_flag);

Who else can be in this critical-section? I don't get what this lock is protecting us from.

src/hotspot/share/gc/g1/g1ConcurrentRefineThread.hpp line 83:

> 81: 
> 82: public:
> 83:   static G1ConcurrentRefineThread* create(G1ConcurrentRefine* cr);

I wonder if the comment for this class "One or more G1 Concurrent Refinement Threads..."  has become obsolete. (AFAICS, this class is a singleton.)

src/hotspot/share/gc/g1/g1ConcurrentRefineWorkTask.cpp line 69:

> 67:     } else if (res == G1RemSet::NoInteresting) {
> 68:       _refine_stats.inc_cards_clean_again();
> 69:     }

A `switch` is probably cleaner.

src/hotspot/share/gc/g1/g1ConcurrentRefineWorkTask.cpp line 78:

> 76:       do_dirty_card(source, dest_card);
> 77:     }
> 78:     return pointer_delta(dirty_r, dirty_l, sizeof(CardValue));

I feel the `pointer_delta` line belongs to the caller. After that, even the entire method can be inlined to the caller.

YMMV.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979666477
PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979678325
PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979699376
PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979695999
PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979705019
PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1979709682