RFR: 8265436: G1: Improve gc+phases log output during full gc

[Hamlin Li]

On Mon, 19 Apr 2021 12:28:58 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   can I have reviews for this fix to remove recently introduced log messages from `debug` to `trace` level?
> 
> If you select `gc+phases=debug` logging with [JDK-8262068](https://bugs.openjdk.java.net/browse/JDK-8262068), you get:
> 
>     [0.695s][debug][gc,phases] GC(51) Phase 1: Weak Processing 0.309ms
>     [0.695s][debug][gc,phases] GC(51) ClassLoaderData 0.003ms
>     [0.695s][debug][gc,phases] GC(51) Trigger cleanups 0.002ms
>     [0.699s][debug][gc,phases] GC(51) Phase 1: Class Unloading and Cleanup 4.056ms
>     [0.699s][info ][gc,phases] GC(51) Phase 1: Mark live objects 7.722ms
>     [0.699s][debug][gc,phases] GC(51) Phase 2: skip compaction region index: 0, live words: 125610
>     [repeated for every region skipped] 
> 
> the last per-region message is quite annyoing and just fills up logs as the code prints one line per affected region (which may be a lot), so I suggest to move it to `trace` level. I did suggest to add some nice phase 2 region summary in [JDK-8265437](https://bugs.openjdk.java.net/browse/JDK-8265437), but that is out of scope for this change.
> 
> Testing: local testing
> 
> Thanks,
>   Thomas

just realized, the test code needs to be adjusted accordingly.
'''
public class TestG1SkipCompaction {
    public static void runTest() throws Exception {
        final String[] arguments = {
            "-XX:+UseG1GC",
            "-XX:MarkSweepDeadRatio=3",
            "-Xmx8m",
            "-Xms8M",
            "-Xlog:gc+phases=debug",            // here, from debug to trace
            "-XX:G1HeapRegionSize=1m",
            GCTest.class.getName()
            };
'''

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PR: https://git.openjdk.java.net/jdk/pull/3569