runtime: cache grown goroutine stacks to reduce stack-growth cost by geekswaroop · Pull Request #80137 · golang/go

geekswaroop · 2026-06-24T14:50:50Z

A dead goroutine's stack is freed in gfput unless it is exactly startingStackSize, and gfget re-frees a stack whose size no longer matches. So a goroutine that grew its stack returns that memory as soon as it dies, and the next goroutine to reuse the g must grow it again from scratch.

In production we observe stack growth (copystack) costing ~3.9% of CPU across the fleet, comparable to GC. The cost is dominated by short-lived RPC goroutines that grow to 16-32 KiB and die before the next GC, so their size is never reflected in startingStackSize and each one pays to grow again.

Instead, retain a dead goroutine's grown stack on the per-P free-g list for reuse, freeing only stacks larger than a 128 KiB cap. Ephemeral goroutines reuse an already-grown stack, while long-lived goroutines are still shrunk by GC. The per-P free-g list is already capped at 64 entries, so the additional retained memory is bounded.

BenchmarkIssue18138 spawns short-lived goroutines that grow their stacks and then die. Goroutines whose stacks stay under the 128 KiB cap (depth=64) reuse a grown stack instead of regrowing; those that exceed the cap (depth=1000, ~1 MiB) are largely unaffected:

                     │   vanilla    │             stackcache              │
                     │    sec/op    │   sec/op     vs base                │

Issue18138/depth=64-48 11.012µ ± 2% 4.309µ ± 5% -60.87% (p=0.000 n=10)
Issue18138/depth=1000-48 62.04µ ± 2% 57.30µ ± 3% -7.65% (p=0.000 n=10)
geomean 26.14µ 15.71µ -39.89%

Updates #77893

A dead goroutine's stack is freed in gfput unless it is exactly startingStackSize, and gfget re-frees a stack whose size no longer matches. So a goroutine that grew its stack returns that memory as soon as it dies, and the next goroutine to reuse the g must grow it again from scratch. In production we observe stack growth (copystack) costing ~3.9% of CPU across the fleet, comparable to GC. The cost is dominated by short-lived RPC goroutines that grow to 16-32 KiB and die before the next GC, so their size is never reflected in startingStackSize and each one pays to grow again. Instead, retain a dead goroutine's grown stack on the per-P free-g list for reuse, freeing only stacks larger than a 128 KiB cap. Ephemeral goroutines reuse an already-grown stack, while long-lived goroutines are still shrunk by GC. The per-P free-g list is already capped at 64 entries, so the additional retained memory is bounded. BenchmarkIssue18138 spawns short-lived goroutines that grow their stacks and then die. Goroutines whose stacks stay under the 128 KiB cap (depth=64) reuse a grown stack instead of regrowing; those that exceed the cap (depth=1000, ~1 MiB) are largely unaffected: │ vanilla │ stackcache │ │ sec/op │ sec/op vs base │ Issue18138/depth=64-48 11.012µ ± 2% 4.309µ ± 5% -60.87% (p=0.000 n=10) Issue18138/depth=1000-48 62.04µ ± 2% 57.30µ ± 3% -7.65% (p=0.000 n=10) geomean 26.14µ 15.71µ -39.89% Updates golang#77893

gopherbot · 2026-06-24T15:18:59Z

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gopherbot · 2026-06-24T15:39:21Z

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