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Blog post: New conversion from cgroup v1 CPU shares to v2 CPU weight #52793

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@iholder101 iholder101 commented Oct 19, 2025

Description

Add blog post: new cgroup v1 to v2 CPU weight conversion formula.

Document what was done in kubernetes/kubernetes#131216 with proposed quadratic formula for better CPU weight granularity and priority alignment.

Issue

Closes: #52639.

@k8s-ci-robot k8s-ci-robot added cncf-cla: yes Indicates the PR's author has signed the CNCF CLA. area/blog Issues or PRs related to the Kubernetes Blog subproject labels Oct 19, 2025
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@iholder101 iholder101 marked this pull request as draft October 19, 2025 13:19
@k8s-ci-robot k8s-ci-robot added the do-not-merge/work-in-progress Indicates that a PR should not merge because it is a work in progress. label Oct 19, 2025
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@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch 2 times, most recently from 5ad59ff to 9599c25 Compare October 19, 2025 13:32
@iholder101 iholder101 marked this pull request as ready for review October 19, 2025 13:32
@k8s-ci-robot k8s-ci-robot removed the do-not-merge/work-in-progress Indicates that a PR should not merge because it is a work in progress. label Oct 19, 2025
@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch from 9599c25 to d045e7d Compare October 19, 2025 13:32
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iholder101 commented Oct 19, 2025

@lmktfy I've tried to add a mathematical graph as an image. This is pretty inelegant. Do you have an idea for adding graphs in a more idiomatic way? Thanks!

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iholder101 commented Oct 19, 2025

/test all

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SergeyKanzhelev
SergeyKanzhelev reviewed Oct 20, 2025
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content/en/blog/_posts/2025-10-25-new-cgroup-v1-to-v2-conversion-formula.md Outdated
After a while, cgroup v1 was stared being replaced by its successor,
cgroup v2. In cgroup v2, the concept of CPU shares (which ranges from 2 to
262144, or from 2^1 to 2^18) was replaced with CPU weight (which ranges from
1 to 10000, or 10^10 to 10^4).
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Suggested change
1 to 10000, or 10^10 to 10^4).
1 to 10000, or 10^1 to 10^4).

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You can use inline maths formulae BTW - see the pages about contributing to Kubernetes documentation.

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@iholder101 iholder101 Oct 21, 2025

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@SergeyKanzhelev done! (it's actually 10^0 :) this is confusing).

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@iholder101 iholder101 Oct 21, 2025

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@lmktfy done.
Also, as asked in #52793 (comment), WDYT about the inlined pictures?

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Thanks; here's some early feedback. We'll try to assign a blog writing buddy soon.

content/en/blog/_posts/2025-10-25-new-cgroup-v1-to-v2-conversion-formula.md Outdated
After a while, cgroup v1 was stared being replaced by its successor,
cgroup v2. In cgroup v2, the concept of CPU shares (which ranges from 2 to
262144, or from 2^1 to 2^18) was replaced with CPU weight (which ranges from
1 to 10000, or 10^10 to 10^4).
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You can use inline maths formulae BTW - see the pages about contributing to Kubernetes documentation.

@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch from d045e7d to 1db2891 Compare October 21, 2025 08:46
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iholder101 commented Oct 21, 2025

Thanks a lot everyone for your reviews!

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content/en/blog/_posts/2025-10-25-new-cgroup-v1-to-v2-conversion-formula/index.md Outdated
Comment on lines 136 to 134
This change was implemented as an OCI-level implementation.
In other words, this is not implemented Kubernetes itself, therefore the
adoption of the new conversion formula depends solely on the OCI runtime
adoption.
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since we vendor the library in kubernetes I believe kubernetes is affected 1.35 and above as well for pod level cpu.weight, no? @SergeyKanzhelev

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Yes, I think k8s is affected. But I didn't look deeper on what the effect will be and whether 1.35 will support both calculations.

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I wonder where this function is used in cadvisor: google/cadvisor#3737

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ok, it looks like that helper is not being used. @haircommander do you see any more issues here?

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we do use a copy of the function here https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/cm/cgroup_v2_manager_linux.go#L174

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we do use a copy of the function here kubernetes/kubernetes@master/pkg/kubelet/cm/cgroup_v2_manager_linux.go#L174

Good point!
I can make a PR to change this conversion function.

@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch 2 times, most recently from fbfa379 to 83e49a4 Compare October 23, 2025 12:04
lmktfy
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content/en/blog/_posts/2025-10-25-new-cgroup-v1-to-v2-conversion-formula.md Outdated
requesting 1 CPU has equal priority with system processes that live outside
of Kubernetes' scope.
However, in cgroup v2, the default CPU weight is `100`, but the current
formula converts 1 CPU (1024m) to only `~39` weight - less than 40% of the
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Did you mean the approximately equals symbol here? We can use the real symbol, ≈.

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@iholder101 iholder101 Oct 29, 2025

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Good point! Done

@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch 2 times, most recently from f2b314d to fa463f4 Compare October 29, 2025 09:02
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iholder101 commented Oct 29, 2025

@lmktfy I believe all is addressed. PTAL

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Reviewed for general proofreading and made some suggestions to align with the style guide, please let me know if you have any questions or concerns! :)

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SergeyKanzhelev approved these changes Oct 30, 2025
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/lgtm

approving for sig node content.

@k8s-ci-robot k8s-ci-robot added the lgtm "Looks good to me", indicates that a PR is ready to be merged. label Oct 30, 2025
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k8s-ci-robot commented Oct 30, 2025

LGTM label has been added.

Git tree hash: 33dbe84b0d2a91e51e2d10f1ffd72fcba40dca52

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: SergeyKanzhelev
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@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch from fa463f4 to 870b38d Compare October 30, 2025 09:12
@iholder101
Blog post: New conversion from cgroup v1 CPU shares to v2 CPU weight
40a6197 
Signed-off-by: Itamar Holder <iholder@redhat.com>
@k8s-ci-robot k8s-ci-robot removed the lgtm "Looks good to me", indicates that a PR is ready to be merged. label Oct 30, 2025
@iholder101 iholder101 force-pushed the blog/cgroup-share-to-weight-conversion branch from 870b38d to 40a6197 Compare October 30, 2025 09:12
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iholder101 commented Oct 30, 2025

@Serenity611 thanks for your review! I believe all is addressed.

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iholder101 commented Nov 2, 2025

I see that feature blog freeze is behind us. Does this have implications regarding this post?
I think it'd be valuable to make it publish in the v1.35 timeframe since the changes already apply.

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SergeyKanzhelev commented Nov 4, 2025

/lgtm

@k8s-ci-robot k8s-ci-robot added the lgtm "Looks good to me", indicates that a PR is ready to be merged. label Nov 4, 2025
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k8s-ci-robot commented Nov 4, 2025

LGTM label has been added.

Git tree hash: 3514edf1a3fe6b89cf0a4500497a01b712cdbe25

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Describe the migration from systemd's cgroup v1 CPU shares to v2 CPU weight formula

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