8378176: Concurrent GC worker threads may suffer from priority inversion

[fisk]

GC worker threads have elevated OS priorities, which has been found to cause priority inversion.

Increasing the priority of a thread is not something that should be done lightly. It can backfire quite a lot when facing priority inversion. Also, the thought is often that you want one set of threads in your process to be prioritized higher than another set of threads in your process. Except, priorities are global, so it becomes hard to reason about not just the priority of threads within the process, but also whether they are really more important than other threads on the system, or not.

These issues are greater with concurrent GC threads that interact not just with the other GC threads, but also with application threads. The risk for priority inversion is higher then.

Other than the difficulty of getting priorities right, elevating priorities is not possible on Linux and Mac OS, without super user privileges, which seemingly makes any potential "benefits" kind of limited to windows. Yet, running through our suite of benchmarks, no benefits have actually been sighted. Instead, I have seen priority inversion problems with ZGC.

Having concurrent worker threads have blanket high priority without inspecting everything they do with great care, is outright dangerous. This patch makes it so that concurrent GC workers have normal OS priority.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8378176: Concurrent GC worker threads may suffer from priority inversion (Bug - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/30242/head:pull/30242
$ git checkout pull/30242

Update a local copy of the PR:
$ git checkout pull/30242
$ git pull https://git.openjdk.org/jdk.git pull/30242/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 30242

View PR using the GUI difftool:
$ git pr show -t 30242

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/30242.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back eosterlund! 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]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

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

• hotspot
• shenandoah

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.

[fisk]

/label add hotspot-gc
/label remove hotspot
/label remove shenandoah

[openjdk]

@fisk
The hotspot-gc label was successfully added.

[openjdk]

@fisk
The hotspot label was successfully removed.

[openjdk]

@fisk
The shenandoah label was successfully removed.

[mlbridge]

Webrevs

• 00: Full (6e941196)