RFR: 8310031: Parallel: Implement better work distribution for large object arrays in old gen [v15]

[Albert Mingkun Yang]

On Fri, 6 Oct 2023 11:17:12 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> This pr introduces parallel scanning of large object arrays in the old generation containing roots for young collections of Parallel GC. This allows for better distribution of the actual work (following the array references) as opposed to "stealing" from other task queues which can lead to inverse scaling demonstrated by small tests (attached to JDK-8310031) and also observed in gerrit production systems.
>> 
>> The algorithm to share scanning large arrays is supposed to be a straight
>> forward extension of the scheme implemented in
>> `PSCardTable::scavenge_contents_parallel`.
>> 
>> - A worker scans the part of a large array located in its stripe
>> 
>> - Except for the end of the large array reaching into a stripe which is scanned by the thread owning the previous stripe. This is just what the current implementation does: it skips objects crossing into the stripe.
>> 
>> - For this it is necessary that large arrays cover at least 3 stripes (see `PSCardTable::large_obj_arr_min_words`)
>>   
>> The implementation also makes use of the precise card marks for arrays. Only dirty regions are actually scanned.
>> 
>> #### Performance testing
>> 
>> ##### BigArrayInOldGenRR.java
>> 
>> [BigArrayInOldGenRR.java](https://bugs.openjdk.org/secure/attachment/104422/BigArrayInOldGenRR.java) is a micro benchmark that assigns new objects to a large array in a loop. Creating new array elements triggers young collections. In each collection the large array is scanned because of its references to the new elements in the young generation. The benchmark score is the geometric mean of the duration of the last 5 young collections (lower is better).
>> 
>> [BigArrayInOldGenRR.pdf](https://cr.openjdk.org/~rrich/webrevs/8310031/BigArrayInOldGenRR.pdf)([BigArrayInOldGenRR.ods](https://cr.openjdk.org/~rrich/webrevs/8310031/BigArrayInOldGenRR.ods)) presents the benchmark results with 1 to 64 gc threads.
>> 
>> Observations
>> 
>> * JDK22 scales inversely. Adding gc threads prolongues young collections. With 32 threads young collections take ~15x longer than single threaded.
>> 
>> * Fixed JDK22 scales well. Adding gc theads reduces the duration of young collections. With 32 threads young collections are 5x shorter than single threaded.
>> 
>> * With just 1 gc thread there is a regression. Young collections are 1.5x longer with the fix. I assume the reason is that the iteration over the array elements is interrupted at the end of a stripe which makes it less efficient. The prize for parallelization is paid ...
>
> Richard Reingruber has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Missed acquire semantics
>  - Overlap scavenge with pre-scavenge

I find pre-processing card-table removes much complexity in determining which (part of) obj belongs to current stripe. However, synchronizing with actual scavenging introduce some complexity.

The fact that `find_first_clean_card` copies the cached-obj-start is easy to miss and hard to reason IMO.

> we would have a read only copy of the card table only for the current stripe.

It would still require pre-processing card-table, right? Otherwise, I don't see how one can work around the "interference" across stripes. Maybe this can simplify the impl of `find_first_clean_card`.

I am not too concerned about the regression observed for "large (32K) non-array instances", because that pattern is not common in java and the pause-time is still reasonable (<100ms). The long-term optimization (or the redemption of the extra-mem-requirement) I have in mind is to use 1 bit (instead of 1 byte) for a card -- Parallel requires only a boolean info for a particular card. One can even pre-alloc two card-tables now that each card-table is 1/8 of its original size, to avoid calling malloc inside young-gc-pause.

My preference is some simple code without much regression. Ofc, this is quite subjective.

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PR Comment: https://git.openjdk.org/jdk/pull/14846#issuecomment-1750541087