Switch from log-linear histograms to log-based histograms
Log-based histograms provide lower estimation error for the same number of buckets compared to log-linear histograms. For example, the current Histogram implementation splits each decimal range (10^n .. 10^(n+1)] into 18 buckets. These buckets have the following bounds: - for log-linear histogram: (1 .. 1.5], (1.5 .. 2], (2 .. 2.5], ... (9.5 .. 10] - for log-based histogram: (1 .. 1.136], (1.136 .. 1.292], ... (8.799 ... 10] The maximum estimated error for log-linear histogram is reached in the first bucket per each decimal range and equals to 1.5-1=0.5 or 50%. The maximum estimated error for log-based histogram is constant across buckets and equals to 1.136-1=0.136 or 13.6%. This means that log-based histogram improves histogram accuracy by up to 50%/13.6% = 3.6 times when using the same number of buckets. Further reading - https://linuxczar.net/blog/2020/08/13/histogram-error/
This commit is contained in:
Aliaksandr Valialkin
parent
43691b65a0
commit
e7f78fa63c
191
histogram.go
191
histogram.go
@ -9,14 +9,15 @@ import (
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)
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const (
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e10Min = -9
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e10Max = 18
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decimalMultiplier = 2
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bucketSize = 9 * decimalMultiplier
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bucketsCount = e10Max - e10Min
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decimalPrecision = 1e-12
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e10Min = -9
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e10Max = 18
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bucketsPerDecimal = 18
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decimalBucketsCount = e10Max - e10Min
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bucketsCount = decimalBucketsCount * bucketsPerDecimal
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)
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var bucketMultiplier = math.Pow(10, 1.0/bucketsPerDecimal)
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// Histogram is a histogram for non-negative values with automatically created buckets.
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//
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// See https://medium.com/@valyala/improving-histogram-usability-for-prometheus-and-grafana-bc7e5df0e350
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@ -48,9 +49,8 @@ type Histogram struct {
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// Mu gurantees synchronous update for all the counters and sum.
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mu sync.Mutex
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buckets [bucketsCount]*histogramBucket
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decimalBuckets [decimalBucketsCount]*[bucketsPerDecimal]uint64
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zeros uint64
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lower uint64
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upper uint64
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@ -65,15 +65,14 @@ func (h *Histogram) Reset() {
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}
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func (h *Histogram) resetLocked() {
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for _, hb := range h.buckets[:] {
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if hb == nil {
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for _, db := range h.decimalBuckets[:] {
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if db == nil {
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continue
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}
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for offset := range hb.counts[:] {
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hb.counts[offset] = 0
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for i := range db[:] {
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db[i] = 0
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}
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}
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h.zeros = 0
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h.lower = 0
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h.upper = 0
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}
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@ -86,31 +85,31 @@ func (h *Histogram) Update(v float64) {
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// Skip NaNs and negative values.
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return
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}
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bucketIdx, offset := getBucketIdxAndOffset(v)
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bucketIdx := (math.Log10(v) - e10Min) * bucketsPerDecimal
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idx := uint(bucketIdx)
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if bucketIdx == float64(idx) {
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// Edge case for 10^n values, which must go to the lower bucket
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// according to Prometheus logic for `le`-based histograms.
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idx--
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}
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decimalBucketIdx := idx / bucketsPerDecimal
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offset := idx % bucketsPerDecimal
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h.mu.Lock()
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h.updateLocked(v, bucketIdx, offset)
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h.mu.Unlock()
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}
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func (h *Histogram) updateLocked(v float64, bucketIdx int, offset uint) {
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h.sum += v
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if bucketIdx < 0 {
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// Special cases for zero, too small or too big value
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if offset == 0 {
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h.zeros++
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} else if offset == 1 {
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h.lower++
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} else {
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h.upper++
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h.lower++
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} else if bucketIdx >= bucketsCount {
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h.upper++
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} else {
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db := h.decimalBuckets[decimalBucketIdx]
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if db == nil {
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var b [bucketsPerDecimal]uint64
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db = &b
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h.decimalBuckets[decimalBucketIdx] = db
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}
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return
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db[offset]++
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}
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hb := h.buckets[bucketIdx]
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if hb == nil {
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hb = &histogramBucket{}
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h.buckets[bucketIdx] = hb
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}
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hb.counts[offset]++
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h.mu.Unlock()
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}
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// VisitNonZeroBuckets calls f for all buckets with non-zero counters.
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@ -121,38 +120,25 @@ func (h *Histogram) updateLocked(v float64, bucketIdx int, offset uint) {
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// with `le` (less or equal) labels.
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func (h *Histogram) VisitNonZeroBuckets(f func(vmrange string, count uint64)) {
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h.mu.Lock()
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h.visitNonZeroBucketsLocked(f)
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h.mu.Unlock()
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}
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func (h *Histogram) visitNonZeroBucketsLocked(f func(vmrange string, count uint64)) {
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if h.zeros > 0 {
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vmrange := getVMRange(-1, 0)
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f(vmrange, h.zeros)
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}
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if h.lower > 0 {
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vmrange := getVMRange(-1, 1)
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f(vmrange, h.lower)
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f(lowerBucketRange, h.lower)
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}
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for bucketIdx, hb := range h.buckets[:] {
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if hb == nil {
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for decimalBucketIdx, db := range h.decimalBuckets[:] {
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if db == nil {
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continue
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}
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for offset, count := range hb.counts[:] {
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for offset, count := range db[:] {
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if count > 0 {
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vmrange := getVMRange(bucketIdx, uint(offset))
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bucketIdx := decimalBucketIdx*bucketsPerDecimal + offset
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vmrange := getVMRange(bucketIdx)
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f(vmrange, count)
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}
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}
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}
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if h.upper > 0 {
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vmrange := getVMRange(-1, 2)
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f(vmrange, h.upper)
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f(upperBucketRange, h.upper)
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}
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}
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type histogramBucket struct {
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counts [bucketSize]uint64
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h.mu.Unlock()
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}
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// NewHistogram creates and returns new histogram with the given name.
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@ -193,43 +179,27 @@ func (h *Histogram) UpdateDuration(startTime time.Time) {
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h.Update(d)
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}
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func getVMRange(bucketIdx int, offset uint) string {
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func getVMRange(bucketIdx int) string {
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bucketRangesOnce.Do(initBucketRanges)
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if bucketIdx < 0 {
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if offset > 2 {
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panic(fmt.Errorf("BUG: offset must be in range [0...2] for negative bucketIdx; got %d", offset))
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}
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return bucketRanges[offset]
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}
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idx := 3 + uint(bucketIdx)*bucketSize + offset
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return bucketRanges[idx]
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return bucketRanges[bucketIdx]
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}
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func initBucketRanges() {
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bucketRanges[0] = "0...0"
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bucketRanges[1] = fmt.Sprintf("0...%.1fe%d", 1.0, e10Min)
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bucketRanges[2] = fmt.Sprintf("%.1fe%d...+Inf", 1.0, e10Max)
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idx := 3
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start := fmt.Sprintf("%.1fe%d", 1.0, e10Min)
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for bucketIdx := 0; bucketIdx < bucketsCount; bucketIdx++ {
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for offset := 0; offset < bucketSize; offset++ {
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e10 := e10Min + bucketIdx
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m := 1 + float64(offset+1)/decimalMultiplier
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if math.Abs(m-10) < decimalPrecision {
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m = 1
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e10++
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}
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end := fmt.Sprintf("%.1fe%d", m, e10)
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bucketRanges[idx] = start + "..." + end
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idx++
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start = end
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}
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v := math.Pow10(e10Min)
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start := fmt.Sprintf("%.3e", v)
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for i := 0; i < bucketsCount; i++ {
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v *= bucketMultiplier
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end := fmt.Sprintf("%.3e", v)
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bucketRanges[i] = start + "..." + end
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start = end
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}
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}
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var (
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// 3 additional buckets for zero, lower and upper.
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bucketRanges [3 + bucketsCount*bucketSize]string
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lowerBucketRange = fmt.Sprintf("0...%.3e", math.Pow10(e10Min))
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upperBucketRange = fmt.Sprintf("%.3e...+Inf", math.Pow10(e10Max))
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bucketRanges [bucketsCount]string
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bucketRangesOnce sync.Once
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)
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@ -238,21 +208,21 @@ func (h *Histogram) marshalTo(prefix string, w io.Writer) {
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h.VisitNonZeroBuckets(func(vmrange string, count uint64) {
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tag := fmt.Sprintf("vmrange=%q", vmrange)
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metricName := addTag(prefix, tag)
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name, filters := splitMetricName(metricName)
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fmt.Fprintf(w, "%s_bucket%s %d\n", name, filters, count)
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name, labels := splitMetricName(metricName)
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fmt.Fprintf(w, "%s_bucket%s %d\n", name, labels, count)
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countTotal += count
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})
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if countTotal == 0 {
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return
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}
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name, filters := splitMetricName(prefix)
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name, labels := splitMetricName(prefix)
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sum := h.getSum()
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if float64(int64(sum)) == sum {
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fmt.Fprintf(w, "%s_sum%s %d\n", name, filters, int64(sum))
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fmt.Fprintf(w, "%s_sum%s %d\n", name, labels, int64(sum))
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} else {
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fmt.Fprintf(w, "%s_sum%s %g\n", name, filters, sum)
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fmt.Fprintf(w, "%s_sum%s %g\n", name, labels, sum)
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}
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fmt.Fprintf(w, "%s_count%s %d\n", name, filters, countTotal)
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fmt.Fprintf(w, "%s_count%s %d\n", name, labels, countTotal)
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}
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func (h *Histogram) getSum() float64 {
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@ -261,46 +231,3 @@ func (h *Histogram) getSum() float64 {
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h.mu.Unlock()
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return sum
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}
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func getBucketIdxAndOffset(v float64) (int, uint) {
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if v < 0 {
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panic(fmt.Errorf("BUG: v must be positive; got %g", v))
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}
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if v == 0 {
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return -1, 0
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}
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if math.IsInf(v, 1) {
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return -1, 2
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}
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e10 := int(math.Floor(math.Log10(v)))
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bucketIdx := e10 - e10Min
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if bucketIdx < 0 {
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return -1, 1
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}
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if bucketIdx >= bucketsCount {
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if bucketIdx == bucketsCount && math.Abs(math.Pow10(e10)-v) < decimalPrecision {
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// Adjust m to be on par with Prometheus 'le' buckets (aka 'less or equal')
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return bucketsCount - 1, bucketSize - 1
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}
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return -1, 2
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}
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m := ((v / math.Pow10(e10)) - 1) * decimalMultiplier
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offset := int(m)
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if offset < 0 {
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offset = 0
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} else if offset >= bucketSize {
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offset = bucketSize - 1
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}
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if math.Abs(float64(offset)-m) < decimalPrecision {
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// Adjust offset to be on par with Prometheus 'le' buckets (aka 'less or equal')
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offset--
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if offset < 0 {
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bucketIdx--
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offset = bucketSize - 1
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if bucketIdx < 0 {
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return -1, 1
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}
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}
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}
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return bucketIdx, uint(offset)
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}
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@ -10,81 +10,21 @@ import (
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"time"
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)
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func TestGetBucketIdxAndOffset(t *testing.T) {
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f := func(v float64, bucketIdxExpected int, offsetExpected uint) {
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t.Helper()
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bucketIdx, offset := getBucketIdxAndOffset(v)
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if bucketIdx != bucketIdxExpected {
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t.Fatalf("unexpected bucketIdx for %g; got %d; want %d", v, bucketIdx, bucketIdxExpected)
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}
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if offset != offsetExpected {
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t.Fatalf("unexpected offset for %g; got %d; want %d", v, offset, offsetExpected)
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}
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}
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const step = 1.0 / decimalMultiplier
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const prec = 2 * decimalPrecision
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f(0, -1, 0)
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f(math.Pow10(e10Min-10), -1, 1)
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f(math.Pow10(e10Min-1), -1, 1)
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f(3*math.Pow10(e10Min-1), -1, 1)
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f(9*math.Pow10(e10Min-1), -1, 1)
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f(9.999*math.Pow10(e10Min-1), -1, 1)
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f(math.Pow10(e10Min), -1, 1)
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f((1+prec)*math.Pow10(e10Min), 0, 0)
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f((1+step)*math.Pow10(e10Min), 0, 0)
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f((1+step+prec)*math.Pow10(e10Min), 0, 1)
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f((1+2*step+prec)*math.Pow10(e10Min), 0, 2)
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f((1+3*step+prec)*math.Pow10(e10Min), 0, 3)
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f(math.Pow10(e10Min+1), 0, bucketSize-1)
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f((1+prec)*math.Pow10(e10Min+1), 1, 0)
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f((1+step)*math.Pow10(e10Min+1), 1, 0)
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f((1+step+prec)*math.Pow10(e10Min+1), 1, 1)
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f(0.1, -e10Min-2, bucketSize-1)
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f((1+prec)*0.1, -e10Min-1, 0)
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f((1+step)*0.1, -e10Min-1, 0)
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f((1+step+prec)*0.1, -e10Min-1, 1)
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f((1+(bucketSize-1)*step)*0.1, -e10Min-1, bucketSize-2)
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f((1+(bucketSize-1)*step+prec)*0.1, -e10Min-1, bucketSize-1)
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f(math.Pow10(e10Max-2), bucketsCount-3, bucketSize-1)
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f((1+prec)*math.Pow10(e10Max-2), bucketsCount-2, 0)
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f(math.Pow10(e10Max-1), bucketsCount-2, bucketSize-1)
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f((1+prec)*math.Pow10(e10Max-1), bucketsCount-1, 0)
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f((1+(bucketSize-1)*step)*math.Pow10(e10Max-1), bucketsCount-1, bucketSize-2)
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f((1+(bucketSize-1)*step+prec)*math.Pow10(e10Max-1), bucketsCount-1, bucketSize-1)
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f(math.Pow10(e10Max), bucketsCount-1, bucketSize-1)
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f((1+prec)*math.Pow10(e10Max), -1, 2)
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f((1+3*step+prec)*math.Pow10(e10Max), -1, 2)
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f(math.Inf(1), -1, 2)
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f(999, 11, 17)
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f(1000, 11, 17)
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f(1001, 12, 0)
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f(1002, 12, 0)
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f(1003, 12, 0)
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}
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func TestGetVMRange(t *testing.T) {
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f := func(bucketIdx int, offset uint, vmrangeExpected string) {
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f := func(bucketIdx int, vmrangeExpected string) {
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t.Helper()
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vmrange := getVMRange(bucketIdx, offset)
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vmrange := getVMRange(bucketIdx)
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if vmrange != vmrangeExpected {
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t.Fatalf("unexpected vmrange for bucketIdx=%d, offset=%d; got %s; want %s", bucketIdx, offset, vmrange, vmrangeExpected)
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t.Fatalf("unexpected vmrange for bucketIdx=%d; got %s; want %s", bucketIdx, vmrange, vmrangeExpected)
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}
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}
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const step = 1.0 / decimalMultiplier
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f(-1, 0, "0...0")
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f(-1, 1, fmt.Sprintf("0...1.0e%d", e10Min))
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f(-1, 2, fmt.Sprintf("1.0e%d...+Inf", e10Max))
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f(0, 0, fmt.Sprintf("1.0e%d...%.1fe%d", e10Min, 1+step, e10Min))
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f(0, 1, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+step, e10Min, 1+2*step, e10Min))
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f(0, bucketSize-2, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-2)*step, e10Min, 1+(bucketSize-1)*step, e10Min))
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f(0, bucketSize-1, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-1)*step, e10Min, 1.0, e10Min+1))
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f(-e10Min, 0, fmt.Sprintf("%.1fe%d...%.1fe%d", 1.0, 0, 1+step, 0))
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f(-e10Min, 1, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+step, 0, 1+2*step, 0))
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f(-e10Min, bucketSize-2, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-2)*step, 0, 1+(bucketSize-1)*step, 0))
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f(-e10Min, bucketSize-1, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-1)*step, 0, 1.0, 1))
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f(bucketsCount-1, bucketSize-2, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-2)*step, e10Max-1, 1+(bucketSize-1)*step, e10Max-1))
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f(bucketsCount-1, bucketSize-1, fmt.Sprintf("%.1fe%d...%.1fe%d", 1+(bucketSize-1)*step, e10Max-1, 1.0, e10Max))
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f(0, "1.000e-09...1.136e-09")
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f(1, "1.136e-09...1.292e-09")
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f(bucketsPerDecimal-1, "8.799e-09...1.000e-08")
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f(bucketsPerDecimal, "1.000e-08...1.136e-08")
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f(bucketsPerDecimal*(-e10Min)-1, "8.799e-01...1.000e+00")
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f(bucketsPerDecimal*(-e10Min), "1.000e+00...1.136e+00")
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f(bucketsPerDecimal*(e10Max-e10Min)-1, "8.799e+17...1.000e+18")
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}
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func TestHistogramSerial(t *testing.T) {
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@ -104,9 +44,29 @@ func TestHistogramSerial(t *testing.T) {
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h.Update(float64(i))
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}
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// Make sure the histogram prints <prefix>_xbucket on marshalTo call
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testMarshalTo(t, h, "prefix", "prefix_bucket{vmrange=\"9.5e1...1.0e2\"} 3\nprefix_bucket{vmrange=\"1.0e2...1.5e2\"} 50\nprefix_bucket{vmrange=\"1.5e2...2.0e2\"} 50\nprefix_bucket{vmrange=\"2.0e2...2.5e2\"} 17\nprefix_sum 18900\nprefix_count 120\n")
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testMarshalTo(t, h, ` m{foo="bar"}`, "\t m_bucket{foo=\"bar\",vmrange=\"9.5e1...1.0e2\"} 3\n\t m_bucket{foo=\"bar\",vmrange=\"1.0e2...1.5e2\"} 50\n\t m_bucket{foo=\"bar\",vmrange=\"1.5e2...2.0e2\"} 50\n\t m_bucket{foo=\"bar\",vmrange=\"2.0e2...2.5e2\"} 17\n\t m_sum{foo=\"bar\"} 18900\n\t m_count{foo=\"bar\"} 120\n")
|
||||
// Make sure the histogram prints <prefix>_bucket on marshalTo call
|
||||
testMarshalTo(t, h, "prefix", `prefix_bucket{vmrange="8.799e+01...1.000e+02"} 3
|
||||
prefix_bucket{vmrange="1.000e+02...1.136e+02"} 13
|
||||
prefix_bucket{vmrange="1.136e+02...1.292e+02"} 16
|
||||
prefix_bucket{vmrange="1.292e+02...1.468e+02"} 17
|
||||
prefix_bucket{vmrange="1.468e+02...1.668e+02"} 20
|
||||
prefix_bucket{vmrange="1.668e+02...1.896e+02"} 23
|
||||
prefix_bucket{vmrange="1.896e+02...2.154e+02"} 26
|
||||
prefix_bucket{vmrange="2.154e+02...2.448e+02"} 2
|
||||
prefix_sum 18900
|
||||
prefix_count 120
|
||||
`)
|
||||
testMarshalTo(t, h, ` m{foo="bar"}`, ` m_bucket{foo="bar",vmrange="8.799e+01...1.000e+02"} 3
|
||||
m_bucket{foo="bar",vmrange="1.000e+02...1.136e+02"} 13
|
||||
m_bucket{foo="bar",vmrange="1.136e+02...1.292e+02"} 16
|
||||
m_bucket{foo="bar",vmrange="1.292e+02...1.468e+02"} 17
|
||||
m_bucket{foo="bar",vmrange="1.468e+02...1.668e+02"} 20
|
||||
m_bucket{foo="bar",vmrange="1.668e+02...1.896e+02"} 23
|
||||
m_bucket{foo="bar",vmrange="1.896e+02...2.154e+02"} 26
|
||||
m_bucket{foo="bar",vmrange="2.154e+02...2.448e+02"} 2
|
||||
m_sum{foo="bar"} 18900
|
||||
m_count{foo="bar"} 120
|
||||
`)
|
||||
|
||||
// Verify Reset
|
||||
h.Reset()
|
||||
@ -119,13 +79,13 @@ func TestHistogramSerial(t *testing.T) {
|
||||
|
||||
// Verify supported ranges
|
||||
for e10 := -100; e10 < 100; e10++ {
|
||||
for offset := 0; offset < bucketSize; offset++ {
|
||||
m := 1 + float64(offset+1)/decimalMultiplier
|
||||
for offset := 0; offset < bucketsPerDecimal; offset++ {
|
||||
m := 1 + math.Pow(bucketMultiplier, float64(offset))
|
||||
f1 := m * math.Pow10(e10)
|
||||
h.Update(f1)
|
||||
f2 := (m + 0.5/decimalMultiplier) * math.Pow10(e10)
|
||||
f2 := (m + 0.5*bucketMultiplier) * math.Pow10(e10)
|
||||
h.Update(f2)
|
||||
f3 := (m + 2*decimalPrecision) * math.Pow10(e10)
|
||||
f3 := (m + 2*bucketMultiplier) * math.Pow10(e10)
|
||||
h.Update(f3)
|
||||
}
|
||||
}
|
||||
@ -160,7 +120,16 @@ func TestHistogramConcurrent(t *testing.T) {
|
||||
if err != nil {
|
||||
t.Fatal(err)
|
||||
}
|
||||
testMarshalTo(t, h, "prefix", "prefix_bucket{vmrange=\"5.5e-1...6.0e-1\"} 5\nprefix_bucket{vmrange=\"6.5e-1...7.0e-1\"} 5\nprefix_bucket{vmrange=\"7.5e-1...8.0e-1\"} 5\nprefix_bucket{vmrange=\"8.5e-1...9.0e-1\"} 5\nprefix_bucket{vmrange=\"9.5e-1...1.0e0\"} 5\nprefix_bucket{vmrange=\"1.0e0...1.5e0\"} 15\nprefix_sum 38\nprefix_count 40\n")
|
||||
testMarshalTo(t, h, "prefix", `prefix_bucket{vmrange="5.995e-01...6.813e-01"} 5
|
||||
prefix_bucket{vmrange="6.813e-01...7.743e-01"} 5
|
||||
prefix_bucket{vmrange="7.743e-01...8.799e-01"} 5
|
||||
prefix_bucket{vmrange="8.799e-01...1.000e+00"} 10
|
||||
prefix_bucket{vmrange="1.000e+00...1.136e+00"} 5
|
||||
prefix_bucket{vmrange="1.136e+00...1.292e+00"} 5
|
||||
prefix_bucket{vmrange="1.292e+00...1.468e+00"} 5
|
||||
prefix_sum 38
|
||||
prefix_count 40
|
||||
`)
|
||||
|
||||
var labels []string
|
||||
var counts []uint64
|
||||
@ -168,11 +137,19 @@ func TestHistogramConcurrent(t *testing.T) {
|
||||
labels = append(labels, label)
|
||||
counts = append(counts, count)
|
||||
})
|
||||
labelsExpected := []string{"5.5e-1...6.0e-1", "6.5e-1...7.0e-1", "7.5e-1...8.0e-1", "8.5e-1...9.0e-1", "9.5e-1...1.0e0", "1.0e0...1.5e0"}
|
||||
labelsExpected := []string{
|
||||
"5.995e-01...6.813e-01",
|
||||
"6.813e-01...7.743e-01",
|
||||
"7.743e-01...8.799e-01",
|
||||
"8.799e-01...1.000e+00",
|
||||
"1.000e+00...1.136e+00",
|
||||
"1.136e+00...1.292e+00",
|
||||
"1.292e+00...1.468e+00",
|
||||
}
|
||||
if !reflect.DeepEqual(labels, labelsExpected) {
|
||||
t.Fatalf("unexpected labels; got %v; want %v", labels, labelsExpected)
|
||||
}
|
||||
countsExpected := []uint64{5, 5, 5, 5, 5, 15}
|
||||
countsExpected := []uint64{5, 5, 5, 10, 5, 5, 5}
|
||||
if !reflect.DeepEqual(counts, countsExpected) {
|
||||
t.Fatalf("unexpected counts; got %v; want %v", counts, countsExpected)
|
||||
}
|
||||
@ -186,7 +163,7 @@ func TestHistogramWithTags(t *testing.T) {
|
||||
var bb bytes.Buffer
|
||||
WritePrometheus(&bb, false)
|
||||
result := bb.String()
|
||||
namePrefixWithTag := `TestHistogram_bucket{tag="foo",vmrange="1.0e2...1.5e2"} 1` + "\n"
|
||||
namePrefixWithTag := `TestHistogram_bucket{tag="foo",vmrange="1.136e+02...1.292e+02"} 1` + "\n"
|
||||
if !strings.Contains(result, namePrefixWithTag) {
|
||||
t.Fatalf("missing histogram %s in the WritePrometheus output; got\n%s", namePrefixWithTag, result)
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user