8017163: minor question regarding the calculation and logging of card set coarsening statistics

[Tianqi Xia]

Hi Thomas,

No need to apologize, hope you had an enjoyable vacation :)

Thanks for explaining on the first and third printing of coarsening stats,
that helped me clear out some doubts. I will try to take this one.

Regards,
Tianqi

On Tue, May 31, 2022 at 5:06 PM Thomas Schatzl <thomas.schatzl at oracle.com>
wrote:

> Hi,
>
>    first, apologies for the late answer - I've been on vacation last
> week and then forgot a bit about this.
>
> On 26.05.22 18:05, Tianqi Xia wrote:
> > Hi,
> >
> > One more observation on how the coarsening statistics are printed.
> >
> > Right now coarsening stats are printed 3 times during a GC cycle: >
> > The first one happens when G1HeapPrinterMark is constructed, this one
> > obtains its data from the _coarsenings field of G1RemSetSummary, and it
> > covers the coarsening stats from the end of last GC cycle to the
> > beginning of current GC cycle.
> >
> > The second one happens when G1HeapPrinterMark is destructed, its source
> > is the _coarsen_stats and _last_coarsen_stats fields of G1CardSet, and
> > covers coarsening stats from the end of last GC cycle to the end of
> > current GC cycle, with both recent- and all- statistics.
> >
> > The third printing happens immediately after the second one, its source
> > is again the _coarsenings field of G1RemSetSummary, and it covers the
> > coarsening stats from the beginning of current GC cycle to the end of
> > current GC cycle.
> >
> > Based on testing results of BigRamTester and some other benchmarks, data
> > displayed by the first one and the second one are almost identical,
> > despite the fact that the second one is more verbose, and in most cases
> > the third one just displays a bunch of zeros.
>
> The first one should always be a copy of the "Coarsening (recent)"
> statistics (at least with G1SummarizeRSetStatsPeriod=1).
>
> The third one should always print zeros - gc does not (and should not)
> add any references directly to the remembered set.
>
> > Is it possible to just keep the second one and remove others? This
> > change can keep the logging of coarsening stats more concise, and save
> > the effort of maintaining a copy of these data in G1RemSetSummary. >
> > I understand there is nothing wrong with the current implementation,
> > just some thoughts to make the code a little bit cleaner.
>
> I am fine with that although it looses a bit of information with
> G1SummarizeRSetStatsPeriod > 1.
>
> I filed https://bugs.openjdk.java.net/browse/JDK-8287558.
>
> Thanks,
>    Thomas
>
>