diff --git a/prometheus/histogram.go b/prometheus/histogram.go index 8d35f2d8a..8a4f49a4c 100644 --- a/prometheus/histogram.go +++ b/prometheus/histogram.go @@ -1658,7 +1658,10 @@ func addAndResetCounts(hot, cold *histogramCounts) { type nativeExemplars struct { sync.Mutex - ttl time.Duration + // Time-to-live for exemplars, it is set to -1 if exemplars are disabled, that is NativeHistogramMaxExemplars is below 0. + // The ttl is used on insertion to remove an exemplar that is older than ttl, if present. + ttl time.Duration + exemplars []*dto.Exemplar } @@ -1673,6 +1676,7 @@ func makeNativeExemplars(ttl time.Duration, maxCount int) nativeExemplars { if maxCount < 0 { maxCount = 0 + ttl = -1 } return nativeExemplars{ @@ -1682,20 +1686,18 @@ func makeNativeExemplars(ttl time.Duration, maxCount int) nativeExemplars { } func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { - if cap(n.exemplars) == 0 { + if n.ttl == -1 { return } n.Lock() defer n.Unlock() - // The index where to insert the new exemplar. - var nIdx int = -1 - // When the number of exemplars has not yet exceeded or // is equal to cap(n.exemplars), then // insert the new exemplar directly. if len(n.exemplars) < cap(n.exemplars) { + var nIdx int for nIdx = 0; nIdx < len(n.exemplars); nIdx++ { if *e.Value < *n.exemplars[nIdx].Value { break @@ -1705,17 +1707,46 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { return } + if len(n.exemplars) == 1 { + // When the number of exemplars is 1, then + // replace the existing exemplar with the new exemplar. + n.exemplars[0] = e + return + } + // From this point on, the number of exemplars is greater than 1. + // When the number of exemplars exceeds the limit, remove one exemplar. var ( - rIdx int // The index where to remove the old exemplar. - - ot = time.Now() // Oldest timestamp seen. - otIdx = -1 // Index of the exemplar with the oldest timestamp. - - md = -1.0 // Logarithm of the delta of the closest pair of exemplars. - mdIdx = -1 // Index of the older exemplar within the closest pair. - cLog float64 // Logarithm of the current exemplar. - pLog float64 // Logarithm of the previous exemplar. + ot = time.Time{} // Oldest timestamp seen. Initial value doesn't matter as we replace it due to otIdx == -1 in the loop. + otIdx = -1 // Index of the exemplar with the oldest timestamp. + + md = -1.0 // Logarithm of the delta of the closest pair of exemplars. + + // The insertion point of the new exemplar in the exemplars slice after insertion. + // This is calculated purely based on the order of the exemplars by value. + // nIdx == len(n.exemplars) means the new exemplar is to be inserted after the end. + nIdx = -1 + + // rIdx is ultimately the index for the exemplar that we are replacing with the new exemplar. + // The aim is to keep a good spread of exemplars by value and not let them bunch up too much. + // It is calculated in 3 steps: + // 1. First we set rIdx to the index of the older exemplar within the closest pair by value. + // That is the following will be true (on log scale): + // either the exemplar pair on index (rIdx-1, rIdx) or (rIdx, rIdx+1) will have + // the closest values to each other from all pairs. + // For example, suppose the values are distributed like this: + // |-----------x-------------x----------------x----x-----| + // ^--rIdx as this is older. + // Or like this: + // |-----------x-------------x----------------x----x-----| + // ^--rIdx as this is older. + // 2. If there is an exemplar that expired, then we simple reset rIdx to that index. + // 3. We check if by inserting the new exemplar we would create a closer pair at + // (nIdx-1, nIdx) or (nIdx, nIdx+1) and set rIdx to nIdx-1 or nIdx accordingly to + // keep the spread of exemplars by value; otherwise we keep rIdx as it is. + rIdx = -1 + cLog float64 // Logarithm of the current exemplar. + pLog float64 // Logarithm of the previous exemplar. ) for i, exemplar := range n.exemplars { @@ -1726,7 +1757,7 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { } // Find the index at which to insert new the exemplar. - if *e.Value <= *exemplar.Value && nIdx == -1 { + if nIdx == -1 && *e.Value <= *exemplar.Value { nIdx = i } @@ -1738,11 +1769,13 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { } diff := math.Abs(cLog - pLog) if md == -1 || diff < md { + // The closest exemplar pair is at index: i-1, i. + // Choose the exemplar with the older timestamp for replacement. md = diff if n.exemplars[i].Timestamp.AsTime().Before(n.exemplars[i-1].Timestamp.AsTime()) { - mdIdx = i + rIdx = i } else { - mdIdx = i - 1 + rIdx = i - 1 } } @@ -1753,8 +1786,12 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { if nIdx == -1 { nIdx = len(n.exemplars) } + // Here, we have the following relationships: + // n.exemplars[nIdx-1].Value < e.Value (if nIdx > 0) + // e.Value <= n.exemplars[nIdx].Value (if nIdx < len(n.exemplars)) if otIdx != -1 && e.Timestamp.AsTime().Sub(ot) > n.ttl { + // If the oldest exemplar has expired, then replace it with the new exemplar. rIdx = otIdx } else { // In the previous for loop, when calculating the closest pair of exemplars, @@ -1764,23 +1801,26 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) { if nIdx > 0 { diff := math.Abs(elog - math.Log(n.exemplars[nIdx-1].GetValue())) if diff < md { + // The value we are about to insert is closer to the previous exemplar at the insertion point than what we calculated before in rIdx. + // v--rIdx + // |-----------x-n-----------x----------------x----x-----| + // nIdx-1--^ ^--new exemplar value + // Do not make the spread worse, replace nIdx-1 and not rIdx. md = diff - mdIdx = nIdx - if n.exemplars[nIdx-1].Timestamp.AsTime().Before(e.Timestamp.AsTime()) { - mdIdx = nIdx - 1 - } + rIdx = nIdx - 1 } } if nIdx < len(n.exemplars) { diff := math.Abs(math.Log(n.exemplars[nIdx].GetValue()) - elog) if diff < md { - mdIdx = nIdx - if n.exemplars[nIdx].Timestamp.AsTime().Before(e.Timestamp.AsTime()) { - mdIdx = nIdx - } + // The value we are about to insert is closer to the next exemplar at the insertion point than what we calculated before in rIdx. + // v--rIdx + // |-----------x-----------n-x----------------x----x-----| + // new exemplar value--^ ^--nIdx + // Do not make the spread worse, replace nIdx-1 and not rIdx. + rIdx = nIdx } } - rIdx = mdIdx } // Adjust the slice according to rIdx and nIdx. diff --git a/prometheus/histogram_test.go b/prometheus/histogram_test.go index f2fb5bb8c..c2a14ae72 100644 --- a/prometheus/histogram_test.go +++ b/prometheus/histogram_test.go @@ -1049,10 +1049,14 @@ func TestNativeHistogramConcurrency(t *testing.T) { go func(vals []float64) { start.Wait() - for _, v := range vals { + for i, v := range vals { // An observation every 1 to 10 seconds. atomic.AddInt64(&ts, rand.Int63n(10)+1) - his.Observe(v) + if i%2 == 0 { + his.Observe(v) + } else { + his.(ExemplarObserver).ObserveWithExemplar(v, Labels{"foo": "bar"}) + } } end.Done() }(vals)