[External] : Re: Heads-up: Late G1 Barrier Expansion (Draft JEP)

[Roberto Castaneda Lozano]

Hi Andrew,

Thanks for your interest! I am unfortunately not very familiar with Shenandoah and its barrier model, but in principle late barrier expansion should be applicable to any collector where barriers are tightly coupled to individual memory access operations and performance does not depend too much on exposing barrier operation details to the JIT compiler.

If it helps, our prototype is available here: https://github.com/robcasloz/jdk/tree/g1-late-barrier-expansion. Please note that this is early, experimental work and might change significantly as the JEP evolves.

Thanks,

Roberto

________________________________________
From: Andrew Dinn <adinn at redhat.com>
Sent: Friday, February 2, 2024 2:33 PM
To: Roberto Castaneda Lozano; porters-dev at openjdk.org
Subject: [External] : Re: Heads-up: Late G1 Barrier Expansion (Draft JEP)

Hi Roberto,

On 02/02/2024 13:18, Roberto Castaneda Lozano wrote:
> I have written (together with Erik Österlund) a draft JEP for
> simplifying C2's handling of G1 barriers, see
> https://bugs.openjdk.org/browse/JDK-8322295. This is a heads-up that
> the implementation of this JEP requires platform-specific support
> from all OpenJDK ports. While interpreter G1 barrier implementations
> are available for all ports and can be largely reused, the JEP
> additionally requires 1) defining G1-specific ADL instructions and 2)
> implementing platform-specific logic to support runtime calls from
> the barrier code. For ports that already support ZGC, the effort
> should be smaller, as the logic for 2) can be shared between ZGC and
> G1.
>
> To give a rough estimation of the required effort, the x86-64 changes
> in our prototype involve approximately 900 line insertions and 300
> line deletions over 9 files, among which approximately 300 deleted
> and inserted lines correspond to logic factored out from ZGC.

I looked at the proposal and was interested in the approach, not least
because ZGC appears to have traversed the path that this JEP recommends
G1 to follow.

Have you considered whether this same approach might be taken with the
Shenandoah GC? Alternatively, can declare any basic assumptions
regarding how G1 operates that are needed to enable this change which
might therefore need be met by Shenandoah?

Of course, access to the prototype code might help answer those
questions (at least it would help someone better versed in Shenandoah
than me) but a high-level summary of what in the design of G1 and ZGC
makes this approach work or, conversely, might make it fail would, if
available, be a great help.

regards,


Andrew Dinn
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