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

[Aleksey Shipilev]

On 12/18/18 4:52 PM, Kim Barrett wrote:
>> 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.

Okay, another attempt. This restructures the code a little bit, which IMO makes the flow more
understandable. It also eliminates a few flags. Then, the placement for GCTraceTime becomes natural.
Well, almost, because it requires some dance around should_do_full_collection, if we want to include
young-only epilog into timing.

See:
  http://cr.openjdk.java.net/~shade/8215221/webrev.02/

This passes hotspot_tier1 and my ad-hoc tests. If this is the way to go, I would do jdk-submit, etc.
then.

-Aleksey

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