RFR: 8342382: Implementation of JEP G1: Improve Application Throughput with a More Efficient Write-Barrier [v5]

[Thomas Schatzl]

On Mon, 3 Mar 2025 15:19:20 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Can you elaborate on what the "special handling" would be, if we don's set "claimed" for non-committed regions?

the iteration code, would for every region check whether the region is actually committed or not.

The `heap_region_iterate()` API of `G1CollectedHeap` only iterates over committed regions. So only committed regions will be updated in the state table. Later when iterating over the state table, the code uses the array directly, i.e. the claim state of uncommitted regions would be read as uninitialized.

Further, it would be hard to exclude regions committed after the snapshot otherwise (we do not need to iterate over them. Their card table can't contain card marks) as we do not track newly committed regions in the snapshot. We could do, but would be a headache due to memory synchronization because regions can be committed any time.

Imho it is much simpler to reset all the card claims to "already processed" and then make the regions we want to work on claimable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23739#discussion_r1978893134