src/runtime/testdata/testprog/gc.go - go - Git at Google

 // Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package main

 import (
 	"fmt"
 	"math"
 	"os"
 	"runtime"
 	"runtime/debug"
 	"runtime/metrics"
 	"sync"
 	"sync/atomic"
 	"time"
 	"unsafe"
 )

 func init() {
 	register("GCFairness", GCFairness)
 	register("GCFairness2", GCFairness2)
 	register("GCSys", GCSys)
 	register("GCPhys", GCPhys)
 	register("DeferLiveness", DeferLiveness)
 	register("GCZombie", GCZombie)
 	register("GCMemoryLimit", GCMemoryLimit)
 	register("GCMemoryLimitNoGCPercent", GCMemoryLimitNoGCPercent)
 }

 func GCSys() {
 	runtime.GOMAXPROCS(1)
 	memstats := new(runtime.MemStats)
 	runtime.GC()
 	runtime.ReadMemStats(memstats)
 	sys := memstats.Sys

 	runtime.MemProfileRate = 0 // disable profiler

 	itercount := 100000
 	for i := 0; i < itercount; i++ {
 		workthegc()
 	}

 	// Should only be using a few MB.
 	// We allocated 100 MB or (if not short) 1 GB.
 	runtime.ReadMemStats(memstats)
 	if sys > memstats.Sys {
 		sys = 0
 	} else {
 		sys = memstats.Sys - sys
 	}
 	if sys > 16<<20 {
 		fmt.Printf("using too much memory: %d bytes\n", sys)
 		return
 	}
 	fmt.Printf("OK\n")
 }

 var sink []byte

 func workthegc() []byte {
 	sink = make([]byte, 1029)
 	return sink
 }

 func GCFairness() {
 	runtime.GOMAXPROCS(1)
 	f, err := os.Open("/dev/null")
 	if os.IsNotExist(err) {
 		// This test tests what it is intended to test only if writes are fast.
 		// If there is no /dev/null, we just don't execute the test.
 		fmt.Println("OK")
 		return
 	}
 	if err != nil {
 		fmt.Println(err)
 		os.Exit(1)
 	}
 	for i := 0; i < 2; i++ {
 		go func() {
 			for {
 				f.Write([]byte("."))
 			}
 		}()
 	}
 	time.Sleep(10 * time.Millisecond)
 	fmt.Println("OK")
 }

 func GCFairness2() {
 	// Make sure user code can't exploit the GC's high priority
 	// scheduling to make scheduling of user code unfair. See
 	// issue #15706.
 	runtime.GOMAXPROCS(1)
 	debug.SetGCPercent(1)
 	var count [3]int64
 	var sink [3]any
 	for i := range count {
 		go func(i int) {
 			for {
 				sink[i] = make([]byte, 1024)
 				atomic.AddInt64(&count[i], 1)
 			}
 		}(i)
 	}
 	// Note: If the unfairness is really bad, it may not even get
 	// past the sleep.
 	//
 	// If the scheduling rules change, this may not be enough time
 	// to let all goroutines run, but for now we cycle through
 	// them rapidly.
 	//
 	// OpenBSD's scheduler makes every usleep() take at least
 	// 20ms, so we need a long time to ensure all goroutines have
 	// run. If they haven't run after 30ms, give it another 1000ms
 	// and check again.
 	time.Sleep(30 * time.Millisecond)
 	var fail bool
 	for i := range count {
 		if atomic.LoadInt64(&count[i]) == 0 {
 			fail = true
 		}
 	}
 	if fail {
 		time.Sleep(1 * time.Second)
 		for i := range count {
 			if atomic.LoadInt64(&count[i]) == 0 {
 				fmt.Printf("goroutine %d did not run\n", i)
 				return
 			}
 		}
 	}
 	fmt.Println("OK")
 }

 func GCPhys() {
 	// This test ensures that heap-growth scavenging is working as intended.
 	//
 	// It attempts to construct a sizeable "swiss cheese" heap, with many
 	// allocChunk-sized holes. Then, it triggers a heap growth by trying to
 	// allocate as much memory as would fit in those holes.
 	//
 	// The heap growth should cause a large number of those holes to be
 	// returned to the OS.

 	const (
 		// The total amount of memory we're willing to allocate.
 		allocTotal = 32 << 20

 		// The page cache could hide 64 8-KiB pages from the scavenger today.
 		maxPageCache = (8 << 10) * 64
 	)

 	// How big the allocations are needs to depend on the page size.
 	// If the page size is too big and the allocations are too small,
 	// they might not be aligned to the physical page size, so the scavenger
 	// will gloss over them.
 	pageSize := os.Getpagesize()
 	var allocChunk int
 	if pageSize <= 8<<10 {
 		allocChunk = 64 << 10
 	} else {
 		allocChunk = 512 << 10
 	}
 	allocs := allocTotal / allocChunk

 	// Set GC percent just so this test is a little more consistent in the
 	// face of varying environments.
 	debug.SetGCPercent(100)

 	// Set GOMAXPROCS to 1 to minimize the amount of memory held in the page cache,
 	// and to reduce the chance that the background scavenger gets scheduled.
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))

 	// Allocate allocTotal bytes of memory in allocChunk byte chunks.
 	// Alternate between whether the chunk will be held live or will be
 	// condemned to GC to create holes in the heap.
 	saved := make([][]byte, allocs/2+1)
 	condemned := make([][]byte, allocs/2)
 	for i := 0; i < allocs; i++ {
 		b := make([]byte, allocChunk)
 		if i%2 == 0 {
 			saved = append(saved, b)
 		} else {
 			condemned = append(condemned, b)
 		}
 	}

 	// Run a GC cycle just so we're at a consistent state.
 	runtime.GC()

 	// Drop the only reference to all the condemned memory.
 	condemned = nil

 	// Clear the condemned memory.
 	runtime.GC()

 	// At this point, the background scavenger is likely running
 	// and could pick up the work, so the next line of code doesn't
 	// end up doing anything. That's fine. What's important is that
 	// this test fails somewhat regularly if the runtime doesn't
 	// scavenge on heap growth, and doesn't fail at all otherwise.

 	// Make a large allocation that in theory could fit, but won't
 	// because we turned the heap into swiss cheese.
 	saved = append(saved, make([]byte, allocTotal/2))

 	// heapBacked is an estimate of the amount of physical memory used by
 	// this test. HeapSys is an estimate of the size of the mapped virtual
 	// address space (which may or may not be backed by physical pages)
 	// whereas HeapReleased is an estimate of the amount of bytes returned
 	// to the OS. Their difference then roughly corresponds to the amount
 	// of virtual address space that is backed by physical pages.
 	//
 	// heapBacked also subtracts out maxPageCache bytes of memory because
 	// this is memory that may be hidden from the scavenger per-P. Since
 	// GOMAXPROCS=1 here, subtracting it out once is fine.
 	var stats runtime.MemStats
 	runtime.ReadMemStats(&stats)
 	heapBacked := stats.HeapSys - stats.HeapReleased - maxPageCache
 	// If heapBacked does not exceed the heap goal by more than retainExtraPercent
 	// then the scavenger is working as expected; the newly-created holes have been
 	// scavenged immediately as part of the allocations which cannot fit in the holes.
 	//
 	// Since the runtime should scavenge the entirety of the remaining holes,
 	// theoretically there should be no more free and unscavenged memory. However due
 	// to other allocations that happen during this test we may still see some physical
 	// memory over-use.
 	overuse := (float64(heapBacked) - float64(stats.HeapAlloc)) / float64(stats.HeapAlloc)
 	// Check against our overuse threshold, which is what the scavenger always reserves
 	// to encourage allocation of memory that doesn't need to be faulted in.
 	//
 	// Add additional slack in case the page size is large and the scavenger
 	// can't reach that memory because it doesn't constitute a complete aligned
 	// physical page. Assume the worst case: a full physical page out of each
 	// allocation.
 	threshold := 0.1 + float64(pageSize)/float64(allocChunk)
 	if overuse <= threshold {
 		fmt.Println("OK")
 		return
 	}
 	// Physical memory utilization exceeds the threshold, so heap-growth scavenging
 	// did not operate as expected.
 	//
 	// In the context of this test, this indicates a large amount of
 	// fragmentation with physical pages that are otherwise unused but not
 	// returned to the OS.
 	fmt.Printf("exceeded physical memory overuse threshold of %3.2f%%: %3.2f%%\n"+
 		"(alloc: %d, goal: %d, sys: %d, rel: %d, objs: %d)\n", threshold*100, overuse*100,
 		stats.HeapAlloc, stats.NextGC, stats.HeapSys, stats.HeapReleased, len(saved))
 	runtime.KeepAlive(saved)
 	runtime.KeepAlive(condemned)
 }

 // Test that defer closure is correctly scanned when the stack is scanned.
 func DeferLiveness() {
 	var x [10]int
 	escape(&x)
 	fn := func() {
 		if x[0] != 42 {
 			panic("FAIL")
 		}
 	}
 	defer fn()

 	x[0] = 42
 	runtime.GC()
 	runtime.GC()
 	runtime.GC()
 }

 //go:noinline
 func escape(x any) { sink2 = x; sink2 = nil }

 var sink2 any

 // Test zombie object detection and reporting.
 func GCZombie() {
 	// Allocate several objects of unusual size (so free slots are
 	// unlikely to all be re-allocated by the runtime).
 	const size = 190
 	const count = 8192 / size
 	keep := make([]*byte, 0, (count+1)/2)
 	free := make([]uintptr, 0, (count+1)/2)
 	zombies := make([]*byte, 0, len(free))
 	for i := 0; i < count; i++ {
 		obj := make([]byte, size)
 		p := &obj[0]
 		if i%2 == 0 {
 			keep = append(keep, p)
 		} else {
 			free = append(free, uintptr(unsafe.Pointer(p)))
 		}
 	}

 	// Free the unreferenced objects.
 	runtime.GC()

 	// Bring the free objects back to life.
 	for _, p := range free {
 		zombies = append(zombies, (*byte)(unsafe.Pointer(p)))
 	}

 	// GC should detect the zombie objects.
 	runtime.GC()
 	println("failed")
 	runtime.KeepAlive(keep)
 	runtime.KeepAlive(zombies)
 }

 func GCMemoryLimit() {
 	gcMemoryLimit(100)
 }

 func GCMemoryLimitNoGCPercent() {
 	gcMemoryLimit(-1)
 }

 // Test SetMemoryLimit functionality.
 //
 // This test lives here instead of runtime/debug because the entire
 // implementation is in the runtime, and testprog gives us a more
 // consistent testing environment to help avoid flakiness.
 func gcMemoryLimit(gcPercent int) {
 	if oldProcs := runtime.GOMAXPROCS(4); oldProcs < 4 {
 		// Fail if the default GOMAXPROCS isn't at least 4.
 		// Whatever invokes this should check and do a proper t.Skip.
 		println("insufficient CPUs")
 		return
 	}
 	debug.SetGCPercent(gcPercent)

 	const myLimit = 256 << 20
 	if limit := debug.SetMemoryLimit(-1); limit != math.MaxInt64 {
 		print("expected MaxInt64 limit, got ", limit, " bytes instead\n")
 		return
 	}
 	if limit := debug.SetMemoryLimit(myLimit); limit != math.MaxInt64 {
 		print("expected MaxInt64 limit, got ", limit, " bytes instead\n")
 		return
 	}
 	if limit := debug.SetMemoryLimit(-1); limit != myLimit {
 		print("expected a ", myLimit, "-byte limit, got ", limit, " bytes instead\n")
 		return
 	}

 	target := make(chan int64)
 	var wg sync.WaitGroup
 	wg.Add(1)
 	go func() {
 		defer wg.Done()

 		sinkSize := int(<-target / memLimitUnit)
 		for {
 			if len(memLimitSink) != sinkSize {
 				memLimitSink = make([]*[memLimitUnit]byte, sinkSize)
 			}
 			for i := 0; i < len(memLimitSink); i++ {
 				memLimitSink[i] = new([memLimitUnit]byte)
 				// Write to this memory to slow down the allocator, otherwise
 				// we get flaky behavior. See #52433.
 				for j := range memLimitSink[i] {
 					memLimitSink[i][j] = 9
 				}
 			}
 			// Again, Gosched to slow down the allocator.
 			runtime.Gosched()
 			select {
 			case newTarget := <-target:
 				if newTarget == math.MaxInt64 {
 					return
 				}
 				sinkSize = int(newTarget / memLimitUnit)
 			default:
 			}
 		}
 	}()
 	var m [2]metrics.Sample
 	m[0].Name = "/memory/classes/total:bytes"
 	m[1].Name = "/memory/classes/heap/released:bytes"

 	// Don't set this too high, because this is a *live heap* target which
 	// is not directly comparable to a total memory limit.
 	maxTarget := int64((myLimit / 10) * 8)
 	increment := int64((myLimit / 10) * 1)
 	for i := increment; i < maxTarget; i += increment {
 		target <- i

 		// Check to make sure the memory limit is maintained.
 		// We're just sampling here so if it transiently goes over we might miss it.
 		// The internal accounting is inconsistent anyway, so going over by a few
 		// pages is certainly possible. Just make sure we're within some bound.
 		// Note that to avoid flakiness due to #52433 (especially since we're allocating
 		// somewhat heavily here) this bound is kept loose. In practice the Go runtime
 		// should do considerably better than this bound.
 		bound := int64(myLimit + 16<<20)
 		start := time.Now()
 		for time.Now().Sub(start) < 200*time.Millisecond {
 			metrics.Read(m[:])
 			retained := int64(m[0].Value.Uint64() - m[1].Value.Uint64())
 			if retained > bound {
 				print("retained=", retained, " limit=", myLimit, " bound=", bound, "\n")
 				panic("exceeded memory limit by more than bound allows")
 			}
 			runtime.Gosched()
 		}
 	}

 	if limit := debug.SetMemoryLimit(math.MaxInt64); limit != myLimit {
 		print("expected a ", myLimit, "-byte limit, got ", limit, " bytes instead\n")
 		return
 	}
 	println("OK")
 }

 // Pick a value close to the page size. We want to m
 const memLimitUnit = 8000

 var memLimitSink []*[memLimitUnit]byte
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package main

	import (
	"fmt"
	"math"
	"os"
	"runtime"
	"runtime/debug"
	"runtime/metrics"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"
	)

	func init() {
	register("GCFairness", GCFairness)
	register("GCFairness2", GCFairness2)
	register("GCSys", GCSys)
	register("GCPhys", GCPhys)
	register("DeferLiveness", DeferLiveness)
	register("GCZombie", GCZombie)
	register("GCMemoryLimit", GCMemoryLimit)
	register("GCMemoryLimitNoGCPercent", GCMemoryLimitNoGCPercent)
	}

	func GCSys() {
	runtime.GOMAXPROCS(1)
	memstats := new(runtime.MemStats)
	runtime.GC()
	runtime.ReadMemStats(memstats)
	sys := memstats.Sys

	runtime.MemProfileRate = 0 // disable profiler

	itercount := 100000
	for i := 0; i < itercount; i++ {
	workthegc()
	}

	// Should only be using a few MB.
	// We allocated 100 MB or (if not short) 1 GB.
	runtime.ReadMemStats(memstats)
	if sys > memstats.Sys {
	sys = 0
	} else {
	sys = memstats.Sys - sys
	}
	if sys > 16<<20 {
	fmt.Printf("using too much memory: %d bytes\n", sys)
	return
	}
	fmt.Printf("OK\n")
	}

	var sink []byte

	func workthegc() []byte {
	sink = make([]byte, 1029)
	return sink
	}

	func GCFairness() {
	runtime.GOMAXPROCS(1)
	f, err := os.Open("/dev/null")
	if os.IsNotExist(err) {
	// This test tests what it is intended to test only if writes are fast.
	// If there is no /dev/null, we just don't execute the test.
	fmt.Println("OK")
	return
	}
	if err != nil {
	fmt.Println(err)
	os.Exit(1)
	}
	for i := 0; i < 2; i++ {
	go func() {
	for {
	f.Write([]byte("."))
	}
	}()
	}
	time.Sleep(10 * time.Millisecond)
	fmt.Println("OK")
	}

	func GCFairness2() {
	// Make sure user code can't exploit the GC's high priority
	// scheduling to make scheduling of user code unfair. See
	// issue #15706.
	runtime.GOMAXPROCS(1)
	debug.SetGCPercent(1)
	var count [3]int64
	var sink [3]any
	for i := range count {
	go func(i int) {
	for {
	sink[i] = make([]byte, 1024)
	atomic.AddInt64(&count[i], 1)
	}
	}(i)
	}
	// Note: If the unfairness is really bad, it may not even get
	// past the sleep.
	//
	// If the scheduling rules change, this may not be enough time
	// to let all goroutines run, but for now we cycle through
	// them rapidly.
	//
	// OpenBSD's scheduler makes every usleep() take at least
	// 20ms, so we need a long time to ensure all goroutines have
	// run. If they haven't run after 30ms, give it another 1000ms
	// and check again.
	time.Sleep(30 * time.Millisecond)
	var fail bool
	for i := range count {
	if atomic.LoadInt64(&count[i]) == 0 {
	fail = true
	}
	}
	if fail {
	time.Sleep(1 * time.Second)
	for i := range count {
	if atomic.LoadInt64(&count[i]) == 0 {
	fmt.Printf("goroutine %d did not run\n", i)
	return
	}
	}
	}
	fmt.Println("OK")
	}

	func GCPhys() {
	// This test ensures that heap-growth scavenging is working as intended.
	//
	// It attempts to construct a sizeable "swiss cheese" heap, with many
	// allocChunk-sized holes. Then, it triggers a heap growth by trying to
	// allocate as much memory as would fit in those holes.
	//
	// The heap growth should cause a large number of those holes to be
	// returned to the OS.

	const (
	// The total amount of memory we're willing to allocate.
	allocTotal = 32 << 20

	// The page cache could hide 64 8-KiB pages from the scavenger today.
	maxPageCache = (8 << 10) * 64
	)

	// How big the allocations are needs to depend on the page size.
	// If the page size is too big and the allocations are too small,
	// they might not be aligned to the physical page size, so the scavenger
	// will gloss over them.
	pageSize := os.Getpagesize()
	var allocChunk int
	if pageSize <= 8<<10 {
	allocChunk = 64 << 10
	} else {
	allocChunk = 512 << 10
	}
	allocs := allocTotal / allocChunk

	// Set GC percent just so this test is a little more consistent in the
	// face of varying environments.
	debug.SetGCPercent(100)

	// Set GOMAXPROCS to 1 to minimize the amount of memory held in the page cache,
	// and to reduce the chance that the background scavenger gets scheduled.
	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))

	// Allocate allocTotal bytes of memory in allocChunk byte chunks.
	// Alternate between whether the chunk will be held live or will be
	// condemned to GC to create holes in the heap.
	saved := make([][]byte, allocs/2+1)
	condemned := make([][]byte, allocs/2)
	for i := 0; i < allocs; i++ {
	b := make([]byte, allocChunk)
	if i%2 == 0 {
	saved = append(saved, b)
	} else {
	condemned = append(condemned, b)
	}
	}

	// Run a GC cycle just so we're at a consistent state.
	runtime.GC()

	// Drop the only reference to all the condemned memory.
	condemned = nil

	// Clear the condemned memory.
	runtime.GC()

	// At this point, the background scavenger is likely running
	// and could pick up the work, so the next line of code doesn't
	// end up doing anything. That's fine. What's important is that
	// this test fails somewhat regularly if the runtime doesn't
	// scavenge on heap growth, and doesn't fail at all otherwise.

	// Make a large allocation that in theory could fit, but won't
	// because we turned the heap into swiss cheese.
	saved = append(saved, make([]byte, allocTotal/2))

	// heapBacked is an estimate of the amount of physical memory used by
	// this test. HeapSys is an estimate of the size of the mapped virtual
	// address space (which may or may not be backed by physical pages)
	// whereas HeapReleased is an estimate of the amount of bytes returned
	// to the OS. Their difference then roughly corresponds to the amount
	// of virtual address space that is backed by physical pages.
	//
	// heapBacked also subtracts out maxPageCache bytes of memory because
	// this is memory that may be hidden from the scavenger per-P. Since
	// GOMAXPROCS=1 here, subtracting it out once is fine.
	var stats runtime.MemStats
	runtime.ReadMemStats(&stats)
	heapBacked := stats.HeapSys - stats.HeapReleased - maxPageCache
	// If heapBacked does not exceed the heap goal by more than retainExtraPercent
	// then the scavenger is working as expected; the newly-created holes have been
	// scavenged immediately as part of the allocations which cannot fit in the holes.
	//
	// Since the runtime should scavenge the entirety of the remaining holes,
	// theoretically there should be no more free and unscavenged memory. However due
	// to other allocations that happen during this test we may still see some physical
	// memory over-use.
	overuse := (float64(heapBacked) - float64(stats.HeapAlloc)) / float64(stats.HeapAlloc)
	// Check against our overuse threshold, which is what the scavenger always reserves
	// to encourage allocation of memory that doesn't need to be faulted in.
	//
	// Add additional slack in case the page size is large and the scavenger
	// can't reach that memory because it doesn't constitute a complete aligned
	// physical page. Assume the worst case: a full physical page out of each
	// allocation.
	threshold := 0.1 + float64(pageSize)/float64(allocChunk)
	if overuse <= threshold {
	fmt.Println("OK")
	return
	}
	// Physical memory utilization exceeds the threshold, so heap-growth scavenging
	// did not operate as expected.
	//
	// In the context of this test, this indicates a large amount of
	// fragmentation with physical pages that are otherwise unused but not
	// returned to the OS.
	fmt.Printf("exceeded physical memory overuse threshold of %3.2f%%: %3.2f%%\n"+
	"(alloc: %d, goal: %d, sys: %d, rel: %d, objs: %d)\n", threshold100, overuse100,
	stats.HeapAlloc, stats.NextGC, stats.HeapSys, stats.HeapReleased, len(saved))
	runtime.KeepAlive(saved)
	runtime.KeepAlive(condemned)
	}

	// Test that defer closure is correctly scanned when the stack is scanned.
	func DeferLiveness() {
	var x [10]int
	escape(&x)
	fn := func() {
	if x[0] != 42 {
	panic("FAIL")
	}
	}
	defer fn()

	x[0] = 42
	runtime.GC()
	runtime.GC()
	runtime.GC()
	}

	//go:noinline
	func escape(x any) { sink2 = x; sink2 = nil }

	var sink2 any

	// Test zombie object detection and reporting.
	func GCZombie() {
	// Allocate several objects of unusual size (so free slots are
	// unlikely to all be re-allocated by the runtime).
	const size = 190
	const count = 8192 / size
	keep := make([]*byte, 0, (count+1)/2)
	free := make([]uintptr, 0, (count+1)/2)
	zombies := make([]*byte, 0, len(free))
	for i := 0; i < count; i++ {
	obj := make([]byte, size)
	p := &obj[0]
	if i%2 == 0 {
	keep = append(keep, p)
	} else {
	free = append(free, uintptr(unsafe.Pointer(p)))
	}
	}

	// Free the unreferenced objects.
	runtime.GC()

	// Bring the free objects back to life.
	for _, p := range free {
	zombies = append(zombies, (*byte)(unsafe.Pointer(p)))
	}

	// GC should detect the zombie objects.
	runtime.GC()
	println("failed")
	runtime.KeepAlive(keep)
	runtime.KeepAlive(zombies)
	}

	func GCMemoryLimit() {
	gcMemoryLimit(100)
	}

	func GCMemoryLimitNoGCPercent() {
	gcMemoryLimit(-1)
	}

	// Test SetMemoryLimit functionality.
	//
	// This test lives here instead of runtime/debug because the entire
	// implementation is in the runtime, and testprog gives us a more
	// consistent testing environment to help avoid flakiness.
	func gcMemoryLimit(gcPercent int) {
	if oldProcs := runtime.GOMAXPROCS(4); oldProcs < 4 {
	// Fail if the default GOMAXPROCS isn't at least 4.
	// Whatever invokes this should check and do a proper t.Skip.
	println("insufficient CPUs")
	return
	}
	debug.SetGCPercent(gcPercent)

	const myLimit = 256 << 20
	if limit := debug.SetMemoryLimit(-1); limit != math.MaxInt64 {
	print("expected MaxInt64 limit, got ", limit, " bytes instead\n")
	return
	}
	if limit := debug.SetMemoryLimit(myLimit); limit != math.MaxInt64 {
	print("expected MaxInt64 limit, got ", limit, " bytes instead\n")
	return
	}
	if limit := debug.SetMemoryLimit(-1); limit != myLimit {
	print("expected a ", myLimit, "-byte limit, got ", limit, " bytes instead\n")
	return
	}

	target := make(chan int64)
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
	defer wg.Done()

	sinkSize := int(<-target / memLimitUnit)
	for {
	if len(memLimitSink) != sinkSize {
	memLimitSink = make([]*[memLimitUnit]byte, sinkSize)
	}
	for i := 0; i < len(memLimitSink); i++ {
	memLimitSink[i] = new([memLimitUnit]byte)
	// Write to this memory to slow down the allocator, otherwise
	// we get flaky behavior. See #52433.
	for j := range memLimitSink[i] {
	memLimitSink[i][j] = 9
	}
	}
	// Again, Gosched to slow down the allocator.
	runtime.Gosched()
	select {
	case newTarget := <-target:
	if newTarget == math.MaxInt64 {
	return
	}
	sinkSize = int(newTarget / memLimitUnit)
	default:
	}
	}
	}()
	var m [2]metrics.Sample
	m[0].Name = "/memory/classes/total:bytes"
	m[1].Name = "/memory/classes/heap/released:bytes"

	// Don't set this too high, because this is a live heap target which
	// is not directly comparable to a total memory limit.
	maxTarget := int64((myLimit / 10) * 8)
	increment := int64((myLimit / 10) * 1)
	for i := increment; i < maxTarget; i += increment {
	target <- i

	// Check to make sure the memory limit is maintained.
	// We're just sampling here so if it transiently goes over we might miss it.
	// The internal accounting is inconsistent anyway, so going over by a few
	// pages is certainly possible. Just make sure we're within some bound.
	// Note that to avoid flakiness due to #52433 (especially since we're allocating
	// somewhat heavily here) this bound is kept loose. In practice the Go runtime
	// should do considerably better than this bound.
	bound := int64(myLimit + 16<<20)
	start := time.Now()
	for time.Now().Sub(start) < 200*time.Millisecond {
	metrics.Read(m[:])
	retained := int64(m[0].Value.Uint64() - m[1].Value.Uint64())
	if retained > bound {
	print("retained=", retained, " limit=", myLimit, " bound=", bound, "\n")
	panic("exceeded memory limit by more than bound allows")
	}
	runtime.Gosched()
	}
	}

	if limit := debug.SetMemoryLimit(math.MaxInt64); limit != myLimit {
	print("expected a ", myLimit, "-byte limit, got ", limit, " bytes instead\n")
	return
	}
	println("OK")
	}

	// Pick a value close to the page size. We want to m
	const memLimitUnit = 8000

	var memLimitSink []*[memLimitUnit]byte