gctest1.go

package main

import (
	"fmt"
	"runtime"
	"time"
)

func main() {
	// One million slices
	allocs := make([][]byte, 1e6)
	// ... of 1 KiB each
	for i := 0; i < 1e6; i++ {
		allocs[i] = make([]byte, 1024)
		allocs[i][42] = 42 // Make sure the page is in use and allocated to us
	}

	fmt.Println("Allocations done, sleeping.")

	for i := 0; i < 1e6; i++ {
		if allocs[i][42] != 42 {
			panic("wat")
		}
		time.Sleep(time.Second)
		runtime.GC()
	}
}

// $ GODEBUG=gctrace=1 go run gctest1.go 
// ...
// gc 42 @36.317s 5%: 1.3+0+0+0+45 ms clock, 11+0+0+0.049/65/0.15+361 ms cpu, 999->999->999 MB, 999 MB goal, 8 P (forced)

gctest2.go

package main

import (
	"fmt"
	"runtime"
	"time"
)

func main() {
	allocs := make([][]byte, 1e6)
	backing := make([]byte, 1e6*1024)
	for i := 0; i < 1e6; i++ {
		allocs[i] = backing[i*1024 : (i+1)*1024]
		allocs[i][42] = 42 // Make sure the page is in use and allocated to us
	}

	fmt.Println("Allocations done, sleeping.")

	for i := 0; i < 1e6; i++ {
		if allocs[i][42] != 42 {
			panic("wat")
		}
		time.Sleep(time.Second)
		runtime.GC()
	}
}

// $ GODEBUG=gctrace=1 go run gctest2.go 
// gc 42 @42.525s 2%: 0.055+0+0+0+27 ms clock, 0.38+0+0+0.016/42/0.11+191 ms cpu, 999->999->999 MB, 999 MB goal, 8 P (forced)

gctest3.go

package main

import (
	"fmt"
	"runtime"
	"time"
)

func main() {
	backing := make([]byte, 1e6*1024)
	for i := 0; i < 1e6; i++ {
		backing[i*1024+42] = 42 // Make sure the page is in use and allocated to us
	}

	fmt.Println("Allocations done, sleeping.")

	for i := 0; i < 1e6; i++ {
		if backing[i*1024+42] != 42 {
			panic("wat")
		}
		time.Sleep(time.Second)
		runtime.GC()
	}
}

// $ GODEBUG=gctrace=1 go run gctest3.go 
// gc 42 @41.423s 0%: 0.050+0+0+0+0.34 ms clock, 0.35+0+0+0/0.065/0.041+2.4 ms cpu, 976->976->976 MB, 976 MB goal, 8 P (forced)