Add documentation for the revision garbage collector

[rhuss]

What is changing? (Please include as many details as possible.)

Currently, there is no comprehensive user-facing documentation for describing the revision garbage colletor, i.e. the way how unused revisions are removed and which parameters are used to configure the garbage collector.

How will this impact our users?

This will help the admin persona to configure the revision garbage collector according to her needs (+ of course understanding the way how the garbage collector works).

In what release will this take happen (to the best of your knowledge)?

Exists since quite some time already (no exact version known when introduced)

Context

Link to associated PRs or issues from other repos here.

• This is a Knative Serving feature
• The global config parameters influencing the revision garbage collector are described in https://github.com/knative/serving/blob/main/config/core/configmaps/gc.yaml
• Switching on/off the [Responsive Revision Garbage Collector] is described in https://knative.dev/docs/serving/feature-flags/#responsive-revision-garbage-collector
• Use case that require a different, more aggressive revision GC:
  • Redeploying Functions leaves many old revisions in the namespace func#285 (comment)
  • More aggressive Revision GC settings serving#8208

Additional info

This concept might be extended later with namespace or KService specific configuration to allow a Knative installation to serve multiple GC configurations, so the documentation might be extended a bit over time.

[abrennan89]

@rhuss was the documentation for https://knative.dev/docs/serving/feature-flags/#responsive-revision-garbage-collector removed? I don't see anything in the site for this when I search for garbage collector

[rhuss]

@abrennan89 good point, looks like that this flag has been removed in the meantime. I'm not aware of this change, but @knative/serving-writers should know.

[julz]

Yeah, there's no feature flag any more (as of knative/serving#11309) since gc v2 is now the default (and only) option

[rhuss]

@julz thanks for the pointer. But I guess we still would need documentation to explain how the garbage collector works in detail ? Or is this PR then obsolete, too ?

[julz]

No we definitely - imo - still need docs for the feature in general, just no longer for turning it on and off :-)

[abrennan89]

Thanks folks, can someone provide details for docs for this please, since presumably this won't be the same as the content at the original links in this issue?

[julz]

I think the comments in https://github.com/knative/serving/blob/main/config/core/configmaps/gc.yaml are actually a reasonably good start, but I'll happily work with someone to convert it in to a proper doc. Do you want me to try to do a first draft and if so is there a template I should use etc?

[abrennan89]

I think @RichardJJG was working on some doc templates but idk if they were finished / implemented yet.

The only thing I'd ask is that you add it to the admin guide under serving config if it's configmap stuff 😉 Happy for you to take a look if you want to, and I can give a review later if you ping when you need it.
Thanks!

/assign @julz