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// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// GCStats collect information about recent garbage collections.
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LastGC time.Time // time of last collection
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NumGC int64 // number of garbage collections
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PauseTotal time.Duration // total pause for all collections
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Pause []time.Duration // pause history, most recent first
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PauseQuantiles []time.Duration
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// Implemented in package runtime.
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func readGCStats(*[]time.Duration)
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func enableGC(bool) bool
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func setGCPercent(int) int
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// ReadGCStats reads statistics about garbage collection into stats.
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// The number of entries in the pause history is system-dependent;
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// stats.Pause slice will be reused if large enough, reallocated otherwise.
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// ReadGCStats may use the full capacity of the stats.Pause slice.
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// If stats.PauseQuantiles is non-empty, ReadGCStats fills it with quantiles
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// summarizing the distribution of pause time. For example, if
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// len(stats.PauseQuantiles) is 5, it will be filled with the minimum,
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// 25%, 50%, 75%, and maximum pause times.
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func ReadGCStats(stats *GCStats) {
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// Create a buffer with space for at least two copies of the
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// pause history tracked by the runtime. One will be returned
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// to the caller and the other will be used as a temporary buffer
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// for computing quantiles.
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const maxPause = len(((*runtime.MemStats)(nil)).PauseNs)
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if cap(stats.Pause) < 2*maxPause {
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stats.Pause = make([]time.Duration, 2*maxPause)
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// readGCStats fills in the pause history (up to maxPause entries)
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// and then three more: Unix ns time of last GC, number of GC,
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// and total pause time in nanoseconds. Here we depend on the
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// fact that time.Duration's native unit is nanoseconds, so the
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// pauses and the total pause time do not need any conversion.
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readGCStats(&stats.Pause)
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n := len(stats.Pause) - 3
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stats.LastGC = time.Unix(0, int64(stats.Pause[n]))
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stats.NumGC = int64(stats.Pause[n+1])
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stats.PauseTotal = stats.Pause[n+2]
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stats.Pause = stats.Pause[:n]
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if len(stats.PauseQuantiles) > 0 {
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for i := range stats.PauseQuantiles {
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stats.PauseQuantiles[i] = 0
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// There's room for a second copy of the data in stats.Pause.
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// See the allocation at the top of the function.
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sorted := stats.Pause[n : n+n]
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copy(sorted, stats.Pause)
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sort.Sort(byDuration(sorted))
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nq := len(stats.PauseQuantiles) - 1
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for i := 0; i < nq; i++ {
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stats.PauseQuantiles[i] = sorted[len(sorted)*i/nq]
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stats.PauseQuantiles[nq] = sorted[len(sorted)-1]
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type byDuration []time.Duration
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func (x byDuration) Len() int { return len(x) }
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func (x byDuration) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
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func (x byDuration) Less(i, j int) bool { return x[i] < x[j] }
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// SetGCPercent sets the garbage collection target percentage:
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// a collection is triggered when the ratio of freshly allocated data
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// to live data remaining after the previous collection reaches this percentage.
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// SetGCPercent returns the previous setting.
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// The initial setting is the value of the GOGC environment variable
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// at startup, or 100 if the variable is not set.
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// A negative percentage disables garbage collection.
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func SetGCPercent(percent int) int {
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return setGCPercent(percent)
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// FreeOSMemory forces a garbage collection followed by an
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// attempt to return as much memory to the operating system
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// as possible. (Even if this is not called, the runtime gradually
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// returns memory to the operating system in a background task.)