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Clarify computeChunkEndTime's purpose (#9049)

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I was struggling to understand the purpose of this method until I
tweaked the tests, so I decided to write down my observations.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>
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Oleg Zaytsev 2021-07-28 15:09:05 +02:00 committed by GitHub
parent ed24e51e7c
commit f9482c5bf6
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1 changed files with 8 additions and 5 deletions
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13
tsdb/head.go
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@ -2607,8 +2607,9 @@ func (s *memSeries) append(t int64, v float64, appendID uint64, chunkDiskMapper
if c.maxTime >= t {
return false, chunkCreated
}
// If we reach 25% of a chunk's desired sample count, set a definitive time
// at which to start the next chunk.
// If we reach 25% of a chunk's desired sample count, predict an end time
// for this chunk that will try to make samples equally distributed within
// the remaining chunks in the current chunk range.
// At latest it must happen at the timestamp set when the chunk was cut.
if numSamples == samplesPerChunk/4 {
s.nextAt = computeChunkEndTime(c.minTime, c.maxTime, s.nextAt)
@ -2642,12 +2643,14 @@ func (s *memSeries) cleanupAppendIDsBelow(bound uint64) {
// computeChunkEndTime estimates the end timestamp based the beginning of a
// chunk, its current timestamp and the upper bound up to which we insert data.
// It assumes that the time range is 1/4 full.
// Assuming that the samples will keep arriving at the same rate, it will make the
// remaining n chunks within this chunk range (before max) equally sized.
func computeChunkEndTime(start, cur, max int64) int64 {
a := (max - start) / ((cur - start + 1) * 4)
if a == 0 {
n := (max - start) / ((cur - start + 1) * 4)
if n <= 1 {
return max
}
return start + (max-start)/a
return start + (max-start)/n
}
// iterator returns a chunk iterator.