runtime: delete UpdateMemStats, replace with ReadMemStats(&stats).

Unexports runtime.MemStats and rename MemStatsType to MemStats.
The new accessor requires passing a pointer to a user-allocated
MemStats structure.

Fixes #2572.

R=bradfitz, rsc, bradfitz, gustavo
CC=golang-dev, remy
https://golang.org/cl/5616072

src/cmd/godoc/godoc.go

@@ -1510,9 +1510,12 @@ func updateIndex() {
 		log.Printf("index updated (%gs, %d bytes of source, %d files, %d lines, %d unique words, %d spots)",
 			secs, stats.Bytes, stats.Files, stats.Lines, stats.Words, stats.Spots)
 	}
-	log.Printf("before GC: bytes = %d footprint = %d", runtime.MemStats.HeapAlloc, runtime.MemStats.Sys)
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	log.Printf("before GC: bytes = %d footprint = %d", memstats.HeapAlloc, memstats.Sys)
 	runtime.GC()
-	log.Printf("after  GC: bytes = %d footprint = %d", runtime.MemStats.HeapAlloc, runtime.MemStats.Sys)
+	runtime.ReadMemStats(memstats)
+	log.Printf("after  GC: bytes = %d footprint = %d", memstats.HeapAlloc, memstats.Sys)
 }
 
 func indexer() {

src/pkg/encoding/gob/timing_test.go

@@ -53,8 +53,9 @@ func TestCountEncodeMallocs(t *testing.T) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
 	bench := &Bench{7, 3.2, "now is the time", []byte("for all good men")}
-	runtime.UpdateMemStats()
-	mallocs := 0 - runtime.MemStats.Mallocs
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	mallocs := 0 - memstats.Mallocs
 	const count = 1000
 	for i := 0; i < count; i++ {
 		err := enc.Encode(bench)
@@ -62,8 +63,8 @@ func TestCountEncodeMallocs(t *testing.T) {
 			t.Fatal("encode:", err)
 		}
 	}
-	runtime.UpdateMemStats()
-	mallocs += runtime.MemStats.Mallocs
+	runtime.ReadMemStats(memstats)
+	mallocs += memstats.Mallocs
 	fmt.Printf("mallocs per encode of type Bench: %d\n", mallocs/count)
 }
 
@@ -79,8 +80,9 @@ func TestCountDecodeMallocs(t *testing.T) {
 		}
 	}
 	dec := NewDecoder(&buf)
-	runtime.UpdateMemStats()
-	mallocs := 0 - runtime.MemStats.Mallocs
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	mallocs := 0 - memstats.Mallocs
 	for i := 0; i < count; i++ {
 		*bench = Bench{}
 		err := dec.Decode(&bench)
@@ -88,7 +90,7 @@ func TestCountDecodeMallocs(t *testing.T) {
 			t.Fatal("decode:", err)
 		}
 	}
-	runtime.UpdateMemStats()
-	mallocs += runtime.MemStats.Mallocs
+	runtime.ReadMemStats(memstats)
+	mallocs += memstats.Mallocs
 	fmt.Printf("mallocs per decode of type Bench: %d\n", mallocs/count)
 }

src/pkg/expvar/expvar.go

@@ -277,7 +277,9 @@ func cmdline() interface{} {
 }
 
 func memstats() interface{} {
-	return runtime.MemStats
+	stats := new(runtime.MemStats)
+	runtime.ReadMemStats(stats)
+	return *stats
 }
 
 func init() {

src/pkg/fmt/fmt_test.go

@@ -538,13 +538,14 @@ var _ bytes.Buffer
 func TestCountMallocs(t *testing.T) {
 	for _, mt := range mallocTest {
 		const N = 100
-		runtime.UpdateMemStats()
-		mallocs := 0 - runtime.MemStats.Mallocs
+		memstats := new(runtime.MemStats)
+		runtime.ReadMemStats(memstats)
+		mallocs := 0 - memstats.Mallocs
 		for i := 0; i < N; i++ {
 			mt.fn()
 		}
-		runtime.UpdateMemStats()
-		mallocs += runtime.MemStats.Mallocs
+		runtime.ReadMemStats(memstats)
+		mallocs += memstats.Mallocs
 		if mallocs/N > uint64(mt.count) {
 			t.Errorf("%s: expected %d mallocs, got %d", mt.desc, mt.count, mallocs/N)
 		}

src/pkg/net/rpc/server_test.go

@@ -442,8 +442,9 @@ func countMallocs(dial func() (*Client, error), t *testing.T) uint64 {
 	}
 	args := &Args{7, 8}
 	reply := new(Reply)
-	runtime.UpdateMemStats()
-	mallocs := 0 - runtime.MemStats.Mallocs
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	mallocs := 0 - memstats.Mallocs
 	const count = 100
 	for i := 0; i < count; i++ {
 		err := client.Call("Arith.Add", args, reply)
@@ -454,8 +455,8 @@ func countMallocs(dial func() (*Client, error), t *testing.T) uint64 {
 			t.Errorf("Add: expected %d got %d", reply.C, args.A+args.B)
 		}
 	}
-	runtime.UpdateMemStats()
-	mallocs += runtime.MemStats.Mallocs
+	runtime.ReadMemStats(memstats)
+	mallocs += memstats.Mallocs
 	return mallocs / count
 }
 

src/pkg/reflect/all_test.go

@@ -1545,15 +1545,19 @@ func TestAddr(t *testing.T) {
 func noAlloc(t *testing.T, n int, f func(int)) {
 	// once to prime everything
 	f(-1)
-	runtime.MemStats.Mallocs = 0
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	oldmallocs := memstats.Mallocs
 
 	for j := 0; j < n; j++ {
 		f(j)
 	}
 	// A few allocs may happen in the testing package when GOMAXPROCS > 1, so don't
 	// require zero mallocs.
-	if runtime.MemStats.Mallocs > 5 {
-		t.Fatalf("%d mallocs after %d iterations", runtime.MemStats.Mallocs, n)
+	runtime.ReadMemStats(memstats)
+	mallocs := memstats.Mallocs - oldmallocs
+	if mallocs > 5 {
+		t.Fatalf("%d mallocs after %d iterations", mallocs, n)
 	}
 }
 

src/pkg/runtime/gc_test.go

@@ -10,20 +10,21 @@ import (
 )
 
 func TestGcSys(t *testing.T) {
+	memstats := new(runtime.MemStats)
 	runtime.GC()
-	runtime.UpdateMemStats()
-	sys := runtime.MemStats.Sys
+	runtime.ReadMemStats(memstats)
+	sys := memstats.Sys
 
 	for i := 0; i < 1000000; i++ {
 		workthegc()
 	}
 
 	// Should only be using a few MB.
-	runtime.UpdateMemStats()
-	if sys > runtime.MemStats.Sys {
+	runtime.ReadMemStats(memstats)
+	if sys > memstats.Sys {
 		sys = 0
 	} else {
-		sys = runtime.MemStats.Sys - sys
+		sys = memstats.Sys - sys
 	}
 	t.Logf("used %d extra bytes", sys)
 	if sys > 4<<20 {

src/pkg/runtime/malloc.h

@@ -234,7 +234,7 @@ struct MStats
 	} by_size[NumSizeClasses];
 };
 
-#define mstats runtime·MemStats	/* name shared with Go */
+#define mstats runtime·memStats	/* name shared with Go */
 extern MStats mstats;
 
 

src/pkg/runtime/mem.go

@@ -6,9 +6,9 @@ package runtime
 
 import "unsafe"
 
-type MemStatsType struct {
+// A MemStats records statistics about the memory allocator.
+type MemStats struct {
 	// General statistics.
-	// Not locked during update; approximate.
 	Alloc      uint64 // bytes allocated and still in use
 	TotalAlloc uint64 // bytes allocated (even if freed)
 	Sys        uint64 // bytes obtained from system (should be sum of XxxSys below)
@@ -43,7 +43,6 @@ type MemStatsType struct {
 	DebugGC      bool
 
 	// Per-size allocation statistics.
-	// Not locked during update; approximate.
 	// 61 is NumSizeClasses in the C code.
 	BySize [61]struct {
 		Size    uint32
@@ -54,21 +53,17 @@ type MemStatsType struct {
 
 var sizeof_C_MStats uintptr // filled in by malloc.goc
 
+var memStats MemStats
+
 func init() {
-	if sizeof_C_MStats != unsafe.Sizeof(MemStats) {
-		println(sizeof_C_MStats, unsafe.Sizeof(MemStats))
+	if sizeof_C_MStats != unsafe.Sizeof(memStats) {
+		println(sizeof_C_MStats, unsafe.Sizeof(memStats))
 		panic("MStats vs MemStatsType size mismatch")
 	}
 }
 
-// MemStats holds statistics about the memory system.
-// The statistics may be out of date, as the information is
-// updated lazily from per-thread caches.
-// Use UpdateMemStats to bring the statistics up to date.
-var MemStats MemStatsType
-
-// UpdateMemStats brings MemStats up to date.
-func UpdateMemStats()
+// ReadMemStats populates m with memory allocator statistics.
+func ReadMemStats(m *MemStats)
 
 // GC runs a garbage collection.
 func GC()

src/pkg/runtime/mgc0.c

@@ -997,7 +997,7 @@ runtime·gc(int32 force)
 }
 
 void
-runtime·UpdateMemStats(void)
+runtime·ReadMemStats(MStats *stats)
 {
 	// Have to acquire gcsema to stop the world,
 	// because stoptheworld can only be used by
@@ -1007,6 +1007,7 @@ runtime·UpdateMemStats(void)
 	m->gcing = 1;
 	runtime·stoptheworld();
 	cachestats();
+	*stats = mstats;
 	m->gcing = 0;
 	runtime·semrelease(&gcsema);
 	runtime·starttheworld(false);

src/pkg/runtime/pprof/pprof.go

@@ -75,7 +75,8 @@ func WriteHeapProfile(w io.Writer) error {
 
 	// Print memstats information too.
 	// Pprof will ignore, but useful for people.
-	s := &runtime.MemStats
+	s := new(runtime.MemStats)
+	runtime.ReadMemStats(s)
 	fmt.Fprintf(b, "\n# runtime.MemStats\n")
 	fmt.Fprintf(b, "# Alloc = %d\n", s.Alloc)
 	fmt.Fprintf(b, "# TotalAlloc = %d\n", s.TotalAlloc)

src/pkg/strconv/itoa_test.go

@@ -127,11 +127,12 @@ func TestUitoa(t *testing.T) {
 }
 
 func numAllocations(f func()) int {
-	runtime.UpdateMemStats()
-	n0 := runtime.MemStats.Mallocs
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	n0 := memstats.Mallocs
 	f()
-	runtime.UpdateMemStats()
-	return int(runtime.MemStats.Mallocs - n0)
+	runtime.ReadMemStats(memstats)
+	return int(memstats.Mallocs - n0)
 }
 
 var globalBuf [64]byte

test/bench/garbage/parser.go

@@ -73,7 +73,7 @@ func parseDir(dirpath string) map[string]*ast.Package {
 }
 
 func main() {
-	st := &runtime.MemStats
+	st := new(runtime.MemStats)
 	packages = append(packages, packages...)
 	packages = append(packages, packages...)
 	n := flag.Int("n", 4, "iterations")
@@ -83,14 +83,17 @@ func main() {
 
 	var lastParsed []map[string]*ast.Package
 	var t0 time.Time
+	var numGC uint32
+	var pauseTotalNs uint64
 	pkgroot := runtime.GOROOT() + "/src/pkg/"
 	for pass := 0; pass < 2; pass++ {
 		// Once the heap is grown to full size, reset counters.
 		// This hides the start-up pauses, which are much smaller
 		// than the normal pauses and would otherwise make
 		// the average look much better than it actually is.
-		st.NumGC = 0
-		st.PauseTotalNs = 0
+		runtime.ReadMemStats(st)
+		numGC = st.NumGC
+		pauseTotalNs = st.PauseTotalNs
 		t0 = time.Now()
 
 		for i := 0; i < *n; i++ {
@@ -107,6 +110,9 @@ func main() {
 	}
 	t1 := time.Now()
 
+	runtime.ReadMemStats(st)
+	st.NumGC -= numGC
+	st.PauseTotalNs -= pauseTotalNs
 	fmt.Printf("Alloc=%d/%d Heap=%d Mallocs=%d PauseTime=%.3f/%d = %.3f\n",
 		st.Alloc, st.TotalAlloc,
 		st.Sys,
@@ -142,9 +148,7 @@ var packages = []string{
 	"container/list",
 	"container/ring",
 	"crypto/aes",
-	"crypto/blowfish",
 	"crypto/hmac",
-	"crypto/md4",
 	"crypto/md5",
 	"crypto/rand",
 	"crypto/rc4",
@@ -155,7 +159,6 @@ var packages = []string{
 	"crypto/subtle",
 	"crypto/tls",
 	"crypto/x509",
-	"crypto/xtea",
 	"debug/dwarf",
 	"debug/macho",
 	"debug/elf",
@@ -164,7 +167,6 @@ var packages = []string{
 	"encoding/ascii85",
 	"encoding/base64",
 	"encoding/binary",
-	"encoding/git85",
 	"encoding/hex",
 	"encoding/pem",
 	"os/exec",
@@ -193,8 +195,7 @@ var packages = []string{
 	"mime",
 	"net",
 	"os",
-	"os/signal",
-	"patch",
+	"exp/signal",
 	"path",
 	"math/rand",
 	"reflect",
@@ -219,6 +220,5 @@ var packages = []string{
 	"unicode",
 	"unicode/utf8",
 	"unicode/utf16",
-	"websocket",
 	"encoding/xml",
 }

test/bench/garbage/stats.go

@@ -12,7 +12,8 @@ import (
 )
 
 func gcstats(name string, n int, t time.Duration) {
-	st := &runtime.MemStats
+	st := new(runtime.MemStats)
+	runtime.ReadMemStats(st)
 	fmt.Printf("garbage.%sMem Alloc=%d/%d Heap=%d NextGC=%d Mallocs=%d\n", name, st.Alloc, st.TotalAlloc, st.Sys, st.NextGC, st.Mallocs)
 	fmt.Printf("garbage.%s %d %d ns/op\n", name, n, t.Nanoseconds()/int64(n))
 	fmt.Printf("garbage.%sLastPause 1 %d ns/op\n", name, st.PauseNs[(st.NumGC-1)%uint32(len(st.PauseNs))])

test/bench/garbage/tree2.go

@@ -30,6 +30,7 @@ var (
 	heap        *Object
 	calls       [20]int
 	numobjects  int64
+	memstats    runtime.MemStats
 )
 
 func buildHeap() {
@@ -55,10 +56,10 @@ func buildTree(objsize, size float64, depth int) (*Object, float64) {
 
 func gc() {
 	runtime.GC()
-	runtime.UpdateMemStats()
-	pause := runtime.MemStats.PauseTotalNs
-	inuse := runtime.MemStats.Alloc
-	free := runtime.MemStats.TotalAlloc - inuse
+	runtime.ReadMemStats(&memstats)
+	pause := memstats.PauseTotalNs
+	inuse := memstats.Alloc
+	free := memstats.TotalAlloc - inuse
 	fmt.Printf("gc pause: %8.3f ms; collect: %8.0f MB; heapsize: %8.0f MB\n",
 		float64(pause-lastPauseNs)/1e6,
 		float64(free-lastFree)/1048576,
@@ -71,9 +72,9 @@ func main() {
 	flag.Parse()
 	buildHeap()
 	runtime.GOMAXPROCS(*cpus)
-	runtime.UpdateMemStats()
-	lastPauseNs = runtime.MemStats.PauseTotalNs
-	lastFree = runtime.MemStats.TotalAlloc - runtime.MemStats.Alloc
+	runtime.ReadMemStats(&memstats)
+	lastPauseNs = memstats.PauseTotalNs
+	lastFree = memstats.TotalAlloc - memstats.Alloc
 	if *cpuprofile != "" {
 		f, err := os.Create(*cpuprofile)
 		if err != nil {

test/chan/select2.go

@@ -35,14 +35,17 @@ func main() {
 	go sender(c, 100000)
 	receiver(c, dummy, 100000)
 	runtime.GC()
-	runtime.MemStats.Alloc = 0
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	alloc := memstats.Alloc
 
 	// second time shouldn't increase footprint by much
 	go sender(c, 100000)
 	receiver(c, dummy, 100000)
 	runtime.GC()
+	runtime.ReadMemStats(memstats)
 
-	if runtime.MemStats.Alloc > 1e5 {
-		println("BUG: too much memory for 100,000 selects:", runtime.MemStats.Alloc)
+	if memstats.Alloc-alloc > 1e5 {
+		println("BUG: too much memory for 100,000 selects:", memstats.Alloc-alloc)
 	}
 }

test/closure.go

@@ -92,8 +92,9 @@ func main() {
 	go h()
 	check([]int{100, 200, 101, 201, 500, 101, 201, 500})
 
-	runtime.UpdateMemStats()
-        n0 := runtime.MemStats.Mallocs
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	n0 := memstats.Mallocs
 
 	x, y := newfunc(), newfunc()
 	if x(1) != 1 || y(2) != 2 {
@@ -101,8 +102,8 @@ func main() {
 		fail = true
 	}
 
-	runtime.UpdateMemStats()
-        if n0 != runtime.MemStats.Mallocs {
+	runtime.ReadMemStats(memstats)
+	if n0 != memstats.Mallocs {
 		println("newfunc allocated unexpectedly")
 		fail = true
 	}
@@ -110,7 +111,7 @@ func main() {
 	ff(1)
 
 	if fail {
-		panic("fail") 
+		panic("fail")
 	}
 }
 

test/gc2.go

@@ -19,7 +19,9 @@ import (
 
 func main() {
 	const N = 10000
-	st := runtime.MemStats
+	st := new(runtime.MemStats)
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(st)
 	for i := 0; i < N; i++ {
 		c := make(chan int, 10)
 		_ = c
@@ -33,8 +35,8 @@ func main() {
 		}
 	}
 
-	runtime.UpdateMemStats()
-	obj := runtime.MemStats.HeapObjects - st.HeapObjects
+	runtime.ReadMemStats(memstats)
+	obj := memstats.HeapObjects - st.HeapObjects
 	if obj > N/5 {
 		fmt.Println("too many objects left:", obj)
 		os.Exit(1)

test/init1.go

@@ -16,10 +16,11 @@ func init() {
 	c := make(chan int)
 	go send(c)
 	<-c
-	
-	const chunk = 1<<20
-	runtime.UpdateMemStats()
-	sys := runtime.MemStats.Sys	
+
+	const chunk = 1 << 20
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	sys := memstats.Sys
 	b := make([]byte, chunk)
 	for i := range b {
 		b[i] = byte(i%10 + '0')
@@ -28,8 +29,8 @@ func init() {
 	for i := 0; i < 1000; i++ {
 		x = []byte(s)
 	}
-	runtime.UpdateMemStats()
-	sys1 := runtime.MemStats.Sys
+	runtime.ReadMemStats(memstats)
+	sys1 := memstats.Sys
 	if sys1-sys > chunk*50 {
 		println("allocated 1000 chunks of", chunk, "and used ", sys1-sys, "memory")
 	}
@@ -41,4 +42,3 @@ func send(c chan int) {
 
 func main() {
 }
-

test/malloc1.go

@@ -17,9 +17,10 @@ import (
 var chatty = flag.Bool("v", false, "chatty")
 
 func main() {
+	memstats := new(runtime.MemStats)
 	runtime.Free(runtime.Alloc(1))
-	runtime.UpdateMemStats()
+	runtime.ReadMemStats(memstats)
 	if *chatty {
-		fmt.Printf("%+v %v\n", runtime.MemStats, uint64(0))
+		fmt.Printf("%+v %v\n", memstats, uint64(0))
 	}
 }

test/mallocrand.go

@@ -21,8 +21,9 @@ var footprint uint64
 var allocated uint64
 
 func bigger() {
-	runtime.UpdateMemStats()
-	if f := runtime.MemStats.Sys; footprint < f {
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	if f := memstats.Sys; footprint < f {
 		footprint = f
 		if *chatty {
 			println("Footprint", footprint, " for ", allocated)

test/mallocrep.go

@@ -16,10 +16,12 @@ import (
 var chatty = flag.Bool("v", false, "chatty")
 
 var oldsys uint64
+var memstats runtime.MemStats
 
 func bigger() {
-	runtime.UpdateMemStats()
-	if st := runtime.MemStats; oldsys < st.Sys {
+	st := &memstats
+	runtime.ReadMemStats(st)
+	if oldsys < st.Sys {
 		oldsys = st.Sys
 		if *chatty {
 			println(st.Sys, " system bytes for ", st.Alloc, " Go bytes")
@@ -32,26 +34,26 @@ func bigger() {
 }
 
 func main() {
-	runtime.GC()               // clean up garbage from init
-	runtime.UpdateMemStats()   // first call can do some allocations
-	runtime.MemProfileRate = 0 // disable profiler
-	runtime.MemStats.Alloc = 0 // ignore stacks
+	runtime.GC()                    // clean up garbage from init
+	runtime.ReadMemStats(&memstats) // first call can do some allocations
+	runtime.MemProfileRate = 0      // disable profiler
+	stacks := memstats.Alloc        // ignore stacks
 	flag.Parse()
 	for i := 0; i < 1<<7; i++ {
 		for j := 1; j <= 1<<22; j <<= 1 {
 			if i == 0 && *chatty {
 				println("First alloc:", j)
 			}
-			if a := runtime.MemStats.Alloc; a != 0 {
+			if a := memstats.Alloc - stacks; a != 0 {
 				println("no allocations but stats report", a, "bytes allocated")
 				panic("fail")
 			}
 			b := runtime.Alloc(uintptr(j))
-			runtime.UpdateMemStats()
-			during := runtime.MemStats.Alloc
+			runtime.ReadMemStats(&memstats)
+			during := memstats.Alloc - stacks
 			runtime.Free(b)
-			runtime.UpdateMemStats()
-			if a := runtime.MemStats.Alloc; a != 0 {
+			runtime.ReadMemStats(&memstats)
+			if a := memstats.Alloc - stacks; a != 0 {
 				println("allocated ", j, ": wrong stats: during=", during, " after=", a, " (want 0)")
 				panic("fail")
 			}

test/mallocrep1.go

@@ -20,7 +20,7 @@ var reverse = flag.Bool("r", false, "reverse")
 var longtest = flag.Bool("l", false, "long test")
 
 var b []*byte
-var stats = &runtime.MemStats
+var stats = new(runtime.MemStats)
 
 func OkAmount(size, n uintptr) bool {
 	if n < size {
@@ -42,7 +42,7 @@ func AllocAndFree(size, count int) {
 	if *chatty {
 		fmt.Printf("size=%d count=%d ...\n", size, count)
 	}
-	runtime.UpdateMemStats()
+	runtime.ReadMemStats(stats)
 	n1 := stats.Alloc
 	for i := 0; i < count; i++ {
 		b[i] = runtime.Alloc(uintptr(size))
@@ -51,13 +51,13 @@ func AllocAndFree(size, count int) {
 			println("lookup failed: got", base, n, "for", b[i])
 			panic("fail")
 		}
-		runtime.UpdateMemStats()
+		runtime.ReadMemStats(stats)
 		if stats.Sys > 1e9 {
 			println("too much memory allocated")
 			panic("fail")
 		}
 	}
-	runtime.UpdateMemStats()
+	runtime.ReadMemStats(stats)
 	n2 := stats.Alloc
 	if *chatty {
 		fmt.Printf("size=%d count=%d stats=%+v\n", size, count, *stats)
@@ -75,17 +75,17 @@ func AllocAndFree(size, count int) {
 			panic("fail")
 		}
 		runtime.Free(b[i])
-		runtime.UpdateMemStats()
+		runtime.ReadMemStats(stats)
 		if stats.Alloc != uint64(alloc-n) {
 			println("free alloc got", stats.Alloc, "expected", alloc-n, "after free of", n)
 			panic("fail")
 		}
-		if runtime.MemStats.Sys > 1e9 {
+		if stats.Sys > 1e9 {
 			println("too much memory allocated")
 			panic("fail")
 		}
 	}
-	runtime.UpdateMemStats()
+	runtime.ReadMemStats(stats)
 	n4 := stats.Alloc
 
 	if *chatty {