Perform incremental rollups instead of rollups at snapshot (#4410)

Perform incremental rollups instead of rollups at snapshot

Author: parasssh

posting/mvcc.go

@@ -21,20 +21,105 @@ import (
 	"encoding/hex"
 	"math"
 	"strconv"
+	"sync"
 	"sync/atomic"
+	"time"
 
 	"github.com/dgraph-io/badger/v2"
+	bpb "github.com/dgraph-io/badger/v2/pb"
 	"github.com/dgraph-io/dgo/v2/protos/api"
 	"github.com/dgraph-io/dgraph/protos/pb"
 	"github.com/dgraph-io/dgraph/x"
+	"github.com/dgraph-io/ristretto/z"
+	"github.com/golang/glog"
 	"github.com/pkg/errors"
 )
 
+// incrRollupi is used to batch keys for rollup incrementally.
+type incrRollupi struct {
+	// keysCh is populated with batch of 64 keys that needs to be rolled up during reads
+	keysCh chan *[][]byte
+	// keysPool is sync.Pool to share the batched keys to rollup.
+	keysPool *sync.Pool
+}
+
 var (
 	// ErrTsTooOld is returned when a transaction is too old to be applied.
 	ErrTsTooOld = errors.Errorf("Transaction is too old")
+
+	// IncrRollup is used to batch keys for rollup incrementally.
+	IncrRollup = &incrRollupi{
+		keysCh: make(chan *[][]byte),
+		keysPool: &sync.Pool{
+			New: func() interface{} {
+				return new([][]byte)
+			},
+		},
+	}
 )
 
+// rollUpKey takes the given key's posting lists, rolls it up and writes back to badger
+func (ir *incrRollupi) rollUpKey(writer *TxnWriter, key []byte) error {
+	l, err := GetNoStore(key)
+	if err != nil {
+		return err
+	}
+
+	kvs, err := l.Rollup()
+	if err != nil {
+		return err
+	}
+
+	return writer.Write(&bpb.KVList{Kv: kvs})
+}
+
+func (ir *incrRollupi) addKeyToBatch(key []byte) {
+	batch := ir.keysPool.Get().(*[][]byte)
+	*batch = append(*batch, key)
+	if len(*batch) < 64 {
+		ir.keysPool.Put(batch)
+		return
+	}
+
+	select {
+	case ir.keysCh <- batch:
+	default:
+		// Drop keys and build the batch again. Lossy behavior.
+		*batch = (*batch)[:0]
+		ir.keysPool.Put(batch)
+	}
+}
+
+// Process will rollup batches of 64 keys in a go routine.
+func (ir *incrRollupi) Process() {
+	m := make(map[uint64]int64) // map hash(key) to ts. hash(key) to limit the size of the map.
+	limiter := time.NewTicker(100 * time.Millisecond)
+	writer := NewTxnWriter(pstore)
+
+	for batch := range ir.keysCh {
+		currTs := time.Now().Unix()
+		for _, key := range *batch {
+			hash := z.MemHash(key)
+			if elem, ok := m[hash]; !ok || (currTs-elem >= 10) {
+				// Key not present or Key present but last roll up was more than 10 sec ago.
+				// Add/Update map and rollup.
+				m[hash] = currTs
+				if err := ir.rollUpKey(writer, key); err != nil {
+					glog.Warningf("Error %v rolling up key %v\n", err, key)
+					continue
+				}
+			}
+		}
+		// clear the batch and put it back in Sync keysPool
+		*batch = (*batch)[:0]
+		ir.keysPool.Put(batch)
+
+		// throttle to 1 batch = 64 rollups per 100 ms.
+		<-limiter.C
+	}
+	// keysCh is closed. This should never happen.
+}
+
 // ShouldAbort returns whether the transaction should be aborted.
 func (txn *Txn) ShouldAbort() bool {
 	if txn == nil {
@@ -144,6 +229,8 @@ func ReadPostingList(key []byte, it *badger.Iterator) (*List, error) {
 	l := new(List)
 	l.key = key
 	l.plist = new(pb.PostingList)
+	const maxDeltaCount = 2
+	deltaCount := 0
 
 	// Iterates from highest Ts to lowest Ts
 	for it.Valid() {
@@ -188,6 +275,7 @@ func ReadPostingList(key []byte, it *badger.Iterator) (*List, error) {
 			if err != nil {
 				return nil, err
 			}
+			deltaCount++
 		case BitSchemaPosting:
 			return nil, errors.Errorf(
 				"Trying to read schema in ReadPostingList for key: %s", hex.Dump(key))
@@ -200,6 +288,11 @@ func ReadPostingList(key []byte, it *badger.Iterator) (*List, error) {
 		}
 		it.Next()
 	}
+
+	if deltaCount >= maxDeltaCount {
+		IncrRollup.addKeyToBatch(key)
+	}
+
 	return l, nil
 }
 

worker/draft.go

@@ -420,13 +420,13 @@ func (n *node) processRollups() {
 			if readTs <= last {
 				break // Break out of the select case.
 			}
-			if err := n.rollupLists(readTs); err != nil {
+			if err := n.calcTabletSizes(readTs); err != nil {
 				// If we encounter error here, we don't need to do anything about
 				// it. Just let the user know.
 				glog.Errorf("Error while rolling up lists at %d: %v\n", readTs, err)
 			} else {
 				last = readTs // Update last only if we succeeded.
-				glog.Infof("List rollup at Ts %d: OK.\n", readTs)
+				glog.Infof("Last rollup at Ts %d: OK.\n", readTs)
 			}
 		}
 	}
@@ -1008,20 +1008,19 @@ func listWrap(kv *bpb.KV) *bpb.KVList {
 	return &bpb.KVList{Kv: []*bpb.KV{kv}}
 }
 
-// rollupLists would consolidate all the deltas that constitute one posting
-// list, and write back a complete posting list.
-func (n *node) rollupLists(readTs uint64) error {
-	writer := posting.NewTxnWriter(pstore)
+// calcTabletSizes updates the tablet sizes for the keys.
+func (n *node) calcTabletSizes(readTs uint64) error {
+	// We can now discard all invalid versions of keys below this ts.
+	pstore.SetDiscardTs(readTs)
+
+	if !n.AmLeader() {
+		// Only leader needs to calculate the tablet sizes.
+		return nil
+	}
 
-	// We're doing rollups. We should use this opportunity to calculate the tablet sizes.
-	amLeader := n.AmLeader()
 	m := new(sync.Map)
 
 	addTo := func(key []byte, delta int64) {
-		if !amLeader {
-			// Only leader needs to calculate the tablet sizes.
-			return
-		}
 		pk, err := x.Parse(key)
 
 		// Type keys should not count for tablet size calculations.
@@ -1043,7 +1042,7 @@ func (n *node) rollupLists(readTs uint64) error {
 	}
 
 	stream := pstore.NewStreamAt(readTs)
-	stream.LogPrefix = "Rolling up"
+	stream.LogPrefix = "Tablet Size Calculation"
 	stream.ChooseKey = func(item *badger.Item) bool {
 		switch item.UserMeta() {
 		case posting.BitSchemaPosting, posting.BitCompletePosting, posting.BitEmptyPosting:
@@ -1052,75 +1051,53 @@ func (n *node) rollupLists(readTs uint64) error {
 		case x.ByteUnused:
 			return false
 		default:
-			return true
+			// not doing rollups anymore.
+			return false
 		}
 	}
-	var numKeys uint64
-	stream.KeyToList = func(key []byte, itr *badger.Iterator) (*bpb.KVList, error) {
-		l, err := posting.ReadPostingList(key, itr)
-		if err != nil {
-			return nil, err
-		}
-		atomic.AddUint64(&numKeys, 1)
-		kvs, err := l.Rollup()
-
-		// If there are multiple keys, the posting list was split into multiple
-		// parts. The key of the first part is the right key to use for tablet
-		// size calculations.
-		for _, kv := range kvs {
-			addTo(kvs[0].Key, int64(kv.Size()))
-		}
 
-		return &bpb.KVList{Kv: kvs}, err
+	stream.KeyToList = func(key []byte, itr *badger.Iterator) (*bpb.KVList, error) {
+		return nil, nil // no-op
 	}
 	stream.Send = func(list *bpb.KVList) error {
-		return writer.Write(list)
+		return nil
 	}
 	if err := stream.Orchestrate(context.Background()); err != nil {
 		return err
 	}
-	if err := writer.Flush(); err != nil {
-		return err
-	}
-	// For all the keys, let's see if they're in the LRU cache. If so, we can roll them up.
-	glog.Infof("Rolled up %d keys. Done", atomic.LoadUint64(&numKeys))
 
-	// We can now discard all invalid versions of keys below this ts.
-	pstore.SetDiscardTs(readTs)
-
-	if amLeader {
-		// Only leader sends the tablet size updates to Zero. No one else does.
-		// doSendMembership is also being concurrently called from another goroutine.
-		go func() {
-			tablets := make(map[string]*pb.Tablet)
-			var total int64
-			m.Range(func(key, val interface{}) bool {
-				pred := key.(string)
-				size := atomic.LoadInt64(val.(*int64))
-				tablets[pred] = &pb.Tablet{
-					GroupId:   n.gid,
-					Predicate: pred,
-					Space:     size,
-				}
-				total += size
-				return true
-			})
-			// Update Zero with the tablet sizes. If Zero sees a tablet which does not belong to
-			// this group, it would send instruction to delete that tablet. There's an edge case
-			// here if the followers are still running Rollup, and happen to read a key before and
-			// write after the tablet deletion, causing that tablet key to resurface. Then, only the
-			// follower would have that key, not the leader.
-			// However, if the follower then becomes the leader, we'd be able to get rid of that
-			// key then. Alternatively, we could look into cancelling the Rollup if we see a
-			// predicate deletion.
-			if err := groups().doSendMembership(tablets); err != nil {
-				glog.Warningf("While sending membership to Zero. Error: %v", err)
-			} else {
-				glog.V(2).Infof("Sent tablet size update to Zero. Total size: %s",
-					humanize.Bytes(uint64(total)))
+	// Only leader sends the tablet size updates to Zero. No one else does.
+	// doSendMembership is also being concurrently called from another goroutine.
+	go func() {
+		tablets := make(map[string]*pb.Tablet)
+		var total int64
+		m.Range(func(key, val interface{}) bool {
+			pred := key.(string)
+			size := atomic.LoadInt64(val.(*int64))
+			tablets[pred] = &pb.Tablet{
+				GroupId:   n.gid,
+				Predicate: pred,
+				Space:     size,
 			}
-		}()
-	}
+			total += size
+			return true
+		})
+		// Update Zero with the tablet sizes. If Zero sees a tablet which does not belong to
+		// this group, it would send instruction to delete that tablet. There's an edge case
+		// here if the followers are still running Rollup, and happen to read a key before and
+		// write after the tablet deletion, causing that tablet key to resurface. Then, only the
+		// follower would have that key, not the leader.
+		// However, if the follower then becomes the leader, we'd be able to get rid of that
+		// key then. Alternatively, we could look into cancelling the Rollup if we see a
+		// predicate deletion.
+		if err := groups().doSendMembership(tablets); err != nil {
+			glog.Warningf("While sending membership to Zero. Error: %v", err)
+		} else {
+			glog.V(2).Infof("Sent tablet size update to Zero. Total size: %s",
+				humanize.Bytes(uint64(total)))
+		}
+	}()
+
 	return nil
 }
 
@@ -1455,6 +1432,7 @@ func (n *node) InitAndStartNode() {
 	go n.processRollups()
 	go n.processApplyCh()
 	go n.BatchAndSendMessages()
+	go posting.IncrRollup.Process()
 	go n.Run()
 }