RFR: 8346920: Serial: Support allocation in old generation before GC

[Albert Mingkun Yang]

On Mon, 20 Jan 2025 20:27:28 GMT, Kirk Pepperdine <kirk at kodewerk.com> wrote:

> If young gen is properly tuned, GC overhead is generally minimalized.

That approach is inflexible and requires trial and error, as it depends on knowing the allocation rate. The GC percentage (`GCTimeRatio`) provides a more direct way to express that requirement or intention.

> Using GCTimeRatio is way down on that list given the current implementations.

Serial doesn't support `GCTimeRatio`; this PR aims to add support for it in Serial. In the long run, as I envision it, once Serial implements auto-heap-resizing, `MaxGCPauseMillis` and `GCTimeRatio` will become the two primary flags for controlling GC.

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PR Comment: https://git.openjdk.org/jdk/pull/22899#issuecomment-2604129892