RFR: 8268122: Add specific gc cause for G1 full collections [v2]

[Stefan Johansson]

On Mon, 7 Jun 2021 08:44:41 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Lgtm.
> 
> Pre-existing: The two changed tests seem to be exactly the same....

The "G1ConcurrentMark" test is run with `-XX:+ExplicitGCInvokesConcurrent` so there is a small difference in that we will get an explicit young GC with `System.gc()` as the cause.

I just pushed a small change to move the new cause up to under the "G1 Evacuation Pause" in the enum.

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PR: https://git.openjdk.java.net/jdk/pull/4357