Merge pull request #457 from prometheus/beorn7/histogram
Lock-free atomic observations in Histograms!
This commit is contained in:
Björn Rabenstein
GitHub
commit
b5bfa0eb2c
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@ -16,7 +16,9 @@ package prometheus
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import (
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"fmt"
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"math"
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"runtime"
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"sort"
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"sync"
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"sync/atomic"
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"github.com/golang/protobuf/proto"
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@ -200,28 +202,49 @@ func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogr
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}
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}
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}
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// Finally we know the final length of h.upperBounds and can make counts.
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h.counts = make([]uint64, len(h.upperBounds))
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// Finally we know the final length of h.upperBounds and can make counts
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// for both states:
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h.counts[0].buckets = make([]uint64, len(h.upperBounds))
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h.counts[1].buckets = make([]uint64, len(h.upperBounds))
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h.init(h) // Init self-collection.
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return h
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}
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type histogram struct {
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type histogramCounts struct {
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// sumBits contains the bits of the float64 representing the sum of all
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// observations. sumBits and count have to go first in the struct to
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// guarantee alignment for atomic operations.
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// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
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sumBits uint64
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count uint64
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buckets []uint64
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}
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type histogram struct {
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selfCollector
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// Note that there is no mutex required.
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desc *Desc
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desc *Desc
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writeMtx sync.Mutex // Only used in the Write method.
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upperBounds []float64
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counts []uint64
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// Two counts, one is "hot" for lock-free observations, the other is
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// "cold" for writing out a dto.Metric.
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counts [2]histogramCounts
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hotIdx int // Index of currently-hot counts. Only used within Write.
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// This is a complicated one. For lock-free yet atomic observations, we
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// need to save the total count of observations again, combined with the
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// index of the currently-hot counts struct, so that we can perform the
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// operation on both values atomically. The least significant bit
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// defines the hot counts struct. The remaining 63 bits represent the
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// total count of observations. This happens under the assumption that
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// the 63bit count will never overflow. Rationale: An observations takes
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// about 30ns. Let's assume it could happen in 10ns. Overflowing the
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// counter will then take at least (2^63)*10ns, which is about 3000
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// years.
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countAndHotIdx uint64
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labelPairs []*dto.LabelPair
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}
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@ -241,36 +264,113 @@ func (h *histogram) Observe(v float64) {
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// 100 buckets: 78.1 ns/op linear - binary 54.9 ns/op
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// 300 buckets: 154 ns/op linear - binary 61.6 ns/op
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i := sort.SearchFloat64s(h.upperBounds, v)
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if i < len(h.counts) {
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atomic.AddUint64(&h.counts[i], 1)
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// We increment h.countAndHotIdx by 2 so that the counter in the upper
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// 63 bits gets incremented by 1. At the same time, we get the new value
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// back, which we can use to find the currently-hot counts.
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n := atomic.AddUint64(&h.countAndHotIdx, 2)
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hotCounts := &h.counts[n%2]
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if i < len(h.upperBounds) {
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atomic.AddUint64(&hotCounts.buckets[i], 1)
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}
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atomic.AddUint64(&h.count, 1)
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for {
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oldBits := atomic.LoadUint64(&h.sumBits)
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oldBits := atomic.LoadUint64(&hotCounts.sumBits)
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newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
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if atomic.CompareAndSwapUint64(&h.sumBits, oldBits, newBits) {
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if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
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break
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}
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}
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// Increment count last as we take it as a signal that the observation
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// is complete.
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atomic.AddUint64(&hotCounts.count, 1)
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}
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func (h *histogram) Write(out *dto.Metric) error {
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his := &dto.Histogram{}
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buckets := make([]*dto.Bucket, len(h.upperBounds))
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var (
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his = &dto.Histogram{}
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buckets = make([]*dto.Bucket, len(h.upperBounds))
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hotCounts, coldCounts *histogramCounts
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count uint64
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)
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his.SampleSum = proto.Float64(math.Float64frombits(atomic.LoadUint64(&h.sumBits)))
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his.SampleCount = proto.Uint64(atomic.LoadUint64(&h.count))
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var count uint64
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// For simplicity, we mutex the rest of this method. It is not in the
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// hot path, i.e. Observe is called much more often than Write. The
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// complication of making Write lock-free isn't worth it.
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h.writeMtx.Lock()
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defer h.writeMtx.Unlock()
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// This is a bit arcane, which is why the following spells out this if
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// clause in English:
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//
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// If the currently-hot counts struct is #0, we atomically increment
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// h.countAndHotIdx by 1 so that from now on Observe will use the counts
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// struct #1. Furthermore, the atomic increment gives us the new value,
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// which, in its most significant 63 bits, tells us the count of
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// observations done so far up to and including currently ongoing
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// observations still using the counts struct just changed from hot to
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// cold. To have a normal uint64 for the count, we bitshift by 1 and
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// save the result in count. We also set h.hotIdx to 1 for the next
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// Write call, and we will refer to counts #1 as hotCounts and to counts
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// #0 as coldCounts.
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//
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// If the currently-hot counts struct is #1, we do the corresponding
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// things the other way round. We have to _decrement_ h.countAndHotIdx
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// (which is a bit arcane in itself, as we have to express -1 with an
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// unsigned int...).
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if h.hotIdx == 0 {
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count = atomic.AddUint64(&h.countAndHotIdx, 1) >> 1
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h.hotIdx = 1
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hotCounts = &h.counts[1]
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coldCounts = &h.counts[0]
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} else {
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count = atomic.AddUint64(&h.countAndHotIdx, ^uint64(0)) >> 1 // Decrement.
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h.hotIdx = 0
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hotCounts = &h.counts[0]
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coldCounts = &h.counts[1]
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}
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// Now we have to wait for the now-declared-cold counts to actually cool
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// down, i.e. wait for all observations still using it to finish. That's
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// the case once the count in the cold counts struct is the same as the
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// one atomically retrieved from the upper 63bits of h.countAndHotIdx.
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for {
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if count == atomic.LoadUint64(&coldCounts.count) {
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break
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}
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runtime.Gosched() // Let observations get work done.
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}
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his.SampleCount = proto.Uint64(count)
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his.SampleSum = proto.Float64(math.Float64frombits(atomic.LoadUint64(&coldCounts.sumBits)))
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var cumCount uint64
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for i, upperBound := range h.upperBounds {
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count += atomic.LoadUint64(&h.counts[i])
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cumCount += atomic.LoadUint64(&coldCounts.buckets[i])
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buckets[i] = &dto.Bucket{
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CumulativeCount: proto.Uint64(count),
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CumulativeCount: proto.Uint64(cumCount),
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UpperBound: proto.Float64(upperBound),
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}
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}
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his.Bucket = buckets
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out.Histogram = his
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out.Label = h.labelPairs
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// Finally add all the cold counts to the new hot counts and reset the cold counts.
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atomic.AddUint64(&hotCounts.count, count)
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atomic.StoreUint64(&coldCounts.count, 0)
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for {
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oldBits := atomic.LoadUint64(&hotCounts.sumBits)
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newBits := math.Float64bits(math.Float64frombits(oldBits) + his.GetSampleSum())
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if atomic.CompareAndSwapUint64(&hotCounts.sumBits, oldBits, newBits) {
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atomic.StoreUint64(&coldCounts.sumBits, 0)
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break
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}
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}
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for i := range h.upperBounds {
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atomic.AddUint64(&hotCounts.buckets[i], atomic.LoadUint64(&coldCounts.buckets[i]))
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atomic.StoreUint64(&coldCounts.buckets[i], 0)
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}
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return nil
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}
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@ -17,6 +17,7 @@ import (
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"math"
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"math/rand"
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"reflect"
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"runtime"
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"sort"
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"sync"
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"testing"
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@ -346,3 +347,47 @@ func TestBuckets(t *testing.T) {
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t.Errorf("linear buckets: got %v, want %v", got, want)
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}
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}
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func TestHistogramAtomicObserve(t *testing.T) {
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var (
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quit = make(chan struct{})
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his = NewHistogram(HistogramOpts{
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Buckets: []float64{0.5, 10, 20},
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})
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)
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defer func() { close(quit) }()
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observe := func() {
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for {
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select {
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case <-quit:
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return
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default:
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his.Observe(1)
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}
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}
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}
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go observe()
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go observe()
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go observe()
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for i := 0; i < 100; i++ {
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m := &dto.Metric{}
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if err := his.Write(m); err != nil {
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t.Fatal("unexpected error writing histogram:", err)
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}
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h := m.GetHistogram()
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if h.GetSampleCount() != uint64(h.GetSampleSum()) ||
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h.GetSampleCount() != h.GetBucket()[1].GetCumulativeCount() ||
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h.GetSampleCount() != h.GetBucket()[2].GetCumulativeCount() {
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t.Fatalf(
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"inconsistent counts in histogram: count=%d sum=%f buckets=[%d, %d]",
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h.GetSampleCount(), h.GetSampleSum(),
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h.GetBucket()[1].GetCumulativeCount(), h.GetBucket()[2].GetCumulativeCount(),
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)
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}
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runtime.Gosched()
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}
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}
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