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Update comments with more explanations
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Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
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krajorama committed Sep 4, 2024
1 parent 504566f commit d6b8c89
Showing 1 changed file with 46 additions and 17 deletions.
63 changes: 46 additions & 17 deletions prometheus/histogram.go
Original file line number Diff line number Diff line change
Expand Up @@ -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
}

Expand Down Expand Up @@ -1690,13 +1693,11 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
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
Expand All @@ -1716,11 +1717,34 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {

// When the number of exemplars exceeds the limit, remove one exemplar.
var (
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.
rIdx = -1 // Index of the older exemplar within the closest pair and where we need to insert the new 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.
)
Expand All @@ -1745,7 +1769,7 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
}
diff := math.Abs(cLog - pLog)
if md == -1 || diff < md {
// The closest exemplar pair is this: |exemplar.[i] - n.exemplars[i-1].Value| is minimal.
// 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()) {
Expand All @@ -1763,7 +1787,8 @@ func (n *nativeExemplars) addExemplar(e *dto.Exemplar) {
nIdx = len(n.exemplars)
}
// Here, we have the following relationships:
// n.exemplars[nIdx-1].Value < e.Value <= n.exemplars[nIdx].Value
// 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.
Expand All @@ -1776,19 +1801,23 @@ 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 closest exemplar pair is this: |e.Value - n.exemplars[nIdx-1].Value| is minimal.
// Assume that the exemplar we are inserting has a newer timestamp. This is not always
// true, due to concurrency, but it's a good enough approximation.
// 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
rIdx = nIdx - 1
}
}
if nIdx < len(n.exemplars) {
diff := math.Abs(math.Log(n.exemplars[nIdx].GetValue()) - elog)
if diff < md {
// The closest exemplar pair is this: |n.exemplars[nIdx].Value - e.Value| is minimal.
// Assume that the exemplar we are inserting has a newer timestamp. This is not always
// true, due to concurrency, but it's a good enough approximation.
// 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
}
}
Expand Down

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