mvcc: allow large concurrent reads under light write workload

internal/mvcc/backend/backend.go

@@ -271,7 +271,12 @@ func (b *backend) run() {
 			b.batchTx.CommitAndStop()
 			return
 		}
-		b.batchTx.Commit()
+		b.batchTx.Lock()
+		pending := b.batchTx.pending
+		b.batchTx.Unlock()
+		if pending != 0 {
+			b.batchTx.Commit()
+		}
 		t.Reset(b.batchInterval)
 	}
 }
@@ -288,15 +293,7 @@ func (b *backend) Commits() int64 {
 }
 
 func (b *backend) Defrag() error {
-	err := b.defrag()
-	if err != nil {
-		return err
-	}
-
-	// commit to update metadata like db.size
-	b.batchTx.Commit()
-
-	return nil
+	return b.defrag()
 }
 
 func (b *backend) defrag() error {
@@ -310,11 +307,7 @@ func (b *backend) defrag() error {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
-	// block concurrent read requests while resetting tx
-	b.readTx.mu.Lock()
-	defer b.readTx.mu.Unlock()
-
-	b.batchTx.unsafeCommit(true)
+	b.batchTx.commit(true)
 	b.batchTx.tx = nil
 
 	tmpdb, err := bolt.Open(b.db.Path()+".tmp", 0600, boltOpenOptions)

internal/mvcc/backend/batch_tx.go

@@ -98,6 +98,7 @@ func unsafeRange(c *bolt.Cursor, key, endKey []byte, limit int64) (keys [][]byte
 		isMatch = func(b []byte) bool { return bytes.Equal(b, key) }
 		limit = 1
 	}
+
 	for ck, cv := c.Seek(key); ck != nil && isMatch(ck); ck, cv = c.Next() {
 		vs = append(vs, cv)
 		keys = append(keys, ck)
@@ -158,17 +159,6 @@ func (t *batchTx) commit(stop bool) {
 	// commit the last tx
 	if t.tx != nil {
 		if t.pending == 0 && !stop {
-			t.backend.mu.RLock()
-			defer t.backend.mu.RUnlock()
-
-			// t.tx.DB()==nil if 'CommitAndStop' calls 'batchTx.commit(true)',
-			// which initializes *bolt.Tx.db and *bolt.Tx.meta as nil; panics t.tx.Size().
-			// Server must make sure 'batchTx.commit(false)' does not follow
-			// 'batchTx.commit(true)' (e.g. stopping backend, and inflight Hash call).
-			size := t.tx.Size()
-			db := t.tx.DB()
-			atomic.StoreInt64(&t.backend.size, size)
-			atomic.StoreInt64(&t.backend.sizeInUse, size-(int64(db.Stats().FreePageN)*int64(db.Info().PageSize)))
 			return
 		}