8379127: G1 does not properly consider existing surviving data when sizing Eden

[walulyai]

Hi all,

Please review this change, which reduces the likelihood of evacuation failures by considering the required evacuation space when sizing the young generation and selecting old CSet candidate groups.

Currently, G1 considers only predicted pause time when selecting old CSet regions for evacuation. This change extends the selection criteria to also account for predicted bytes to copy during evacuation.

A copy budget for old CSet candidates is derived after reserving space for young CSet evacuation. This budget applies to both candidates from marking and retained regions.

When selecting regions for optional collection, consider only the pause-time budget. Copy-space constraints are checked later, when optional regions are selected for evacuation.

We override both time and copy-space budgets to meet the required minimum old CSet size. Thus, the budgets only apply for additional selection after selecting the minimum regions required for evacuation progress.

Young-generation sizing is also refactored to use the same evacuation predictions: both pause time and bytes to copy. This reserves headroom for required evacuation work when sizing the young generation.

Testing: Mach5 Tiers 1-3

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Issue

• JDK-8379127: G1 does not properly consider existing surviving data when sizing Eden (Bug - P3)

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