RFR (11u, S): backport JDK-8247615 Initialize the bytes left for the heap sampler

[Volker Simonis]

Hi Jc,

the downport looks fine to me except the extra empty line you've
inserted in threadHeapSampler.hpp:

     _collectors_present = 0;
+
+
+    // Call this after _rnd is initialized to initialize _bytes_until_sample.

Thank you and best regards,
Volker

PS: as a general rule of thumb, a RFR for a downport should include
the following:
- a link to the original bug (JBS link)
- a link to the original change (i.e. a link to the Mercurial change
set, not to the original webrev)
- a link to the webrev with the new change
- if your changes to "original change" are significant an explanation
why they are required. A diff in webrev format of the "new change"
against "original change" might be helpful in such a case.

On Wed, Jul 29, 2020 at 8:09 PM Jean Christophe Beyler
<jcbeyler at google.com> wrote:
>
> Hi all,
>
> Any chance to get a review on this? Did I perhaps missed something?
>
> Thanks again for your help!
> Jc
>
> On Thu, Jul 23, 2020 at 10:43 AM Jean Christophe Beyler <jcbeyler at google.com>
> wrote:
>
> > Hi all,
> >
> > First time I request a backport so if I am missing anything, please let me
> > know :)
> >
> > I am trying to port this webrev into 11u:
> >
> > Summary:
> > This fixes a sampling bias for the first allocations and ensures that the
> > sampling rate is correct from the start.
> > It is safe to apply as the whole code is protected behind flags and only
> > is used when JVMTI enables the heap monitoring.
> > We added a test that tickles this code on purpose.
> >
> > Porting issues:
> > Finally, the patch does not apply cleanly due to slight differences with
> > the files. This webrev provides the clean to JDK11u:
> > http://cr.openjdk.java.net/~jcbeyler/8247615/webrev.02_11/
> >
> > - That webrev compiles and tests for HeapMonitor (with the new test) pass:
> >     make run-test
> > TEST="test/hotspot/jtreg/serviceability/jvmti/HeapMonitor"
> >
> > compared to what I submitted to head:
> > http://cr.openjdk.java.net/~jcbeyler/8247615/webrev.02/
> >
> > Let me know what else I can do or provide!
> >
> > Thanks for all your help,
> > Jc
> >
> > Ps: I sent this email before I subscribed so one is waiting in moderation;
> > if this shows up in double, I apologize :)
> >
>
>
> --
>
> Thanks,
> Jc