perf(metarepos): add a pool for []*mrpb.Report

This change adds the pool for []*mrpb.Report, which is reportQueuePool.

```
BenchmarkReportQueuePool/WithoutPool-16         	  763526	      1573 ns/op	    8192 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithoutPool-16         	  745612	      1574 ns/op	    8192 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithoutPool-16         	  738848	      1569 ns/op	    8192 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithoutPool-16         	  765339	      1563 ns/op	    8192 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithoutPool-16         	  739354	      1567 ns/op	    8192 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithPool-16            	 4445330	       270.2 ns/op	      24 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithPool-16            	 4342182	       271.9 ns/op	      24 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithPool-16            	 4517366	       267.3 ns/op	      24 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithPool-16            	 4440441	       263.5 ns/op	      24 B/op	       1 allocs/op
BenchmarkReportQueuePool/WithPool-16            	 4666981	       263.1 ns/op	      24 B/op	       1 allocs/op

```

internal/metarepos/raft_metadata_repository.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/gogo/status"
 	"go.etcd.io/etcd/pkg/fileutil"
 	"go.etcd.io/etcd/raft"
 	"go.etcd.io/etcd/raft/raftpb"
@@ -21,8 +22,6 @@ import (
 	"google.golang.org/grpc/health"
 	"google.golang.org/grpc/health/grpc_health_v1"
 
-	"github.com/gogo/status"
-
 	"github.com/kakao/varlog/internal/reportcommitter"
 	"github.com/kakao/varlog/pkg/types"
 	"github.com/kakao/varlog/pkg/util/container/set"
@@ -111,7 +110,7 @@ func NewRaftMetadataRepository(opts ...Option) *RaftMetadataRepository {
 		commitC:       make(chan *committedEntry, 4096),
 		rnConfChangeC: make(chan raftpb.ConfChange, 1),
 		rnProposeC:    make(chan []byte),
-		reportQueue:   make([]*mrpb.Report, 0, 1024),
+		reportQueue:   mrpb.NewReportQueue(),
 		runner:        runner.New("mr", cfg.logger),
 		sw:            stopwaiter.New(),
 		tmStub:        tmStub,
@@ -351,7 +350,7 @@ func (mr *RaftMetadataRepository) processReport(ctx context.Context) {
 			if num > 0 {
 				reports = mrpb.NewReports(mr.nodeID, time.Now())
 				reports.Reports = mr.reportQueue
-				mr.reportQueue = make([]*mrpb.Report, 0, 1024)
+				mr.reportQueue = mrpb.NewReportQueue()
 			}
 			mr.muReportQueue.Unlock()
 

proto/mrpb/raft_entry.go

@@ -22,7 +22,34 @@ func NewReports(nodeID types.NodeID, ts time.Time) *Reports {
 
 func (rs *Reports) Release() {
 	if rs != nil {
+		rq := (ReportQueue)(rs.Reports)
+		rq.Release()
 		*rs = Reports{}
 		reportsPool.Put(rs)
 	}
 }
+
+const (
+	reportQueueSize = 1024
+)
+
+type ReportQueue []*Report
+
+var reportQueuePool = sync.Pool{
+	New: func() any {
+		q := make(ReportQueue, 0, reportQueueSize)
+		return &q
+	},
+}
+
+func NewReportQueue() ReportQueue {
+	rq := reportQueuePool.Get().(*ReportQueue)
+	return *rq
+}
+
+func (rq *ReportQueue) Release() {
+	if rq != nil {
+		*rq = (*rq)[0:0:reportQueueSize]
+		reportQueuePool.Put(rq)
+	}
+}

proto/mrpb/raft_entry_test.go

@@ -0,0 +1,84 @@
+package mrpb
+
+import (
+	"math/rand"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/kakao/varlog/pkg/types"
+)
+
+func TestReports(t *testing.T) {
+	const nid = types.NodeID(1)
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	for i := 0; i < 1e4; i++ {
+		reports := NewReports(nid, time.Now())
+		require.Empty(t, reports.Reports)
+
+		queue := NewReportQueue()
+		require.Empty(t, queue)
+		require.Equal(t, reportQueueSize, cap(queue))
+		numReports := rng.Intn(reportQueueSize*2) + 1
+		for j := 0; j < numReports; j++ {
+			queue = append(queue, &Report{})
+		}
+		require.Len(t, queue, numReports)
+		reports.Reports = queue
+
+		reports.Release()
+	}
+}
+
+func TestReportQueuePool(t *testing.T) {
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	for i := 0; i < 1e4; i++ {
+		queue := NewReportQueue()
+		require.Empty(t, queue)
+		require.Equal(t, reportQueueSize, cap(queue))
+
+		numReports := rng.Intn(reportQueueSize*2) + 1
+		for j := 0; j < numReports; j++ {
+			queue = append(queue, &Report{})
+		}
+		require.Len(t, queue, numReports)
+
+		queue.Release()
+	}
+}
+
+func BenchmarkReportQueuePool(b *testing.B) {
+	const numReports = 128
+	predefinedReports := make([]*Report, numReports)
+	for i := 0; i < numReports; i++ {
+		predefinedReports[i] = &Report{}
+	}
+	reports := &Reports{}
+
+	reports.Reports = make([]*Report, 0, reportQueueSize)
+	b.ResetTimer()
+	b.Run("WithoutPool", func(b *testing.B) {
+		for n := 0; n < b.N; n++ {
+			for j := 0; j < numReports; j++ {
+				reports.Reports = append(reports.Reports, predefinedReports[j])
+			}
+			reports.Reports = make([]*Report, 0, reportQueueSize)
+		}
+	})
+
+	reports.Reports = NewReportQueue()
+	b.ResetTimer()
+	b.Run("WithPool", func(b *testing.B) {
+		for n := 0; n < b.N; n++ {
+			for j := 0; j < numReports; j++ {
+				reports.Reports = append(reports.Reports, predefinedReports[j])
+			}
+			rq := (ReportQueue)(reports.Reports)
+			rq.Release()
+			reports.Reports = NewReportQueue()
+		}
+	})
+}