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// 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 (
func init() {
register("GCFairness", GCFairness)
register("GCFairness2", GCFairness2)
register("GCSys", GCSys)
register("GCPhys", GCPhys)
register("DeferLiveness", DeferLiveness)
register("GCZombie", GCZombie)
func GCSys() {
memstats := new(runtime.MemStats)
sys := memstats.Sys
runtime.MemProfileRate = 0 // disable profiler
itercount := 100000
for i := 0; i < itercount; i++ {
// Should only be using a few MB.
// We allocated 100 MB or (if not short) 1 GB.
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)
var sink []byte
func workthegc() []byte {
sink = make([]byte, 1029)
return sink
func GCFairness() {
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.
if err != nil {
for i := 0; i < 2; i++ {
go func() {
for {
time.Sleep(10 * time.Millisecond)
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.
var count [3]int64
var sink [3]interface{}
for i := range count {
go func(i int) {
for {
sink[i] = make([]byte, 1024)
atomic.AddInt64(&count[i], 1)
// 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)
func GCPhys() {
// This test ensures that heap-growth scavenging is working as intended.
// It sets up a specific scenario: it allocates two pairs of objects whose
// sizes sum to size. One object in each pair is "small" (though must be
// large enough to be considered a large object by the runtime) and one is
// large. The small objects are kept while the large objects are freed,
// creating two large unscavenged holes in the heap. The heap goal should
// also be small as a result (so size must be at least as large as the
// minimum heap size). We then allocate one large object, bigger than both
// pairs of objects combined. This allocation, because it will tip
// HeapSys-HeapReleased well above the heap goal, should trigger heap-growth
// scavenging and scavenge most, if not all, of the large holes we created
// earlier.
const (
// Size must be also large enough to be considered a large
// object (not in any size-segregated span).
size = 4 << 20
split = 64 << 10
objects = 2
// The page cache could hide 64 8-KiB pages from the scavenger today.
maxPageCache = (8 << 10) * 64
// Reduce GOMAXPROCS down to 4 if it's greater. We need to bound the amount
// of memory held in the page cache because the scavenger can't reach it.
// The page cache will hold at most maxPageCache of memory per-P, so this
// bounds the amount of memory hidden from the scavenger to 4*maxPageCache
// at most.
maxProcs = 4
// Set GOGC so that this test operates under consistent assumptions.
procs := runtime.GOMAXPROCS(-1)
if procs > maxProcs {
defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(maxProcs))
procs = runtime.GOMAXPROCS(-1)
// Save objects which we want to survive, and condemn objects which we don't.
// Note that we condemn objects in this way and release them all at once in
// order to avoid having the GC start freeing up these objects while the loop
// is still running and filling in the holes we intend to make.
saved := make([][]byte, 0, objects+1)
condemned := make([][]byte, 0, objects)
for i := 0; i < 2*objects; i++ {
if i%2 == 0 {
saved = append(saved, make([]byte, split))
} else {
condemned = append(condemned, make([]byte, size-split))
condemned = nil
// Clean up the heap. This will free up every other object created above
// (i.e. everything in condemned) creating holes in the heap.
// Also, if the condemned objects are still being swept, its possible that
// the scavenging that happens as a result of the next allocation won't see
// the holes at all. We call runtime.GC() twice here so that when we allocate
// our large object there's no race with sweeping.
// Perform one big allocation which should also scavenge any holes.
// The heap goal will rise after this object is allocated, so it's very
// important that we try to do all the scavenging in a single allocation
// that exceeds the heap goal. Otherwise the rising heap goal could foil our
// test.
saved = append(saved, make([]byte, objects*size))
// Clean up the heap again just to put it in a known state.
// 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.
var stats runtime.MemStats
heapBacked := stats.HeapSys - stats.HeapReleased
// 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)
// Compute the threshold.
// In theory, this threshold should just be zero, but that's not possible in practice.
// Firstly, the runtime's page cache can hide up to maxPageCache of free memory from the
// scavenger per P. To account for this, we increase the threshold by the ratio between the
// total amount the runtime could hide from the scavenger to the amount of memory we expect
// to be able to scavenge here, which is (size-split)*objects. This computation is the crux
// GOMAXPROCS above; if GOMAXPROCS is too high the threshold just becomes 100%+ since the
// amount of memory being allocated is fixed. Then we add 5% to account for noise, such as
// other allocations this test may have performed that we don't explicitly account for The
// baseline threshold here is around 11% for GOMAXPROCS=1, capping out at around 30% for
threshold := 0.05 + float64(procs)*maxPageCache/float64((size-split)*objects)
if overuse <= threshold {
// 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))
// Test that defer closure is correctly scanned when the stack is scanned.
func DeferLiveness() {
var x [10]int
fn := func() {
if x[0] != 42 {
defer fn()
x[0] = 42
func escape(x interface{}) { sink2 = x; sink2 = nil }
var sink2 interface{}
// 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.
// Bring the free objects back to life.
for _, p := range free {
zombies = append(zombies, (*byte)(unsafe.Pointer(p)))
// GC should detect the zombie objects.