RFR: Unconditional conditional card marking

[Aleksey Shipilev]

On Fri, 21 Apr 2023 18:25:46 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> Changes look good.
> 
> Historically, however, conditional card marks were found to reduce performance, but may be there is newer data that shows that it's better to use conditional card marks to reduce cache coherency traffic as the core counts and cache sizes have both gone up?

The last time I checked (10 years ago!), the break-even was somewhere around 4..8 active mutator (benchmark) threads. With 512 bytes per byte of card, 64 bytes per cache line, the "collision window" is about 32K of Java heap, which is "only" 32K collision regions on 1 GB heap. A rather tiny part of the heap is usually under heavy card updates (basically, old space), so actual collision window is much smaller...

> Can we collect some fresh relative performance numbers?

Yes, I would like to go upstream and argue that in 2023, at Parallel should default to `UseCondCardMark = true`, if not just drop the flag completely :)

I can table this PR until we have a discussion about the current status of conditional card marking upstream. But then again, G1 enables it always, so...

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

PR Comment: https://git.openjdk.org/shenandoah/pull/264#issuecomment-1518238280