For memory store KV with history of 1 we were scanning for our next first when we did not have to.

Signed-off-by: Derek Collison <derek@nats.io>

server/jetstream_test.go

@@ -17359,3 +17359,34 @@ func Benchmark_JetStream10x1kWorker(b *testing.B) {
 func Benchmark_JetStream4x512Worker(b *testing.B) {
 	benchJetStreamWorkersAndBatch(b, 4, 512)
 }
+
+func TestJetStreamKVMemoryStorePerf(t *testing.T) {
+	// Comment out to run, holding place for now.
+	t.SkipNow()
+
+	s := RunBasicJetStreamServer()
+	if config := s.JetStreamConfig(); config != nil {
+		defer removeDir(t, config.StoreDir)
+	}
+	defer s.Shutdown()
+
+	nc, js := jsClientConnect(t, s)
+	defer nc.Close()
+
+	kv, err := js.CreateKeyValue(&nats.KeyValueConfig{Bucket: "TEST", History: 1, Storage: nats.MemoryStorage})
+	require_NoError(t, err)
+
+	start := time.Now()
+	for i := 0; i < 100_000; i++ {
+		_, err := kv.PutString(fmt.Sprintf("foo.%d", i), "HELLO")
+		require_NoError(t, err)
+	}
+	fmt.Printf("Took %v for first run\n", time.Since(start))
+
+	start = time.Now()
+	for i := 0; i < 100_000; i++ {
+		_, err := kv.PutString(fmt.Sprintf("foo.%d", i), "HELLO")
+		require_NoError(t, err)
+	}
+	fmt.Printf("Took %v for second run\n", time.Since(start))
+}

server/memstore.go

@@ -753,7 +753,12 @@ func (ms *memStore) removeSeqPerSubject(subj string, seq uint64) {
 	if seq != ss.First {
 		return
 	}
-	// TODO(dlc) - Might want to optimize this.
+	// If we know we only have 1 msg left don't need to search for next first.
+	if ss.Msgs == 1 {
+		ss.First = ss.Last
+		return
+	}
+	// TODO(dlc) - Might want to optimize this longer term.
 	for tseq := seq + 1; tseq <= ss.Last; tseq++ {
 		if sm := ms.msgs[tseq]; sm != nil && sm.subj == subj {
 			ss.First = tseq