RFR (XS) 8215221: Serial GC misreports young GC time

[Kim Barrett]

> On Dec 11, 2018, at 3:23 PM, Aleksey Shipilev <shade at redhat.com> wrote:
> 
> Bug:
>  https://bugs.openjdk.java.net/browse/JDK-8215221
> 
> One of our guys pointed out the oddity in Serial GC logs: "Pause Young" includes the timing for Full
> collection, and wrecks up GCId order as well. This is because GCTraceTime-s are nested on those
> paths, and putting them in proper order, closer to collect_generation, fixes it.
> 
> The downside is that some of the cleanup actions are now outside the GCTraceTime. Those look
> innocuous to omit to me, though.

ClassLoaderDataGraph::purge() doesn’t look so innocuous.  I haven’t
looked hard at other stuff being omitted from accounting by this change.

It seems to me that fixing this reporting problem might require some
restructuring of this rather complicated and flag-filled function.

> Fix:
>  http://cr.openjdk.java.net/~shade/8215221/webrev.01/
> 
> Testing: hotspot tier1, eyeballing GC logs, jdk-submit (running)
> 
> Thanks,
> -Aleksey