RFR: 8311883: [Genshen] Adaptive tenuring threshold

[Kelvin Nilsen]

On Thu, 15 Jun 2023 20:08:58 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

> JDK-8311883 [GenShen] Adaptive tenuring
> 
> I am opening this previously draft PR for formal preliminary review. It has already benefited from review feedback from a code walkthrough of an earlier version of the code. Most of that feedback and the corrections thereof are to be found in the comments in this PR. I have addressed a large majority of those comments, and am working on the last one that I plan to address as part of this PR. For the ones that I don't plan to address in this PR, I will create follow up tickets. Those will be added in the responses for the remaining feedback comments recorded in this PR's conversation.
> 
> Preliminary testing w/SPECjbb didn't yield reliable performance data from which to infer any performance improvements stemming from enabling adaptive tenuring. I believe that was because of the way SPECjbb is run, which causes excessive degenerate and full gc's. I plan to collect SPECjbb numbers with a fixed lower max HBIR so as to be able to discern performance differences from this change, as well as Extremem workloads. Those will be added here once ready over the next few days.

I've run this through some Extremem workloads.  Good news is I see no regressions.  On the other hand, I am not yet seeing huge benefit.  (I may be seeing a decrease in degenerated cycles, but need to run a few more tests to be sure.)

One concern is that we are still improperly identifying promotions as mortality.  I'm attaching a log with some of my comments preceded by ;; at the start of lines.  See line 3421 of the log, for example.  IN GC(16), we chose tenure age 2.
Then we promoted in place 420 regions.  This caused us to believe there was high mortality of ages 3-6, but really there was no mortality and only promotions.  In GC(17), we should have stayed with tenure age 2.

I think the way to fix this is to only scan from 1 to the current tenure age when you select a new tenure age.  If there is no mortality at the current tenure age, then we can set the new tenure age to 1 + current tenure age.
[auto-tenure.out.txt](https://github.com/openjdk/shenandoah/files/12031253/auto-tenure.out.txt)

I'm thinking we would want to make -XX:+ShenandoahGenerationalAdaptiveTenuring -XX:-ShenandoahGenerationalCensusAtEvac the default behavior in this PR.  Since we do not yet have a large community of GenShen production users, are there reasons not to make these the defaults?

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PR Comment: https://git.openjdk.org/shenandoah/pull/289#issuecomment-1633041261
PR Comment: https://git.openjdk.org/shenandoah/pull/289#issuecomment-1633044020