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// Copyright 2013 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 debug
import (
// GCStats collect information about recent garbage collections.
type GCStats struct {
LastGC time.Time // time of last collection
NumGC int64 // number of garbage collections
PauseTotal time.Duration // total pause for all collections
Pause []time.Duration // pause history, most recent first
PauseEnd []time.Time // pause end times history, most recent first
PauseQuantiles []time.Duration
// ReadGCStats reads statistics about garbage collection into stats.
// The number of entries in the pause history is system-dependent;
// stats.Pause slice will be reused if large enough, reallocated otherwise.
// ReadGCStats may use the full capacity of the stats.Pause slice.
// If stats.PauseQuantiles is non-empty, ReadGCStats fills it with quantiles
// summarizing the distribution of pause time. For example, if
// len(stats.PauseQuantiles) is 5, it will be filled with the minimum,
// 25%, 50%, 75%, and maximum pause times.
func ReadGCStats(stats *GCStats) {
// Create a buffer with space for at least two copies of the
// pause history tracked by the runtime. One will be returned
// to the caller and the other will be used as transfer buffer
// for end times history and as a temporary buffer for
// computing quantiles.
const maxPause = len(((*runtime.MemStats)(nil)).PauseNs)
if cap(stats.Pause) < 2*maxPause+3 {
stats.Pause = make([]time.Duration, 2*maxPause+3)
// readGCStats fills in the pause and end times histories (up to
// maxPause entries) and then three more: Unix ns time of last GC,
// number of GC, and total pause time in nanoseconds. Here we
// depend on the fact that time.Duration's native unit is
// nanoseconds, so the pauses and the total pause time do not need
// any conversion.
n := len(stats.Pause) - 3
stats.LastGC = time.Unix(0, int64(stats.Pause[n]))
stats.NumGC = int64(stats.Pause[n+1])
stats.PauseTotal = stats.Pause[n+2]
n /= 2 // buffer holds pauses and end times
stats.Pause = stats.Pause[:n]
if cap(stats.PauseEnd) < maxPause {
stats.PauseEnd = make([]time.Time, 0, maxPause)
stats.PauseEnd = stats.PauseEnd[:0]
for _, ns := range stats.Pause[n : n+n] {
stats.PauseEnd = append(stats.PauseEnd, time.Unix(0, int64(ns)))
if len(stats.PauseQuantiles) > 0 {
if n == 0 {
for i := range stats.PauseQuantiles {
stats.PauseQuantiles[i] = 0
} else {
// There's room for a second copy of the data in stats.Pause.
// See the allocation at the top of the function.
sorted := stats.Pause[n : n+n]
copy(sorted, stats.Pause)
sort.Slice(sorted, func(i, j int) bool { return sorted[i] < sorted[j] })
nq := len(stats.PauseQuantiles) - 1
for i := 0; i < nq; i++ {
stats.PauseQuantiles[i] = sorted[len(sorted)*i/nq]
stats.PauseQuantiles[nq] = sorted[len(sorted)-1]
// SetGCPercent sets the garbage collection target percentage:
// a collection is triggered when the ratio of freshly allocated data
// to live data remaining after the previous collection reaches this percentage.
// SetGCPercent returns the previous setting.
// The initial setting is the value of the GOGC environment variable
// at startup, or 100 if the variable is not set.
// A negative percentage disables garbage collection.
func SetGCPercent(percent int) int {
return int(setGCPercent(int32(percent)))
// FreeOSMemory forces a garbage collection followed by an
// attempt to return as much memory to the operating system
// as possible. (Even if this is not called, the runtime gradually
// returns memory to the operating system in a background task.)
func FreeOSMemory() {
// SetMaxStack sets the maximum amount of memory that
// can be used by a single goroutine stack.
// If any goroutine exceeds this limit while growing its stack,
// the program crashes.
// SetMaxStack returns the previous setting.
// The initial setting is 1 GB on 64-bit systems, 250 MB on 32-bit systems.
// There may be a system-imposed maximum stack limit regardless
// of the value provided to SetMaxStack.
// SetMaxStack is useful mainly for limiting the damage done by
// goroutines that enter an infinite recursion. It only limits future
// stack growth.
func SetMaxStack(bytes int) int {
return setMaxStack(bytes)
// SetMaxThreads sets the maximum number of operating system
// threads that the Go program can use. If it attempts to use more than
// this many, the program crashes.
// SetMaxThreads returns the previous setting.
// The initial setting is 10,000 threads.
// The limit controls the number of operating system threads, not the number
// of goroutines. A Go program creates a new thread only when a goroutine
// is ready to run but all the existing threads are blocked in system calls, cgo calls,
// or are locked to other goroutines due to use of runtime.LockOSThread.
// SetMaxThreads is useful mainly for limiting the damage done by
// programs that create an unbounded number of threads. The idea is
// to take down the program before it takes down the operating system.
func SetMaxThreads(threads int) int {
return setMaxThreads(threads)
// SetPanicOnFault controls the runtime's behavior when a program faults
// at an unexpected (non-nil) address. Such faults are typically caused by
// bugs such as runtime memory corruption, so the default response is to crash
// the program. Programs working with memory-mapped files or unsafe
// manipulation of memory may cause faults at non-nil addresses in less
// dramatic situations; SetPanicOnFault allows such programs to request
// that the runtime trigger only a panic, not a crash.
// The runtime.Error that the runtime panics with may have an additional method:
// Addr() uintptr
// If that method exists, it returns the memory address which triggered the fault.
// The results of Addr are best-effort and the veracity of the result
// may depend on the platform.
// SetPanicOnFault applies only to the current goroutine.
// It returns the previous setting.
func SetPanicOnFault(enabled bool) bool {
return setPanicOnFault(enabled)
// WriteHeapDump writes a description of the heap and the objects in
// it to the given file descriptor.
// WriteHeapDump suspends the execution of all goroutines until the heap
// dump is completely written. Thus, the file descriptor must not be
// connected to a pipe or socket whose other end is in the same Go
// process; instead, use a temporary file or network socket.
// The heap dump format is defined at
func WriteHeapDump(fd uintptr)
// SetTraceback sets the amount of detail printed by the runtime in
// the traceback it prints before exiting due to an unrecovered panic
// or an internal runtime error.
// The level argument takes the same values as the GOTRACEBACK
// environment variable. For example, SetTraceback("all") ensure
// that the program prints all goroutines when it crashes.
// See the package runtime documentation for details.
// If SetTraceback is called with a level lower than that of the
// environment variable, the call is ignored.
func SetTraceback(level string)