runtime: slowdown with green tea garbage collector #74923
Description
Go version
go version go1.25rc3 darwin/arm64
Output of go env in your module/workspace:
AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/bep/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/bep/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_g/j3j21hts4fn7__h04w2x8gb40000gn/T/go-build2628208990=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/bep/dev/go/gohugoio/hugo/go.mod'
GOMODCACHE='/Users/bep/dev/gomod_cache'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/bep/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org'
GOROOT='/Users/bep/sdk/go1.25rc3'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/Users/bep/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/bep/sdk/go1.25rc3/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.25rc3'
GOWORK=''
PKG_CONFIG='pkg-config'What did you do?
Build Hugo in two versions:
- With export GOEXPERIMENT=greenteagc
- With export GOEXPERIMENT=nogreenteagc
Then build this project https://github.com/bep/many-big-json-in-assets with both binaries:
HUGO_MEMORYLIMIT=8 hugo_greenteagc --logLevel info
HUGO_MEMORYLIMIT=8 hugo_nogreenteagc --logLevel info
What did you see happen?
I have repeated the above multiple times, and I get consistently build times around 40 seconds with geentea enabled, and around 20 seconds with it disabled.
Note that this has been reported before using rc1, and closed as fixed in #74375 -- but that points at a particular issue with nil checks on Linux.
I have tested the above on my MacBook Pro M1 with 32 GB memory.
What did you expect to see?
I expected the new garbage collector to be more effective, and at least not twice as ineffective, for the above case.