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Unconditionally preserve all marks during GC #6

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@rkennke rkennke commented May 18, 2021

When storing the Klass* in the object header, we need to preserve all marks during GC, because otherwise we'd be loosing the Klass*.

This means that the PreservedMarks structure is repurposed as a full (reverse) forwarding table. Reverse because instead of mapping object->forwardee, and leaving the mark alone, it maps object->mark and stores the forwardee in the header, which might be a better idea because it means we can have a most compact table.

Shenandoah doesn't have this problem, except during full-GC, and ZGC has its own forwarding table.

This change will likely affect performance, but I haven't checked by how much, yet. It's probably more useful to see this in the context of actually reduced header.

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@bridgekeeper bridgekeeper bot commented May 18, 2021

👋 Welcome back rkennke! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

@openjdk openjdk bot added the rfr label May 18, 2021
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@rkennke rkennke requested review from shipilev and stefank May 18, 2021
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@shipilev shipilev left a comment

I have questions about the asserts.

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@shipilev shipilev left a comment

Looks fine to me as experimental code.

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@openjdk openjdk bot commented May 19, 2021

@rkennke This change now passes all automated pre-integration checks.

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Unconditionally preserve all marks during GC

Reviewed-by: shade

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@rkennke rkennke commented May 19, 2021

Looks fine to me as experimental code.

However, I noticed that we (will) have another problem during GC: For sliding compaction, while objects are forwarded, we will also require the Klass* at the same time e.g. for heap traversal. Which means, we cannot simply override the Klass* in the header, not even temporarily. Solutions to this could be avoiding klass-based traversal (e.g. using bitmaps, but serial/parallel GC don't use a bitmap), or an external forwarding table. If we go for the forwarding table, which is what I'm currently leaning to, we don't need the preserved-marks anymore. I need to think this through a little bit.

@rkennke rkennke mentioned this pull request May 21, 2021
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@rkennke rkennke commented Aug 19, 2021

I am closing this because it is no longer needed.

@rkennke rkennke closed this Aug 19, 2021
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