storage: avoid copying marshalled RaftCommand when encoding

Informs #36347.

This change avoids the unnecessary allocation and memory copy present in
Raft command encoding. This extra work is expensive for large commands
like `AddSSTable` requests. Even for smaller requests, this work was
still a serious problem because it took place under heavily contended
locks. For instance, the encoding in `defaultSubmitProposalLocked` took
place under the Replica mutex, which serializes all Raft proposals for
a Range. The other two locations fixed took place under the Raft mutex.
While less heavily contended, this was still slowing down the Raft
processing goroutine.

This is a less dramatic version of a change I've been working on. The
full change lifts the slice allocation and most of the RaftCommand proto
marshalling all the way out of `defaultSubmitProposalLocked` and out of
the `Replica.mu` critical section. This commit gets us part of the way
there sets the stage for the rest of the change.

Release note: None

pkg/storage/replica_raft.go

@@ -355,7 +355,9 @@ func (r *Replica) submitProposalLocked(p *ProposalData) error {
 }
 
 func defaultSubmitProposalLocked(r *Replica, p *ProposalData) error {
-	data, err := protoutil.Marshal(p.command)
+	cmdSize := p.command.Size()
+	data := make([]byte, raftCommandPrefixLen+cmdSize)
+	_, err := protoutil.MarshalToWithoutFuzzing(p.command, data[raftCommandPrefixLen:])
 	if err != nil {
 		return err
 	}
@@ -369,7 +371,7 @@ func defaultSubmitProposalLocked(r *Replica, p *ProposalData) error {
   RaftCommand.ReplicatedEvalResult:          %d
   RaftCommand.ReplicatedEvalResult.Delta:    %d
   RaftCommand.WriteBatch:                    %d
-`, p.Request.Summary(), len(data),
+`, p.Request.Summary(), cmdSize,
 			p.command.ProposerReplica.Size(),
 			p.command.ReplicatedEvalResult.Size(),
 			p.command.ReplicatedEvalResult.Delta.Size(),
@@ -383,7 +385,7 @@ func defaultSubmitProposalLocked(r *Replica, p *ProposalData) error {
 	// blips or worse, and it's good to be able to pick them from traces.
 	//
 	// TODO(tschottdorf): can we mark them so lightstep can group them?
-	if size := len(data); size > largeProposalEventThresholdBytes {
+	if size := cmdSize; size > largeProposalEventThresholdBytes {
 		log.Eventf(p.ctx, "proposal is large: %s", humanizeutil.IBytes(int64(size)))
 	}
 
@@ -406,7 +408,7 @@ func defaultSubmitProposalLocked(r *Replica, p *ProposalData) error {
 
 		confChangeCtx := ConfChangeContext{
 			CommandID: string(p.idKey),
-			Payload:   data,
+			Payload:   data[raftCommandPrefixLen:], // chop off prefix
 			Replica:   crt.Replica,
 		}
 		encodedCtx, err := protoutil.Marshal(&confChangeCtx)
@@ -427,24 +429,26 @@ func defaultSubmitProposalLocked(r *Replica, p *ProposalData) error {
 		})
 	}
 
-	return r.withRaftGroupLocked(true, func(raftGroup *raft.RawNode) (bool, error) {
-		encode := encodeRaftCommandV1
-		if p.command.ReplicatedEvalResult.AddSSTable != nil {
-			if p.command.ReplicatedEvalResult.AddSSTable.Data == nil {
-				return false, errors.New("cannot sideload empty SSTable")
-			}
-			encode = encodeRaftCommandV2
-			r.store.metrics.AddSSTableProposals.Inc(1)
-			log.Event(p.ctx, "sideloadable proposal detected")
-		}
+	if log.V(4) {
+		log.Infof(p.ctx, "proposing command %x: %s", p.idKey, p.Request.Summary())
+	}
 
-		if log.V(4) {
-			log.Infof(p.ctx, "proposing command %x: %s", p.idKey, p.Request.Summary())
+	encodingVersion := raftVersionStandard
+	if p.command.ReplicatedEvalResult.AddSSTable != nil {
+		if p.command.ReplicatedEvalResult.AddSSTable.Data == nil {
+			return errors.New("cannot sideload empty SSTable")
 		}
+		encodingVersion = raftVersionSideloaded
+		r.store.metrics.AddSSTableProposals.Inc(1)
+		log.Event(p.ctx, "sideloadable proposal detected")
+	}
+	encodeRaftCommandPrefix(data[:raftCommandPrefixLen], encodingVersion, p.idKey)
+
+	return r.withRaftGroupLocked(true, func(raftGroup *raft.RawNode) (bool, error) {
 		// We're proposing a command so there is no need to wake the leader if
 		// we're quiesced.
 		r.unquiesceLocked()
-		return false /* unquiesceAndWakeLeader */, raftGroup.Propose(encode(p.idKey, data))
+		return false /* unquiesceAndWakeLeader */, raftGroup.Propose(data)
 	})
 }
 

pkg/storage/replica_raft_quiesce.go

@@ -97,7 +97,8 @@ func (r *Replica) unquiesceAndWakeLeaderLocked() {
 		r.store.unquiescedReplicas.Unlock()
 		r.maybeCampaignOnWakeLocked(ctx)
 		// Propose an empty command which will wake the leader.
-		_ = r.mu.internalRaftGroup.Propose(encodeRaftCommandV1(makeIDKey(), nil))
+		data := encodeRaftCommand(raftVersionStandard, makeIDKey(), nil)
+		_ = r.mu.internalRaftGroup.Propose(data)
 	}
 }
 

pkg/storage/replica_raftstorage.go

@@ -1030,8 +1030,6 @@ type raftCommandEncodingVersion byte
 //
 // TODO(bdarnell): is this commandID still appropriate for our needs?
 const (
-	// The prescribed length for each command ID.
-	raftCommandIDLen = 8
 	// The initial Raft command version, used for all regular Raft traffic.
 	raftVersionStandard raftCommandEncodingVersion = 0
 	// A proposal containing an SSTable which preferably should be sideloaded
@@ -1040,6 +1038,10 @@ const (
 	// Raft log it necessary to inline the payload first as it has usually
 	// been sideloaded.
 	raftVersionSideloaded raftCommandEncodingVersion = 1
+	// The prescribed length for each command ID.
+	raftCommandIDLen = 8
+	// The prescribed length of each encoded command's prefix.
+	raftCommandPrefixLen = 1 + raftCommandIDLen
 	// The no-split bit is now unused, but we still apply the mask to the first
 	// byte of the command for backward compatibility.
 	//
@@ -1048,27 +1050,26 @@ const (
 	raftCommandNoSplitMask = raftCommandNoSplitBit - 1
 )
 
-func encodeRaftCommandV1(commandID storagebase.CmdIDKey, command []byte) []byte {
-	return encodeRaftCommand(raftVersionStandard, commandID, command)
-}
-
-func encodeRaftCommandV2(commandID storagebase.CmdIDKey, command []byte) []byte {
-	return encodeRaftCommand(raftVersionSideloaded, commandID, command)
-}
-
-// encode a command ID, an encoded storagebase.RaftCommand, and
-// whether the command contains a split.
 func encodeRaftCommand(
 	version raftCommandEncodingVersion, commandID storagebase.CmdIDKey, command []byte,
 ) []byte {
+	b := make([]byte, raftCommandPrefixLen+len(command))
+	encodeRaftCommandPrefix(b[:raftCommandPrefixLen], version, commandID)
+	copy(b[raftCommandPrefixLen:], command)
+	return b
+}
+
+func encodeRaftCommandPrefix(
+	b []byte, version raftCommandEncodingVersion, commandID storagebase.CmdIDKey,
+) {
 	if len(commandID) != raftCommandIDLen {
 		panic(fmt.Sprintf("invalid command ID length; %d != %d", len(commandID), raftCommandIDLen))
 	}
-	x := make([]byte, 1, 1+raftCommandIDLen+len(command))
-	x[0] = byte(version)
-	x = append(x, []byte(commandID)...)
-	x = append(x, command...)
-	return x
+	if len(b) != raftCommandPrefixLen {
+		panic(fmt.Sprintf("invalid command prefix length; %d != %d", len(b), raftCommandPrefixLen))
+	}
+	b[0] = byte(version)
+	copy(b[1:], []byte(commandID))
 }
 
 // DecodeRaftCommand splits a raftpb.Entry.Data into its commandID and

pkg/storage/replica_sideload.go

@@ -147,14 +147,15 @@ func maybeSideloadEntriesImpl(
 			strippedCmd.ReplicatedEvalResult.AddSSTable.Data = nil
 
 			{
-				var err error
-				data, err = protoutil.Marshal(&strippedCmd)
+				data = make([]byte, raftCommandPrefixLen+strippedCmd.Size())
+				encodeRaftCommandPrefix(data[:raftCommandPrefixLen], raftVersionSideloaded, cmdID)
+				_, err := protoutil.MarshalToWithoutFuzzing(&strippedCmd, data[raftCommandPrefixLen:])
 				if err != nil {
 					return nil, 0, errors.Wrap(err, "while marshaling stripped sideloaded command")
 				}
+				ent.Data = data
 			}
 
-			ent.Data = encodeRaftCommandV2(cmdID, data)
 			log.Eventf(ctx, "writing payload at index=%d term=%d", ent.Index, ent.Term)
 			if err = sideloaded.Put(ctx, ent.Index, ent.Term, dataToSideload); err != nil {
 				return nil, 0, err
@@ -226,11 +227,13 @@ func maybeInlineSideloadedRaftCommand(
 	}
 	command.ReplicatedEvalResult.AddSSTable.Data = sideloadedData
 	{
-		data, err := protoutil.Marshal(&command)
+		data := make([]byte, raftCommandPrefixLen+command.Size())
+		encodeRaftCommandPrefix(data[:raftCommandPrefixLen], raftVersionSideloaded, cmdID)
+		_, err := protoutil.MarshalToWithoutFuzzing(&command, data[raftCommandPrefixLen:])
 		if err != nil {
 			return nil, err
 		}
-		ent.Data = encodeRaftCommandV2(cmdID, data)
+		ent.Data = data
 	}
 	return &ent, nil
 }

pkg/storage/split_delay_helper.go

@@ -52,7 +52,8 @@ func (sdh *splitDelayHelper) ProposeEmptyCommand(ctx context.Context) {
 	_ = r.withRaftGroup(true /* campaignOnWake */, func(rawNode *raft.RawNode) (bool, error) {
 		// NB: intentionally ignore the error (which can be ErrProposalDropped
 		// when there's an SST inflight).
-		_ = rawNode.Propose(encodeRaftCommandV1(makeIDKey(), nil))
+		data := encodeRaftCommand(raftVersionStandard, makeIDKey(), nil)
+		_ = rawNode.Propose(data)
 		// NB: we need to unquiesce as the group might be quiesced.
 		return true /* unquiesceAndWakeLeader */, nil
 	})