Unexpected rusage data of child process

[si9ma]

What version of Go are you using (go version)?

$ go version

go version go1.12.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"

GOBIN=""

GOCACHE="/home/si9ma/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="/home/si9ma/Go"

GOPROXY=""

GORACE=""

GOROOT="/usr/lib/go"

GOTMPDIR=""

GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD=""

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build113116426=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I use follow code to get ru_maxrss data:

package main

import (
	"fmt"
	"os/exec"
	"syscall"
)

func main() {
	cmd := exec.Command("/usr/bin/ls")
	if err := cmd.Run(); err != nil {
		fmt.Println("error", err)
	}

	ru := cmd.ProcessState.SysUsage().(*syscall.Rusage)
	fmt.Println(ru.Maxrss)
}

the result is 1960

However, if I import some other package,eg:

package main

import (
	"fmt"
	"os/exec"
	"syscall"

	_ "github.com/gin-gonic/gin"
	_ "github.com/go-redis/redis"
	_ "github.com/go-sql-driver/mysql"
)

func main() {
	cmd := exec.Command("/usr/bin/ls")
	if err := cmd.Run(); err != nil {
		fmt.Println("error", err)
	}

	ru := cmd.ProcessState.SysUsage().(*syscall.Rusage)
	fmt.Println(ru.Maxrss)
}

I will get the result 8928

What did you expect to see?

I just want to get the rusage data of child process

What did you see instead?

It seems that child process inherit the rusage data of parent process.

[ianlancetaylor]

That is how Unix systems work. A new executable is started by calling fork and then execve. The RSS size after fork is inevitably similar to the RSS size of the parent process. The max RSS size reported by SysUsage is therefore never less than the RSS size of the parent process. You will see the same behavior in a C program. The field is only useful for measuring large child processes, not small ones.