[IMPROVED] Last msg lookup (KV Get) when subject is a literal subject (…

…#4232)

When messages were very small and the key space was very large the
performance of last message gets in the store layer (both file and
memory) would degrade.

If the subject is literal we can optimize and avoid sequence scans that
are needed when multiple subject states need to be considered.

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

Resolves #4221

server/filestore.go

@@ -4665,7 +4665,16 @@ func (fs *fileStore) loadLast(subj string, sm *StoreMsg) (lsm *StoreMsg, err err
 			mb.mu.Unlock()
 			return nil, err
 		}
-		_, _, l := mb.filteredPendingLocked(subj, wc, mb.first.seq)
+		var l uint64
+		// Optimize if subject is not a wildcard.
+		if !wc {
+			if ss := mb.fss[subj]; ss != nil {
+				l = ss.Last
+			}
+		}
+		if l == 0 {
+			_, _, l = mb.filteredPendingLocked(subj, wc, mb.first.seq)
+		}
 		if l > 0 {
 			if mb.cacheNotLoaded() {
 				if err := mb.loadMsgsWithLock(); err != nil {

server/memstore.go

@@ -864,6 +864,10 @@ func (ms *memStore) LoadLastMsg(subject string, smp *StoreMsg) (*StoreMsg, error
 
 	if subject == _EMPTY_ || subject == fwcs {
 		sm, ok = ms.msgs[ms.state.LastSeq]
+	} else if subjectIsLiteral(subject) {
+		if ss := ms.fss[subject]; ss != nil && ss.Msgs > 0 {
+			sm, ok = ms.msgs[ss.Last]
+		}
 	} else if ss := ms.filteredStateLocked(1, subject, true); ss.Msgs > 0 {
 		sm, ok = ms.msgs[ss.Last]
 	}
@@ -895,8 +899,8 @@ func (ms *memStore) LoadNextMsg(filter string, wc bool, start uint64, smp *Store
 
 	isAll := filter == _EMPTY_ || filter == fwcs
 
-	// Skip scan of mb.fss is number of messages in the block are less than
-	// 1/2 the number of subjects in mb.fss. Or we have a wc and lots of fss entries.
+	// Skip scan of ms.fss is number of messages in the block are less than
+	// 1/2 the number of subjects in ms.fss. Or we have a wc and lots of fss entries.
 	const linearScanMaxFSS = 256
 	doLinearScan := isAll || 2*int(ms.state.LastSeq-start) < len(ms.fss) || (wc && len(ms.fss) > linearScanMaxFSS)