docs/CHANGELOG.md: run at least 4 background mergers on systems with less than 4 CPU cores

This reduces the probability of sudden spike in the number of small parts when all the background mergers
are busy with big merges.

docs/CHANGELOG.md

@@ -15,6 +15,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 
 ## v1.87.x long-time support release (LTS)
 
+* BUGFIX: reduce the probability of sudden increase in the number of small parts on systems with small number of CPU cores.
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation.html): suppress `series after dedup` error message in logs when `-remoteWrite.streamAggr.dedupInterval` command-line flag is set at [vmagent](https://docs.victoriametrics.com/vmgent.html) or when `-streamAggr.dedupInterval` command-line flag is set at [single-node VictoriaMetrics](https://docs.victoriametrics.com/).
 
 ## [v1.87.5](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.87.5)

lib/mergeset/table.go

@@ -923,10 +923,8 @@ func newPartWrapperFromInmemoryPart(mp *inmemoryPart, flushToDiskDeadline time.T
 }
 
 func (tb *Table) startMergeWorkers() {
-	// Start a merge worker per available CPU core.
 	// The actual number of concurrent merges is limited inside mergeWorker() below.
-	workersCount := cgroup.AvailableCPUs()
-	for i := 0; i < workersCount; i++ {
+	for i := 0; i < cap(mergeWorkersLimitCh); i++ {
 		tb.wg.Add(1)
 		go func() {
 			tb.mergeWorker()
@@ -1403,7 +1401,18 @@ func (tb *Table) nextMergeIdx() uint64 {
 	return atomic.AddUint64(&tb.mergeIdx, 1)
 }
 
-var mergeWorkersLimitCh = make(chan struct{}, cgroup.AvailableCPUs())
+var mergeWorkersLimitCh = make(chan struct{}, getWorkersCount())
+
+func getWorkersCount() int {
+	n := cgroup.AvailableCPUs()
+	if n < 4 {
+		// Allow at least 4 merge workers on systems with small CPUs count
+		// in order to guarantee that background merges can be continued
+		// when multiple workers are busy with big merges.
+		n = 4
+	}
+	return n
+}
 
 func openParts(path string) ([]*partWrapper, error) {
 	// The path can be missing after restoring from backup, so create it if needed.

lib/storage/partition.go

@@ -1004,7 +1004,7 @@ func hasActiveMerges(pws []*partWrapper) bool {
 	return false
 }
 
-var mergeWorkersLimitCh = make(chan struct{}, adjustMergeWorkersLimit(getDefaultMergeConcurrency(16)))
+var mergeWorkersLimitCh = make(chan struct{}, getDefaultMergeConcurrency(16))
 
 var bigMergeWorkersLimitCh = make(chan struct{}, getDefaultMergeConcurrency(4))
 
@@ -1013,7 +1013,7 @@ func getDefaultMergeConcurrency(max int) int {
 	if v > max {
 		v = max
 	}
-	return v
+	return adjustMergeWorkersLimit(v)
 }
 
 // SetBigMergeWorkersCount sets the maximum number of concurrent mergers for big blocks.
@@ -1040,20 +1040,18 @@ func SetMergeWorkersCount(n int) {
 }
 
 func adjustMergeWorkersLimit(n int) int {
-	if n < 2 {
-		// Allow at least 2 merge workers on systems with a single CPU core
+	if n < 4 {
+		// Allow at least 4 merge workers on systems with small CPUs count
 		// in order to guarantee that background merges can be continued
-		// when a single worker is busy with the long merge of big parts.
-		return 2
+		// when multiple workers are busy with big merges.
+		n = 4
 	}
 	return n
 }
 
 func (pt *partition) startMergeWorkers() {
-	// Start a merge worker per available CPU core.
 	// The actual number of concurrent merges is limited inside mergeWorker() below.
-	workersCount := cgroup.AvailableCPUs()
-	for i := 0; i < workersCount; i++ {
+	for i := 0; i < cap(mergeWorkersLimitCh); i++ {
 		pt.wg.Add(1)
 		go func() {
 			pt.mergeWorker()