lib/logstorage: improve performance of top and field_values pipes on systems with many CPU cores

- Parallelize mering of per-CPU results.
- Parallelize writing the results to the next pipe.

docs/VictoriaLogs/CHANGELOG.md

@@ -17,7 +17,7 @@ according to [these docs](https://docs.victoriametrics.com/victorialogs/quicksta
 
 * FEATURE: add basic [alerting rules](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/deployment/docker/alerts-vlogs.yml) for VictoriaLogs process. See details at [monitoring docs](https://docs.victoriametrics.com/victorialogs/index.html#monitoring).
 * FEATURE: improve [`stats` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#stats-pipe) performance on systems with many CPU cores when `by(...)` fields contain big number of unique values. For example, `_time:1d | stats by (user_id) count() x` should be executed much faster when `user_id` field contains millions of unique values.
-* FEATURE: improve [`top` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#top-pipe) performance on systems with many CPU cores when it is applied to [log fields](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model) with big number of unique values. For example, `_time:1d | top 5 (user_id)` should be executed much faster when `user_id` field contains millions of unique values.
+* FEATURE: improve performance for [`top`](https://docs.victoriametrics.com/victorialogs/logsql/#top-pipe), [`uniq`](https://docs.victoriametrics.com/victorialogs/logsql/#uniq-pipe) and [`field_values`](https://docs.victoriametrics.com/victorialogs/logsql/#field_values-pipe) pipes on systems with many CPU cores when it is applied to [log fields](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model) with big number of unique values. For example, `_time:1d | top 5 (user_id)` should be executed much faster when `user_id` field contains millions of unique values.
 
 ## [v0.36.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v0.36.0-victorialogs)
 

lib/logstorage/pipe_uniq.go

@@ -4,9 +4,12 @@ import (
 	"fmt"
 	"slices"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"unsafe"
 
+	"github.com/cespare/xxhash/v2"
+
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/encoding"
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
@@ -263,31 +266,63 @@ func (pup *pipeUniqProcessor) flush() error {
 		return fmt.Errorf("cannot calculate [%s], since it requires more than %dMB of memory", pup.pu.String(), pup.maxStateSize/(1<<20))
 	}
 
-	// merge state across shards
-	shards := pup.shards
-	m := shards[0].getM()
-	shards = shards[1:]
-	for i := range shards {
+	// merge state across shards in parallel
+	ms, err := pup.mergeShardsParallel()
+	if err != nil {
+		return err
+	}
 	if needStop(pup.stopCh) {
 		return nil
 	}
 
-		for k, pHitsSrc := range shards[i].getM() {
-			pHits, ok := m[k]
-			if !ok {
-				m[k] = pHitsSrc
-			} else {
-				*pHits += *pHitsSrc
-			}
-		}
+	resetHits := false
+	if limit := pup.pu.limit; limit > 0 {
+		// Trim the number of entries according to the given limit
+		entriesLen := 0
+		result := ms[:0]
+		for _, m := range ms {
+			entriesLen += len(m)
+			if uint64(entriesLen) <= limit {
+				result = append(result, m)
+				continue
 			}
 
-	// There is little sense in returning partial hits when the limit on the number of unique entries is reached.
-	// It is better from UX experience is to return zero hits instead.
-	resetHits := pup.pu.limit > 0 && uint64(len(m)) > pup.pu.limit
+			// There is little sense in returning partial hits when the limit on the number of unique entries is reached,
+			// since arbitrary number of unique entries and hits for these entries could be skipped.
+			// It is better to return zero hits instead of misleading hits results.
+			resetHits = true
+			for k := range m {
+				delete(m, k)
+				entriesLen--
+				if uint64(entriesLen) <= limit {
+					break
+				}
+			}
+			if len(m) > 0 {
+				result = append(result, m)
+			}
+			break
+		}
+		ms = result
+	}
 
-	// write result
+	// Write the calculated stats in parallel to the next pipe.
+	var wg sync.WaitGroup
+	for i, m := range ms {
+		wg.Add(1)
+		go func(workerID uint) {
+			defer wg.Done()
+			pup.writeShardData(workerID, m, resetHits)
+		}(uint(i))
+	}
+	wg.Wait()
+
+	return nil
+}
+
+func (pup *pipeUniqProcessor) writeShardData(workerID uint, m map[string]*uint64, resetHits bool) {
 	wctx := &pipeUniqWriteContext{
+		workerID: workerID,
 		pup:      pup,
 	}
 	byFields := pup.pu.byFields
@@ -311,7 +346,7 @@ func (pup *pipeUniqProcessor) flush() error {
 	if len(byFields) == 0 {
 		for k, pHits := range m {
 			if needStop(pup.stopCh) {
-				return nil
+				return
 			}
 
 			rowFields = rowFields[:0]
@@ -341,7 +376,7 @@ func (pup *pipeUniqProcessor) flush() error {
 		fieldName := byFields[0]
 		for k, pHits := range m {
 			if needStop(pup.stopCh) {
-				return nil
+				return
 			}
 
 			rowFields = append(rowFields[:0], Field{
@@ -354,7 +389,7 @@ func (pup *pipeUniqProcessor) flush() error {
 	} else {
 		for k, pHits := range m {
 			if needStop(pup.stopCh) {
-				return nil
+				return
 			}
 
 			rowFields = rowFields[:0]
@@ -379,18 +414,136 @@ func (pup *pipeUniqProcessor) flush() error {
 	}
 
 	wctx.flush()
+}
 
-	return nil
+func (pup *pipeUniqProcessor) mergeShardsParallel() ([]map[string]*uint64, error) {
+	shards := pup.shards
+	shardsLen := len(shards)
+	if shardsLen == 1 {
+		m := shards[0].getM()
+		var ms []map[string]*uint64
+		if len(m) > 0 {
+			ms = append(ms, m)
+		}
+		return ms, nil
+	}
+
+	var wg sync.WaitGroup
+	perShardMaps := make([][]map[string]*uint64, shardsLen)
+	for i := range shards {
+		wg.Add(1)
+		go func(idx int) {
+			defer wg.Done()
+
+			shardMaps := make([]map[string]*uint64, shardsLen)
+			for i := range shardMaps {
+				shardMaps[i] = make(map[string]*uint64)
+			}
+
+			n := int64(0)
+			nTotal := int64(0)
+			for k, pHits := range shards[idx].getM() {
+				if needStop(pup.stopCh) {
+					return
+				}
+				h := xxhash.Sum64(bytesutil.ToUnsafeBytes(k))
+				m := shardMaps[h%uint64(len(shardMaps))]
+				n += updatePipeUniqMap(m, k, pHits)
+				if n > stateSizeBudgetChunk {
+					if nRemaining := pup.stateSizeBudget.Add(-n); nRemaining < 0 {
+						return
+					}
+					nTotal += n
+					n = 0
+				}
+			}
+			nTotal += n
+			pup.stateSizeBudget.Add(-n)
+
+			perShardMaps[idx] = shardMaps
+
+			// Clean the original map and return its state size budget back.
+			shards[idx].m = nil
+			pup.stateSizeBudget.Add(nTotal)
+		}(i)
+	}
+	wg.Wait()
+	if needStop(pup.stopCh) {
+		return nil, nil
+	}
+	if n := pup.stateSizeBudget.Load(); n < 0 {
+		return nil, fmt.Errorf("cannot calculate [%s], since it requires more than %dMB of memory", pup.pu.String(), pup.maxStateSize/(1<<20))
+	}
+
+	// Merge per-shard entries into perShardMaps[0]
+	for i := range perShardMaps {
+		wg.Add(1)
+		go func(idx int) {
+			defer wg.Done()
+
+			m := perShardMaps[0][idx]
+			for i := 1; i < len(perShardMaps); i++ {
+				n := int64(0)
+				nTotal := int64(0)
+				for k, psg := range perShardMaps[i][idx] {
+					if needStop(pup.stopCh) {
+						return
+					}
+					n += updatePipeUniqMap(m, k, psg)
+					if n > stateSizeBudgetChunk {
+						if nRemaining := pup.stateSizeBudget.Add(-n); nRemaining < 0 {
+							return
+						}
+						nTotal += n
+						n = 0
+					}
+				}
+				nTotal += n
+				pup.stateSizeBudget.Add(-n)
+
+				// Clean the original map and return its state size budget back.
+				perShardMaps[i][idx] = nil
+				pup.stateSizeBudget.Add(nTotal)
+			}
+		}(i)
+	}
+	wg.Wait()
+	if needStop(pup.stopCh) {
+		return nil, nil
+	}
+	if n := pup.stateSizeBudget.Load(); n < 0 {
+		return nil, fmt.Errorf("cannot calculate [%s], since it requires more than %dMB of memory", pup.pu.String(), pup.maxStateSize/(1<<20))
+	}
+
+	// Filter out maps without entries
+	ms := perShardMaps[0]
+	result := ms[:0]
+	for _, m := range ms {
+		if len(m) > 0 {
+			result = append(result, m)
+		}
+	}
+
+	return result, nil
+}
+
+func updatePipeUniqMap(m map[string]*uint64, k string, pHitsSrc *uint64) int64 {
+	pHitsDst := m[k]
+	if pHitsDst != nil {
+		*pHitsDst += *pHitsSrc
+		return 0
+	}
+
+	m[k] = pHitsSrc
+	return int64(unsafe.Sizeof(k) + unsafe.Sizeof(pHitsSrc))
 }
 
 type pipeUniqWriteContext struct {
+	workerID uint
 	pup      *pipeUniqProcessor
 	rcs      []resultColumn
 	br       blockResult
 
-	// rowsWritten is the total number of rows passed to writeRow.
-	rowsWritten uint64
-
 	// rowsCount is the number of rows in the current block
 	rowsCount int
 
@@ -399,11 +552,6 @@ type pipeUniqWriteContext struct {
 }
 
 func (wctx *pipeUniqWriteContext) writeRow(rowFields []Field) {
-	if limit := wctx.pup.pu.limit; limit > 0 && wctx.rowsWritten >= limit {
-		return
-	}
-	wctx.rowsWritten++
-
 	rcs := wctx.rcs
 
 	areEqualColumns := len(rcs) == len(rowFields)
@@ -447,7 +595,7 @@ func (wctx *pipeUniqWriteContext) flush() {
 	// Flush rcs to ppNext
 	br.setResultColumns(rcs, wctx.rowsCount)
 	wctx.rowsCount = 0
-	wctx.pup.ppNext.writeBlock(0, br)
+	wctx.pup.ppNext.writeBlock(wctx.workerID, br)
 	br.reset()
 	for i := range rcs {
 		rcs[i].resetValues()