8391957: [CRaC] Record restore-related metrics

[rvansa]

We are reporting metrics that describe JVM+application state & performance, the warmed-up-ness. The concept of metrics can be useful for engine extensions and management at different moments, though - therefore we can report restore times (native & Java) and current generation as metrics, and configure image constraints sooner than when we are attempting a restore.

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• JDK-8391957: [CRaC] Record restore-related metrics (Task - P4)

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[rvansa]

Note: ticks equal to nanos on Linux, but Windows could use different granularity.

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8391957: [CRaC] Record restore-related metrics

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At the time when this comment was updated there had been 2 new commits pushed to the crac branch:

• df66721: 8391953: [CRaC] crlib_bitmap_comparison_t enum values have no unique prefix
• 9bd5f77: 8391859: [CRaC] Test repeated checkpoint

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[mlbridge]

Webrevs

• 01: Full - Incremental (8f9ce595)
• 00: Full (5251842d)

[TimPushkin]

Are the -1 fallbacks (and initializing _restore_native_end_nanos = -1 at the top of the file) really needed when we guard the reads with _generation > 1?

[rvansa]

The initialization isn't necessary, this is the only read. But about the fallback - do you suggest to include the metric only if _generation > 1? I have followed the same pattern as with vm.uptimeSinceRestore above, rather than omitting this entirely.

Btw., with keep-running this value is set even if this is a booted (not restored) instance. In that case it's a bit confusing (has some low value) but I don't think it's critical.