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runtime: floor cgroup CPU limits in adjustCgroupGOMAXPROCS #76863

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runtime: floor cgroup CPU limits in adjustCgroupGOMAXPROCS #76863

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7 changes: 5 additions & 2 deletions src/runtime/cgroup_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,10 @@ func defaultGOMAXPROCS(ncpu int32) int32 {
//
// 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.

Expand All @@ -109,7 +112,7 @@ func defaultGOMAXPROCS(ncpu int32) int32 {
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)
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