RFR: 8323634: Shenandoah: Document behavior of EvacOOM protocol

[William Kemper]

On Fri, 12 Jan 2024 01:14:24 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> The protocol for handling OOM during evacuation is subtle and critical for correct operation.  This PR does NOT change behavior.  It provided improved documentation of existing behavior.

src/hotspot/share/gc/shenandoah/shenandoahEvacOOMHandler.hpp line 70:

> 68:  *
> 69:  *  1. If we fail to evacuate the entirety of live memory from all cset regions,
> 70:  *     we will transition to STW full gc at the end of the evacuation cycle.  Full GC

First it attempts a degenerated cycle. If we don't believe a degenerated oom-during-evac cycle can ever succeed, we should change this behavior.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17385#discussion_r1450969186