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Merge pull request #1608 from krajorama/index-out-of-range-native-histogram-exemplar 

bugfix: native histogram: exemplars index out of range
This commit is contained in:
George Krajcsovits 2024-09-04 21:21:43 +02:00 committed by GitHub
parent 67121dc55a d6b8c8925b
commit 6e9914db5a
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
2 changed files with 70 additions and 26 deletions
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80
prometheus/histogram.go
View File

@ -1658,7 +1658,10 @@ func addAndResetCounts(hot, cold *histogramCounts) {
type nativeExemplars struct {
sync.Mutex
// 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
@ -1705,15 +1707,44 @@ 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.
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.
mdIdx = -1 // Index of the older exemplar within the closest pair.
// 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.
)
@ -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,
@ -1764,24 +1801,27 @@ 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.
switch {

6
prometheus/histogram_test.go
View File

@ -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)
if i%2 == 0 {
his.Observe(v)
} else {
his.(ExemplarObserver).ObserveWithExemplar(v, Labels{"foo": "bar"})
}
}
end.Done()
}(vals)