all: add -dedup.minScrapeInterval command-line flag for data de-duplication

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/86
Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/278

app/vmselect/netstorage/netstorage.go

@@ -264,7 +264,7 @@ func mergeSortBlocks(dst *Result, sbh sortBlocksHeap) {
 			dst.Timestamps = append(dst.Timestamps, top.Timestamps[top.NextIdx:]...)
 			dst.Values = append(dst.Values, top.Values[top.NextIdx:]...)
 			putSortBlock(top)
-			return
+			break
 		}
 		sbNext := sbh[0]
 		tsNext := sbNext.Timestamps[sbNext.NextIdx]
@@ -285,6 +285,8 @@ func mergeSortBlocks(dst *Result, sbh sortBlocksHeap) {
 			putSortBlock(top)
 		}
 	}
+
+	dst.Timestamps, dst.Values = storage.DeduplicateSamples(dst.Timestamps, dst.Values)
 }
 
 type sortBlock struct {

lib/storage/block.go

@@ -158,6 +158,25 @@ func (b *Block) tooBig() bool {
 	return false
 }
 
+func (b *Block) deduplicateSamplesDuringMerge() {
+	if len(b.values) == 0 {
+		// Nothing to dedup or the data is already marshaled.
+		return
+	}
+	srcTimestamps := b.timestamps[b.nextIdx:]
+	srcValues := b.values[b.nextIdx:]
+	timestamps, values := deduplicateSamplesDuringMerge(srcTimestamps, srcValues)
+	b.timestamps = b.timestamps[:b.nextIdx+len(timestamps)]
+	b.values = b.values[:b.nextIdx+len(values)]
+}
+
+func (b *Block) rowsCount() int {
+	if len(b.values) == 0 {
+		return int(b.bh.RowsCount)
+	}
+	return len(b.values[b.nextIdx:])
+}
+
 // MarshalData marshals the block into binary representation.
 func (b *Block) MarshalData(timestampsBlockOffset, valuesBlockOffset uint64) ([]byte, []byte, []byte) {
 	if len(b.values) == 0 {

lib/storage/block_stream_writer.go

@@ -171,6 +171,8 @@ func (bsw *blockStreamWriter) MustClose() {
 
 // WriteExternalBlock writes b to bsw and updates ph and rowsMerged.
 func (bsw *blockStreamWriter) WriteExternalBlock(b *Block, ph *partHeader, rowsMerged *uint64) {
+	atomic.AddUint64(rowsMerged, uint64(b.rowsCount()))
+	b.deduplicateSamplesDuringMerge()
 	headerData, timestampsData, valuesData := b.MarshalData(bsw.timestampsBlockOffset, bsw.valuesBlockOffset)
 
 	bsw.indexData = append(bsw.indexData, headerData...)
@@ -186,7 +188,6 @@ func (bsw *blockStreamWriter) WriteExternalBlock(b *Block, ph *partHeader, rowsM
 	bsw.valuesBlockOffset += uint64(len(valuesData))
 
 	updatePartHeader(b, ph)
-	atomic.AddUint64(rowsMerged, uint64(b.bh.RowsCount))
 }
 
 func updatePartHeader(b *Block, ph *partHeader) {

lib/storage/dedup.go

@@ -0,0 +1,99 @@
+package storage
+
+import (
+	"flag"
+
+	"github.com/VictoriaMetrics/metrics"
+)
+
+var minScrapeInterval = flag.Duration("dedup.minScrapeInterval", 0, "Remove superflouos samples from time series if they are located closer to each other than this duration. "+
+	"This may be useful for reducing overhead when multiple identically configured Prometheus instances write data to the same VictoriaMetrics. "+
+	"Deduplication is disabled if the `-dedup.minScrapeInterval` is 0")
+
+func getMinDelta() int64 {
+	// Divide minScrapeInterval by 2 in order to preserve proper data points.
+	// For instance, if minScrapeInterval=10, the following time series:
+	//    10 15 19 25 30 34 41
+	// Would be unexpectedly converted to:
+	//    10 25 41
+	// When dividing minScrapeInterval by 2, it will be converted to the expected:
+	//    10 19 30 41
+	return minScrapeInterval.Milliseconds() / 2
+}
+
+// DeduplicateSamples removes samples from src* if they are closer to each other than minScrapeInterval.
+func DeduplicateSamples(srcTimestamps []int64, srcValues []float64) ([]int64, []float64) {
+	if *minScrapeInterval <= 0 {
+		return srcTimestamps, srcValues
+	}
+	minDelta := getMinDelta()
+	if !needsDedup(srcTimestamps, minDelta) {
+		// Fast path - nothing to deduplicate
+		return srcTimestamps, srcValues
+	}
+
+	// Slow path - dedup data points.
+	prevTimestamp := srcTimestamps[0]
+	dstTimestamps := srcTimestamps[:1]
+	dstValues := srcValues[:1]
+	dedups := 0
+	for i := 1; i < len(srcTimestamps); i++ {
+		ts := srcTimestamps[i]
+		if ts-prevTimestamp < minDelta {
+			dedups++
+			continue
+		}
+		dstTimestamps = append(dstTimestamps, ts)
+		dstValues = append(dstValues, srcValues[i])
+		prevTimestamp = ts
+	}
+	dedupsDuringSelect.Add(dedups)
+	return dstTimestamps, dstValues
+}
+
+var dedupsDuringSelect = metrics.NewCounter(`deduplicated_samples_total{type="select"}`)
+
+func deduplicateSamplesDuringMerge(srcTimestamps []int64, srcValues []int64) ([]int64, []int64) {
+	if *minScrapeInterval <= 0 {
+		return srcTimestamps, srcValues
+	}
+	minDelta := getMinDelta()
+	if !needsDedup(srcTimestamps, minDelta) {
+		// Fast path - nothing to deduplicate
+		return srcTimestamps, srcValues
+	}
+
+	// Slow path - dedup data points.
+	prevTimestamp := srcTimestamps[0]
+	dstTimestamps := srcTimestamps[:1]
+	dstValues := srcValues[:1]
+	dedups := 0
+	for i := 1; i < len(srcTimestamps); i++ {
+		ts := srcTimestamps[i]
+		if ts-prevTimestamp < minDelta {
+			dedups++
+			continue
+		}
+		dstTimestamps = append(dstTimestamps, ts)
+		dstValues = append(dstValues, srcValues[i])
+		prevTimestamp = ts
+	}
+	dedupsDuringMerge.Add(dedups)
+	return dstTimestamps, dstValues
+}
+
+var dedupsDuringMerge = metrics.NewCounter(`deduplicated_samples_total{type="merge"}`)
+
+func needsDedup(timestamps []int64, minDelta int64) bool {
+	if len(timestamps) == 0 {
+		return false
+	}
+	prevTimestamp := timestamps[0]
+	for _, ts := range timestamps[1:] {
+		if ts-prevTimestamp < minDelta {
+			return true
+		}
+		prevTimestamp = ts
+	}
+	return false
+}