[go] runtime: use floor instead of ceil for cgroup CPU limits in adjustCgroupGOMAXPROCS
Gerrit Bot (Gerrit)
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Commit message
runtime: use floor instead of ceil for cgroup CPU limits in adjustCgroupGOMAXPROCS
## Summary
Change the computation to use floor instead of ceil, ensuring GOMAXPROCS never exceeds the available cgroup CPU quota under fractional values. Fractional CPU limits now round down, except values less than 2 still round up to 2 to preserve minimum parallelism.
This reduces contention and CPU throttling observed in production deployments with fractional CPU quotas.
Proposal: https://github.com/golang/go/issues/76835
Closes #76835
## Evidences
### Before change
<img width="475" height="800" alt="before" src="https://github.com/user-attachments/assets/51e6f332-4d59-479f-a0d4-3108bfe8db90" />
### After change
<img width="475" height="800" alt="after" src="https://github.com/user-attachments/assets/4d96b2f0-ef0b-4293-a430-d673fa9da852" />
Change diff
diff --git a/src/runtime/cgroup_linux.go b/src/runtime/cgroup_linux.go
index 73e7363..2695e63 100644
--- a/src/runtime/cgroup_linux.go
+++ b/src/runtime/cgroup_linux.go
@@ -89,7 +89,10 @@
//
// 2. The average CPU cgroup throughput limit (average throughput =
// quota/period). A limit less than 2 is rounded up to 2, and any
- // fractional component is rounded up.
+ // fractional component is rounded down. The reason for rounding down is
+ // to avoid oversubscribing the cgroup when fractional quotas are used. Using
+ // floor keeps GOMAXPROCS within the effective quota, reducing the risk of
+ // throttling under sustained load.
//
// TODO: add rationale.
@@ -109,7 +112,7 @@
func adjustCgroupGOMAXPROCS(procs int32, cpu cgroup.CPU) int32 {
limit, ok, err := cgroup.ReadCPULimit(cpu)
if err == nil && ok {
- limit = ceil(limit)
+ limit = floor(limit)
limit = max(limit, 2)
if int32(limit) < procs {
procs = int32(limit)
Change information
- M src/runtime/cgroup_linux.go
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Gopher Robot (Gerrit)
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I spotted some possible problems with your PR:
1. You have a long 259 character line in the commit message body. Please add line breaks to long lines that should be wrapped. Lines in the commit message body should be wrapped at ~76 characters unless needed for things like URLs or tables. (Note: GitHub might render long lines as soft-wrapped, so double-check in the Gerrit commit message shown above.)
2. It looks like you have a properly formated bug reference, but the convention is to put bug references at the bottom of the commit message, even if a bug is also mentioned in the body of the message.
Please address any problems by updating the GitHub PR.
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Marcos P. Sánchez (Gerrit)
Marcos P. Sánchez added 1 comment![]( "Open in Gerrit")
I spotted some possible problems with your PR:
1. You have a long 259 character line in the commit message body. Please add line breaks to long lines that should be wrapped. Lines in the commit message body should be wrapped at ~76 characters unless needed for things like URLs or tables. (Note: GitHub might render long lines as soft-wrapped, so double-check in the Gerrit commit message shown above.)
2. It looks like you have a properly formated bug reference, but the convention is to put bug references at the bottom of the commit message, even if a bug is also mentioned in the body of the message.Please address any problems by updating the GitHub PR.
When complete, mark this comment as 'Done' and click the [blue 'Reply' button](https://go.dev/wiki/GerritBot#i-left-a-reply-to-a-comment-in-gerrit-but-no-one-but-me-can-see-it) above. These findings are based on heuristics; if a finding does not apply, briefly reply here saying so.
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For more details, see:
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- the Go project's [conventions for commit messages](https://go.dev/doc/contribute#commit_messages) that you should follow.
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