8241678: Remove PerfData sampling via StatSampler

[caspernorrbin]

Hi everyone,

This change removes the legacy PerfData sampling mechanism implemented through the StatSampler — an always-on periodic task that runs every 50ms my default. The sampling feature was originally introduced to collect performance counters and timestamps, but has since seen very little use.

For G1/ZGC, the only sampled value is a timestamp (sun.os.hrt.ticks). For Serial/Parallel, it also samples some heap space counters, but these are already updated after each GC cycle, making the sampling redundant. With sampling removed, the PerfDataSamplingInterval flag becomes obsoleted, as it no longer serves any purpose.

The only thing relying on the sampled timestamps is jstat: running jstat -t prints an extra column with the time since VM start. To preserve this funcitonality, we can calculate the timestamps as an offset from the already existing sun.rt.createVmBeginTime instead.

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Change requires CSR request JDK-8241701 to be approved
• Commit message must refer to an issue

Issues

• JDK-8241678: Remove PerfData sampling via StatSampler (Enhancement - P4)
• JDK-8241701: Remove PerfData sampling via StatSampler (CSR)

Reviewers

• Albert Mingkun Yang (@albertnetymk - Reviewer) Review applies to ed3670eb
• Johan Sjölen (@jdksjolen - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back cnorrbin! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

@caspernorrbin This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8241678: Remove PerfData sampling via StatSampler

Reviewed-by: jsjolen, ayang

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 95 new commits pushed to the master branch:

• f30e154: 8357673: remove test serviceability/jvmti/vthread/TestPinCaseWithCFLH
• 0671309: 8357919: Arena::allocate returns segments with address zero if the segment length is zero after JDK-8345687
• 627ef34: 8304065: HttpServer.stop should terminate immediately if no exchanges are in progress
• ... and 92 more: https://git.openjdk.org/jdk/compare/428d33ef3ca0af34d8f164fe9d9b722e81e866a7...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

[openjdk]

@caspernorrbin The following labels will be automatically applied to this pull request:

• hotspot
• serviceability

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

[albertnetymk]

/cc hotspot-gc

[openjdk]

@albertnetymk
The hotspot-gc label was successfully added.

[mlbridge]

Webrevs

• 03: Full (e976ea68)
• 02: Full - Incremental (fb53bd44)
• 01: Full - Incremental (ed3670eb)
• 00: Full (7f8141ba)

[albertnetymk]

There are still a few matches for "hrt.ticks"; don't know if they should be removed (in this PR or a followup).

[caspernorrbin]

Thank you for reviewing @albertnetymk! I'll look into the last traces of "hrt.ticks" to see if they can be removed here.

[albertnetymk]

Reviewed only hotspot changes. Not familiar with other parts.

[jdksjolen]

All of this looks good to me.

It seems like there's no user-visible change, except the removal of the global variable. Is that correct?

[openjdk]

@caspernorrbin this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout statsampler-removal
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

[caspernorrbin]

It seems like there's no user-visible change, except the removal of the global variable. Is that correct?

Besides the global variable, there's a slight change in the behaviour of the perfdata-counters for Serial/Parallel, as mentioned in the PR. The JBS issue has a clarifying comment explaining why this shouldn't have any meaningful impact.

[kevinjwalls]

Hi, looks good.
If we are removing things from jcmd PerfCounter.print output, that could feature in the release note that you have planned. Anybody expecting these counters using jcmd PerfCounter.print or other methods, may not know that they are related to StatSampler and would not realise from the title that this is a relevant change.

$ jcmd 203133 PerfCounter.print | grep hrt
sun.os.hrt.frequency=1000000000
sun.os.hrt.ticks=132389861230

[caspernorrbin]

Hi, looks good.
If we are removing things from jcmd PerfCounter.print output, that could feature in the release note that you have planned. Anybody expecting these counters using jcmd PerfCounter.print or other methods, may not know that they are related to StatSampler and would not realise from the title that this is a relevant change.

Thank you. I will add that to the release note, along with text on how to calculate it instead using offsets (similar to what I did with the jstat counter)

[jdksjolen]

Thank you for the review, @kevinjwalls. Let's integrate it :-).

[caspernorrbin]

Thank you everyone for the reviews!

/integrate

[openjdk]

@caspernorrbin
Your change (at version e976ea6) is now ready to be sponsored by a Committer.

[albertnetymk]

/sponsor

[openjdk]

Going to push as commit 6ebae6c.
Since your change was applied there have been 95 commits pushed to the master branch:

• f30e154: 8357673: remove test serviceability/jvmti/vthread/TestPinCaseWithCFLH
• 0671309: 8357919: Arena::allocate returns segments with address zero if the segment length is zero after JDK-8345687
• 627ef34: 8304065: HttpServer.stop should terminate immediately if no exchanges are in progress
• ... and 92 more: https://git.openjdk.org/jdk/compare/428d33ef3ca0af34d8f164fe9d9b722e81e866a7...master

Your commit was automatically rebased without conflicts.

[openjdk]

@albertnetymk @caspernorrbin Pushed as commit 6ebae6c.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.