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app/vmselect/promql: propagate lower bucket values when fixing a histogram

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In most cases histograms are exposed in sorted manner with lower buckets being first. This means that during scraping buckets with lower bounds have higher chance of being updated earlier than upper ones.

 Previously, values were propagated from upper to lower bounds, which means that in most cases that would produce results higher than expected once all buckets will become updated.
 Propagating from upper bound effectively limits highest value of histogram to the value of previous scrape. Once the data will become consistent in the subsequent evaluation this causes spikes in the result.

 Changing propagation to be from lower to higher buckets reduces value spikes in most cases due to nature of the original inconsistency.

 See: https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4580

Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>
This commit is contained in:
Zakhar Bessarab 2024-06-28 12:43:21 +04:00
parent 547aa6108e
commit 5c7514134b
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GPG key ID: 932B34D6FE062023
3 changed files with 10 additions and 10 deletions
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2
app/vmselect/promql/exec_test.go
View file

@ -4471,7 +4471,7 @@ func TestExecSuccess(t *testing.T) {
),0.01)`
r := netstorage.Result{
MetricName: metricNameExpected,
Values: []float64{18.57, 18.57, 18.57, 18.57, 18.57, 18.57},
Values: []float64{30, 30, 30, 30, 30, 30},
Timestamps: timestampsExpected,
}
r.MetricName.Tags = []storage.Tag{{

10
app/vmselect/promql/transform.go
View file

@ -1061,12 +1061,12 @@ func fixBrokenBuckets(i int, xss []leTimeseries) {
break
}
}
// Substitute lower bucket values with upper values if the lower values are NaN
// or are bigger than the upper bucket values.
vNext := xss[len(xss)-1].ts.Values[i]
for j := len(xss) - 2; j >= 0; j-- {
// Substitute upper bucket values lower bucket values if the upper values are NaN
// or are bigger than the lower bucket values.
vNext := xss[0].ts.Values[0]
for j := 1; j < len(xss); j++ {
v := xss[j].ts.Values[i]
if math.IsNaN(v) || v > vNext {
if math.IsNaN(v) || vNext > v {
xss[j].ts.Values[i] = vNext
} else {
vNext = v

8
app/vmselect/promql/transform_test.go
View file

@ -32,11 +32,11 @@ func TestFixBrokenBuckets(t *testing.T) {
f(nil, []float64{})
f([]float64{1}, []float64{1})
f([]float64{1, 2}, []float64{1, 2})
f([]float64{2, 1}, []float64{1, 1})
f([]float64{2, 1}, []float64{2, 2})
f([]float64{1, 2, 3, nan, nan}, []float64{1, 2, 3, 3, 3})
f([]float64{5, 1, 2, 3, nan}, []float64{1, 1, 2, 3, 3})
f([]float64{1, 5, 2, nan, 6, 3}, []float64{1, 2, 2, 3, 3, 3})
f([]float64{5, 10, 4, 3}, []float64{3, 3, 3, 3})
f([]float64{5, 1, 2, 3, nan}, []float64{5, 5, 5, 5, 5})
f([]float64{1, 5, 2, nan, 6, 3}, []float64{1, 5, 5, 5, 6, 6})
f([]float64{5, 10, 4, 3}, []float64{5, 10, 10, 10})
}
func TestVmrangeBucketsToLE(t *testing.T) {