On Wed, 5 Nov 2025 18:13:55 GMT, Severin Gehwolf <[email protected]> wrote:
> Please review this change to the Linux container detection code updating the
> mapping function of cg v2 to most recent runtimes' implementation. OCI has
> been standardized with cgroup v1 in mind and there is a mapping function in
> place - implemented by container runtimes such as `runc` or `crun` - which
> map the passed `--cpu-shares` value on the engine's `run` command to the
> internal cgroup v2 `cpu.weight` interface file's values.
>
> For runtimes lower than 1.23 (crun) and 1.3.2 (runc) the mapping function was
> (where `f(x)=<cpu.weight value>` and `x=<cpu-shares CLI value>`):
>
>
> f(x) = {(x - 2) \times 9999 + 1 \over 262142}
>
>
> This changed in `crun` >= 1.23 and `runc` >= 1.3.2 to:
>
>
> f(x)=10^{({{\log_{2}(x)}^2 \over 612} + {125 \over 612} \times \log_{2}(x) -
> {7 \over 34})}
>
>
> Therefore, we need to find the inverse of the new quadratic mapping function
> and use that over the inverse of the old mapping function. Since the input
> domain is `[2,262144]` for `--cpu-shares` mapping to `[1,10000]` for
> `cpu.weight` we can discount the negative exponent value as a possible
> solution.
>
> Since there are production systems out there with those runtimes already I
> believe it's time to change the mapping function internal to the JDK to the
> new one in JDK 26. There is a chance that JDK 26 would run on old runtimes in
> a container using cgroup v2, resulting in wrong diagnostics. The chance of
> this happening will decrease over time.
>
> Thoughts?
>
> **Testing:**
> - [x] GHA
> - [x] `test/hotspot/jtreg/containers` and `test/jdk/jdk/internal/platform` on
> cgroup v1 (with an old runtime - code not affected) and on cgroup v2 with a
> new runtime. All tests passed.
This pull request has now been integrated.
Changeset: 5d65c23c
Author: Severin Gehwolf <[email protected]>
URL:
https://git.openjdk.org/jdk/commit/5d65c23cd99b72527dcfab9eb6da9510e7dc6330
Stats: 107 lines in 4 files changed: 90 ins; 0 del; 17 mod
8370492: [Linux] Update cpu shares to cpu.weight mapping function
Reviewed-by: cnorrbin, ayang, syan
-------------
PR: https://git.openjdk.org/jdk/pull/28157
