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Discontiguous MarkCompactSpace support #930

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wenyuzhao opened this issue Aug 31, 2023 · 2 comments · Fixed by #939
Closed

Discontiguous MarkCompactSpace support #930

wenyuzhao opened this issue Aug 31, 2023 · 2 comments · Fixed by #939

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@wenyuzhao
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Currently, the MarkComapctSpace relies on self.common.start, which is only set for contiguous spaces: https://github.com/mmtk/mmtk-core/blob/master/src/policy/markcompactspace.rs#L322

This brings an implicit assumption that the MarkCompactSpace can work correctly only when being configured as contiguous. Although it's constructor actually supports discontiguous layout https://github.com/mmtk/mmtk-core/blob/master/src/policy/markcompactspace.rs#L202-L206.

This is a blocker for the openjdk compressed oops support, as it will enable discontiguous MarkCompactSpace for MarkCompact plan.
@wks
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wks commented Aug 31, 2023

If I read the code of MonotonicPageResource correctly, it chains up all the "contiguous chunks" in a doubly linked list, with CommonPageResource::head_discontiguous_region pointing to the last "contiguous chunks" obtained from Map32. We can follow this linked list and fill up each "contiguous chunks" when computing forwarding pointers.

@wenyuzhao
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If I read the code of MonotonicPageResource correctly, it chains up all the "contiguous chunks" in a doubly linked list, with CommonPageResource::head_discontiguous_region pointing to the last "contiguous chunks" obtained from Map32. We can follow this linked list and fill up each "contiguous chunks" when computing forwarding pointers.

Yeah I'll try to use this to fix it. I did this on Java MMTk before.

@wenyuzhao wenyuzhao mentioned this issue Sep 5, 2023
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github-merge-queue bot pushed a commit that referenced this issue Sep 5, 2023
@wenyuzhao
Discontiguous mark compact space support (#939)
40777ed 
This PR adds discontiguous heap support for mark compact GC, and
rewrites the forwarding pointer calculation and compaction code to be
compactable to both contiguous and discontiguous spaces.

Fixes #930
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Discontiguous mark compact space support wenyuzhao/mmtk-core
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@wks @wenyuzhao