RFR: 8370041: GenShen: Filter young pointers from thread local SATB buffers when only marking old

[William Kemper]

On Fri, 24 Oct 2025 21:00:40 GMT, William Kemper <wkemper at openjdk.org> wrote:

> When GenShen is only marking the old generation, we do not need the SATB mechanism to preserve young pointers. We currently filter these out of the SATB buffers during the final-update-refs and init-mark safepoints. This increases latency and introduces no small amount of complexity. It should be possible to instead filter out these pointers when the SATB buffers are 'compacted' before being 'completed'.
> 
> # Background
> When GenShen is marking the old generation it leaves the SATB barrier enabled. When a young collection interrupts old marking, it creates a situation where a mutator thread could overwrite a field holding a pointer into a collection set region. The SATB barrier will dutifully place this object in the SATB queue. If this pointer makes it into a mark queue, the marking thread will crash. Prior to this change, GenShen filtered out such pointers _after_ the thread local SATB buffers were completed. After this change, such pointers are filtered out _before_ the buffers are completed. This is more inline with the natural way of things.

I have results from an earlier version of this PR that flushed the buffers during `init-mark`. That version showed a consistent 3% improvement on critical _and_ max jops. They also showed a reduction in p99.999 latency for some of the extremem measurements. I will retest with this version.

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PR Comment: https://git.openjdk.org/jdk/pull/27983#issuecomment-3445088766