[11u] RFR: 8254185: Fix Code cache sweeper heuristics for JDK 11

[Lindenmaier, Goetz]

Thanks!

Best regards,
  Goetz.

> -----Original Message-----
> From: jdk-updates-dev <jdk-updates-dev-retn at openjdk.java.net> On Behalf
> Of Nils Eliasson
> Sent: Friday, October 16, 2020 10:38 AM
> To: jdk-updates-dev at openjdk.java.net
> Subject: Re: [11u] RFR: 8254185: Fix Code cache sweeper heuristics for JDK
> 11
> 
> Hi Goetz,
> 
> Yes. With both fixes the sweeper will work fine again.
> 
> Best regards,
> Nils
> 
> On 2020-10-15 12:47, Lindenmaier, Goetz wrote:
> > Hi Nils,
> >
> > Do I understand correctly that after this fix I should also push
> > "8244278: Excessive code cache flushes and sweeps" ?
> > https://bugs.openjdk.java.net/browse/JDK-8244278
> >
> > Best regards,
> >    Goetz.
> >
> >> -----Original Message-----
> >> From: jdk-updates-dev <jdk-updates-dev-retn at openjdk.java.net> On
> Behalf
> >> Of Nils Eliasson
> >> Sent: Friday, October 9, 2020 4:35 PM
> >> To: jdk-updates-dev at openjdk.java.net
> >> Subject: [11u] RFR: 8254185: Fix Code cache sweeper heuristics for JDK 11
> >>
> >> Hi,
> >>
> >> Bug: https://bugs.openjdk.java.net/browse/JDK-8254185
> >> Webrev: http://cr.openjdk.java.net/~neliasso/8254185/webrev.01/
> >>
> >> This fix is a replacement for JDK-8244660 that I consider to risky to
> >> back port.
> >>
> >> This issue can't be seen unless JDK-8254185 is applied. That bug causes
> >> the sweeper to trigger on every code cache allocation. When JDK-
> 8254185
> >> is applied then no sweeps can be observed at all.
> >>
> >> The main issue is that the sweeper thread is never woken. And if it
> >> isn't woken, it can't check the counters to see if it is time to sweep.
> >>
> >> This fix include two changes:
> >>
> >> 1) Notifying waiters on the codecache_lock when a threshold for
> >> reclaimed nmethods has been reached. This fix is enough on its own.
> >>
> >> In JDK 15 I introduced a new lock for the sweeper because a adding a
> >> notify in NMethodSweeper::possibly_enable_sweeper conflicted with the
> >> compiledMethod_lock in add_osr_method. That lock was added when
> >> deoptimization was converted to using handshakes. So in 11 it is still
> >> ok to use the codecache_lock.
> >>
> >> 2) Moving the updates of _total_nof_code_cache_sweeps and
> _last_sweep
> >> inside block that is only run when doing a sweep.
> >> This change just makes the counters update as expected. _last_sweep is
> >> used to trigger sweeps after a number of safepoints. This is counted by
> >> _time_counter. But since updates of _time_counter doesn't wake the
> >> sweeper - it still doesn't work.
> >>
> >> In JDK 15 I removed all of this code since we can't even rely on
> >> safepoints happening at all. I don't want to introduce any bugs in JDK
> >> 11 by mistake - so I am leaving it as it is.
> >>
> >> Please review.
> >>
> >> Best regards,
> >> Nils Eliasson