*: allow fully concurrent large read

etcdserver/v3_server.go

@@ -86,8 +86,6 @@ type Authenticator interface {
 }
 
 func (s *EtcdServer) Range(ctx context.Context, r *pb.RangeRequest) (*pb.RangeResponse, error) {
-	defer warnOfExpensiveReadOnlyRangeRequest(s.getLogger(), time.Now(), r)
-
 	if !r.Serializable {
 		err := s.linearizableReadNotify(ctx)
 		if err != nil {

mvcc/backend/backend.go

@@ -49,6 +49,11 @@ var (
 )
 
 type Backend interface {
+	// ConcurrentReadTx returns a non-blocking read tx that is suitable for large reads.
+	// ConcurrentReadTx call itself will not return until the current BatchTx gets committed to
+	// ensure consistency.
+	ConcurrentReadTx() ReadTx
+
 	ReadTx() ReadTx
 	BatchTx() BatchTx
 
@@ -97,6 +102,8 @@ type backend struct {
 
 	readTx *readTx
 
+	concurrentReadTxCh chan chan ReadTx
+
 	stopc chan struct{}
 	donec chan struct{}
 
@@ -165,6 +172,8 @@ func newBackend(bcfg BackendConfig) *backend {
 			buckets: make(map[string]*bolt.Bucket),
 		},
 
+		concurrentReadTxCh: make(chan chan ReadTx),
+
 		stopc: make(chan struct{}),
 		donec: make(chan struct{}),
 
@@ -184,6 +193,12 @@ func (b *backend) BatchTx() BatchTx {
 
 func (b *backend) ReadTx() ReadTx { return b.readTx }
 
+func (b *backend) ConcurrentReadTx() ReadTx {
+	rch := make(chan ReadTx)
+	b.concurrentReadTxCh <- rch
+	return <-rch
+}
+
 // ForceCommit forces the current batching tx to commit.
 func (b *backend) ForceCommit() {
 	b.batchTx.Commit()
@@ -301,6 +316,25 @@ func (b *backend) run() {
 			b.batchTx.Commit()
 		}
 		t.Reset(b.batchInterval)
+		b.createConcurrentReadTxs()
+	}
+}
+
+func (b *backend) createConcurrentReadTxs() {
+	// do not allow too many concurrent read txs.
+	// TODO: improve this by having a global pending counter?
+	for i := 0; i < 100; i++ {
+		select {
+		case rch := <-b.concurrentReadTxCh:
+			rtx, err := b.db.Begin(false)
+			if err != nil {
+				plog.Fatalf("cannot begin read tx (%s)", err)
+			}
+			rch <- &concurrentReadTx{tx: rtx}
+		default:
+			// no more to create.
+			return
+		}
 	}
 }
 

mvcc/backend/backend_test.go

@@ -300,6 +300,81 @@ func TestBackendWritebackForEach(t *testing.T) {
 	}
 }
 
+// TestBackendConcurrentReadTx checks if the concurrent tx is created correctly.
+func TestBackendConcurrentReadTx(t *testing.T) {
+	b, tmpPath := NewTmpBackend(2*time.Second, 10000)
+	defer cleanup(b, tmpPath)
+
+	var rtx0 ReadTx
+	done := make(chan struct{})
+	go func() {
+		rtx0 = b.ConcurrentReadTx()
+		close(done)
+	}()
+
+	tx := b.BatchTx()
+	tx.Lock()
+	tx.UnsafeCreateBucket([]byte("key"))
+	for i := 0; i < 5; i++ {
+		k := []byte(fmt.Sprintf("%04d", i))
+		tx.UnsafePut([]byte("key"), k, []byte("bar"))
+	}
+	tx.Unlock()
+
+	select {
+	case <-done:
+		t.Fatal("concurrent read tx should block on the last batch tx!")
+	case <-time.After(time.Second):
+	}
+
+	select {
+	case <-done:
+	case <-time.After(4 * time.Second):
+		t.Fatal("commit the last batched tx should unblock concurrent tx!")
+	}
+
+	rtx0.Lock()
+	defer rtx0.Unlock()
+	ks, _ := rtx0.UnsafeRange([]byte("key"), []byte(fmt.Sprintf("%04d", 0)), []byte(fmt.Sprintf("%04d", 5)), 0)
+	if len(ks) != 5 {
+		t.Errorf("got %d keys, expect %d", len(ks), 5)
+	}
+
+	// test if we can create concurrent read while the previous read tx is still open
+	var rtx1 ReadTx
+	done = make(chan struct{})
+	go func() {
+		rtx1 = b.ConcurrentReadTx()
+		rtx1.Lock()
+		rtx1.UnsafeForEach([]byte(""), nil)
+		rtx1.Unlock()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(4 * time.Second):
+		t.Fatal("cannot create concurrent read")
+	}
+
+	done = make(chan struct{})
+	// test if we can create concurrent write while the previous read tx is still open
+	go func() {
+		tx := b.BatchTx()
+		tx.Lock()
+		for i := 0; i < 5; i++ {
+			k := []byte(fmt.Sprintf("%04d", i))
+			tx.UnsafePut([]byte("key"), k, []byte("bar"))
+		}
+		tx.Unlock()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(4 * time.Second):
+		t.Fatal("cannot create concurrent write")
+	}
+}
+
 func cleanup(b Backend, path string) {
 	b.Close()
 	os.Remove(path)

mvcc/backend/concurrent_read_tx.go

@@ -0,0 +1,38 @@
+// Copyright 2018 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	bolt "github.com/coreos/bbolt"
+)
+
+type concurrentReadTx struct {
+	tx *bolt.Tx
+}
+
+func (rt *concurrentReadTx) Lock()   {}
+func (rt *concurrentReadTx) Unlock() { rt.tx.Rollback() }
+
+func (rt *concurrentReadTx) UnsafeRange(bucketName, key, endKey []byte, limit int64) ([][]byte, [][]byte) {
+	bucket := rt.tx.Bucket(bucketName)
+	if bucket == nil {
+		plog.Fatalf("bucket %s does not exist", bucketName)
+	}
+	return unsafeRange(bucket.Cursor(), key, endKey, limit)
+}
+
+func (rt *concurrentReadTx) UnsafeForEach(bucketName []byte, visitor func(k, v []byte) error) error {
+	return unsafeForEach(rt.tx, bucketName, visitor)
+}

mvcc/index.go

@@ -91,10 +91,11 @@ func (ti *treeIndex) keyIndex(keyi *keyIndex) *keyIndex {
 func (ti *treeIndex) visit(key, end []byte, f func(ki *keyIndex)) {
 	keyi, endi := &keyIndex{key: key}, &keyIndex{key: end}
 
-	ti.RLock()
-	defer ti.RUnlock()
+	ti.Lock()
+	clone := ti.tree.Clone()
+	ti.Unlock()
 
-	ti.tree.AscendGreaterOrEqual(keyi, func(item btree.Item) bool {
+	clone.AscendGreaterOrEqual(keyi, func(item btree.Item) bool {
 		if len(endi.key) > 0 && !item.Less(endi) {
 			return false
 		}

mvcc/kvstore_test.go

@@ -640,7 +640,7 @@ func TestTxnBlockBackendForceCommit(t *testing.T) {
 	s := NewStore(zap.NewExample(), b, &lease.FakeLessor{}, nil)
 	defer os.Remove(tmpPath)
 
-	txn := s.Read()
+	txn := s.Write()
 
 	done := make(chan struct{})
 	go func() {
@@ -742,6 +742,7 @@ type fakeBackend struct {
 
 func (b *fakeBackend) BatchTx() backend.BatchTx                                    { return b.tx }
 func (b *fakeBackend) ReadTx() backend.ReadTx                                      { return b.tx }
+func (b *fakeBackend) ConcurrentReadTx() backend.ReadTx                            { return b.tx }
 func (b *fakeBackend) Hash(ignores map[backend.IgnoreKey]struct{}) (uint32, error) { return 0, nil }
 func (b *fakeBackend) Size() int64                                                 { return 0 }
 func (b *fakeBackend) SizeInUse() int64                                            { return 0 }

mvcc/kvstore_txn.go

@@ -21,9 +21,21 @@ import (
 	"go.uber.org/zap"
 )
 
+const (
+	expensiveReadLimit = 1000
+	readonly           = true
+	readwrite          = false
+)
+
 type storeTxnRead struct {
-	s  *store
-	tx backend.ReadTx
+	s        *store
+	tx       backend.ReadTx
+	txlocked bool
+
+	// for creating concurrent read tx when the read is expensive.
+	b backend.Backend
+	// is the transcation readonly?
+	ro bool
 
 	firstRev int64
 	rev      int64
@@ -33,10 +45,15 @@ func (s *store) Read() TxnRead {
 	s.mu.RLock()
 	tx := s.b.ReadTx()
 	s.revMu.RLock()
-	tx.Lock()
 	firstRev, rev := s.compactMainRev, s.currentRev
 	s.revMu.RUnlock()
-	return newMetricsTxnRead(&storeTxnRead{s, tx, firstRev, rev})
+	return newMetricsTxnRead(&storeTxnRead{
+		s:        s,
+		tx:       tx,
+		b:        s.b,
+		ro:       readonly,
+		firstRev: firstRev,
+		rev:      rev})
 }
 
 func (tr *storeTxnRead) FirstRev() int64 { return tr.firstRev }
@@ -47,7 +64,9 @@ func (tr *storeTxnRead) Range(key, end []byte, ro RangeOptions) (r *RangeResult,
 }
 
 func (tr *storeTxnRead) End() {
-	tr.tx.Unlock()
+	if tr.txlocked {
+		tr.tx.Unlock()
+	}
 	tr.s.mu.RUnlock()
 }
 
@@ -64,10 +83,15 @@ func (s *store) Write() TxnWrite {
 	tx := s.b.BatchTx()
 	tx.Lock()
 	tw := &storeTxnWrite{
-		storeTxnRead: storeTxnRead{s, tx, 0, 0},
-		tx:           tx,
-		beginRev:     s.currentRev,
-		changes:      make([]mvccpb.KeyValue, 0, 4),
+		storeTxnRead: storeTxnRead{
+			s:        s,
+			txlocked: true,
+			tx:       tx,
+			ro:       readwrite,
+		},
+		tx:       tx,
+		beginRev: s.currentRev,
+		changes:  make([]mvccpb.KeyValue, 0, 4),
 	}
 	return newMetricsTxnWrite(tw)
 }
@@ -134,6 +158,15 @@ func (tr *storeTxnRead) rangeKeys(key, end []byte, curRev int64, ro RangeOptions
 		limit = len(revpairs)
 	}
 
+	if limit > expensiveReadLimit && !tr.txlocked && tr.ro { // first expensive read in a read only transcation
+		// too many keys to range. upgrade the read transcation to concurrent read tx.
+		tr.tx = tr.b.ConcurrentReadTx()
+	}
+	if !tr.txlocked {
+		tr.tx.Lock()
+		tr.txlocked = true
+	}
+
 	kvs := make([]mvccpb.KeyValue, limit)
 	revBytes := newRevBytes()
 	for i, revpair := range revpairs[:len(kvs)] {