raft: dont allocate slice and sort on every commit

raft/raft.go

@@ -235,6 +235,7 @@ type raft struct {
 	maxInflight int
 	maxMsgSize  uint64
 	prs         map[uint64]*Progress
+	matchBuf    uint64Slice
 
 	state StateType
 
@@ -503,13 +504,23 @@ func (r *raft) bcastHeartbeatWithCtx(ctx []byte) {
 // the commit index changed (in which case the caller should call
 // r.bcastAppend).
 func (r *raft) maybeCommit() bool {
-	// TODO(bmizerany): optimize.. Currently naive
-	mis := make(uint64Slice, 0, len(r.prs))
+	if cap(r.matchBuf) < len(r.prs) {
+		r.matchBuf = make(uint64Slice, len(r.prs))
+	}
+	mis := r.matchBuf[:len(r.prs)]
+	idx := 0
+	sorted := true
 	for id := range r.prs {
-		mis = append(mis, r.prs[id].Match)
+		mis[idx] = r.prs[id].Match
+		if idx > 0 && mis[idx] < mis[idx-1] {
+			sorted = false
+		}
+		idx++
+	}
+	if !sorted {
+		sort.Sort(mis)
 	}
-	sort.Sort(sort.Reverse(mis))
-	mci := mis[r.quorum()-1]
+	mci := mis[len(mis)-r.quorum()]
 	return r.raftLog.maybeCommit(mci, r.Term)
 }