encoding/binary: more memory usage when encoding a slice of structs with smaller length. #66223

kwakubiney · 2024-03-09T16:49:15Z

Go version

go version devel go1.23-e8b5bc63be linux/amd64

Output of `go env` in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/root/.cache/go-build'
GOENV='/root/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/root/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/root/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/root/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/root/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.23-e8b5bc63be Wed Mar 6 01:33:19 2024 +0000'
GODEBUG=''
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/root/go/src/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1931883500=/tmp/go-build -gno-record-gcc-switches'

What did you do?

When using binary.Write for encoding a slice of structs, I encountered some weird behaviour where memory allocations in a particular path was more than I expected.

I wrote some benchmarks in the standard library's encoding/binary package to demonstrate this.

func BenchmarkWriteSlice1000Structs(b *testing.B) {
	slice := make([]Struct, 1000)
	buf := new(bytes.Buffer)
	var w io.Writer = buf
	b.SetBytes(int64(Size(slice)))
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		buf.Reset()
		Write(w, BigEndian, slice)
	}
	b.StopTimer()
}

func BenchmarkWriteSlice10Structs(b *testing.B) {
	slice := make([]Struct, 10)
	buf := new(bytes.Buffer)
	var w io.Writer = buf
	b.SetBytes(int64(Size(slice)))
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		buf.Reset()
		Write(w, BigEndian, slice)
	}
	b.StopTimer()
}

What did you see happen?

I ran both benchmarks and these are the results:

Encoding a slice with 1000 struct elements

root@ubuntu-s-2vcpu-2gb-fra1-01:~/go/src/encoding/binary#  ../../../bin/go test -run='^$' -memprofile memprofile.out -benchmem -bench  BenchmarkWriteSlice1000Structs -count=10
root@ubuntu-s-2vcpu-2gb-fra1-01:~/go/src/encoding/binary# ../../../bin/go tool pprof memprofile.out
File: binary.test
Type: alloc_space
Time: Mar 9, 2024 at 3:27pm (UTC)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 1305.40MB, 99.84% of 1307.48MB total
Dropped 8 nodes (cum <= 6.54MB)
      flat  flat%   sum%        cum   cum%
 1302.13MB 99.59% 99.59%  1304.21MB 99.75%  encoding/binary.Write
    3.27MB  0.25% 99.84%  1307.48MB   100%  encoding/binary.BenchmarkWriteSlice1000Structs
         0     0% 99.84%  1305.31MB 99.83%  testing.(*B).launch
         0     0% 99.84%  1307.48MB   100%  testing.(*B).runN

Encoding a slice with 10 struct elements

root@ubuntu-s-2vcpu-2gb-fra1-01:~/go/src/encoding/binary# > ../../../bin/go test -run='^$' -memprofile memprofile.out -benchmem -bench  BenchmarkWriteSlice10Structs -count=10
warning: GOPATH set to GOROOT (/root/go) has no effect
root@ubuntu-s-2vcpu-2gb-fra1-01:~/go/src/encoding/binary# ../../../bin/go tool pprof  memprofile.out
warning: GOPATH set to GOROOT (/root/go) has no effect
File: binary.test
Type: alloc_space
Time: Mar 9, 2024 at 4:24pm (UTC)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 905.58MB, 100% of 905.58MB total
      flat  flat%   sum%        cum   cum%
  792.58MB 87.52% 87.52%   905.58MB   100%  encoding/binary.Write
     113MB 12.48%   100%      113MB 12.48%  reflect.(*structType).Field
         0     0%   100%   905.58MB   100%  encoding/binary.BenchmarkWriteSlice10Structs
         0     0%   100%      113MB 12.48%  encoding/binary.dataSize
         0     0%   100%      113MB 12.48%  encoding/binary.sizeof
         0     0%   100%      113MB 12.48%  reflect.(*rtype).Field
         0     0%   100%   905.58MB   100%  testing.(*B).launch
         0     0%   100%   905.58MB   100%  testing.(*B).runN
(pprof)

What did you expect to see?

Per the benchmarks, there is a rise in total memory allocated incurred at reflect.(*structType).Field when encoding a slice of 10 struct elements compared to a slice of 1000 struct elements. I expected to see the memory incurred to be at worst, the same if not less when encoding a slice of structs with lesser length. I draw my conclusion from here since we are calling sizeof on the same struct type regardless of the length of the slice.

go/src/encoding/binary/binary.go

Line 483 in 74726de

if s := sizeof(v.Type().Elem()); s >= 0 {

Also, looking at the primary source of the allocations, per the line below, since we are working with the same struct type, I expect the number of allocations here to be the same regardless since both benchmarks are working with the same struct type and hence have the same fields.

go/src/reflect/type.go

Line 1061 in 74726de

f.Index = []int{i}

The text was updated successfully, but these errors were encountered:

kwakubiney · 2024-03-10T14:00:07Z

closing because i misinterpreted the profiling results

kwakubiney changed the title ~~encoding/binary: more memory usage when encoding slices of smaller length.~~ encoding/binary: more memory usage when encoding slices of structs with smaller length. Mar 9, 2024

kwakubiney changed the title ~~encoding/binary: more memory usage when encoding slices of structs with smaller length.~~ encoding/binary: more memory usage when encoding a slice of structs with smaller length. Mar 10, 2024

kwakubiney closed this as completed Mar 10, 2024

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encoding/binary: more memory usage when encoding a slice of structs with smaller length. #66223

encoding/binary: more memory usage when encoding a slice of structs with smaller length. #66223

kwakubiney commented Mar 9, 2024 •

edited

kwakubiney commented Mar 10, 2024 •

edited

encoding/binary: more memory usage when encoding a slice of structs with smaller length. #66223

encoding/binary: more memory usage when encoding a slice of structs with smaller length. #66223

Comments

kwakubiney commented Mar 9, 2024 • edited

Go version

Output of go env in your module/workspace:

What did you do?

What did you see happen?

What did you expect to see?

kwakubiney commented Mar 10, 2024 • edited

kwakubiney commented Mar 9, 2024 •

edited

Output of `go env` in your module/workspace:

kwakubiney commented Mar 10, 2024 •

edited