memdb: use btree for storage

common_test.go

@@ -80,7 +80,7 @@ func newTempDB(t *testing.T, backend BackendType) (db DB, dbDir string) {
 // NOTE: not actually goroutine safe.
 // If you want something goroutine safe, maybe you just want a MemDB.
 type mockDB struct {
-	mtx   sync.RWMutex
+	mtx   sync.Mutex
 	calls map[string]int
 }
 
@@ -90,7 +90,7 @@ func newMockDB() *mockDB {
 	}
 }
 
-func (mdb *mockDB) Mutex() *sync.RWMutex {
+func (mdb *mockDB) Mutex() *sync.Mutex {
 	return &(mdb.mtx)
 }
 
@@ -213,7 +213,8 @@ func benchmarkRangeScans(b *testing.B, db DB, dbSize int64) {
 		bytes := int642Bytes(i)
 		err := db.Set(bytes, bytes)
 		if err != nil {
-			// for performance
+			// require.NoError() is very expensive (according to profiler), so we do a cheap if as
+			// well since this is a tight loop.
 			require.NoError(b, err)
 		}
 	}
@@ -257,6 +258,8 @@ func benchmarkRandomReadsWrites(b *testing.B, db DB) {
 			//fmt.Printf("Set %X -> %X\n", idxBytes, valBytes)
 			err := db.Set(idxBytes, valBytes)
 			if err != nil {
+				// require.NoError() is very expensive (according to profiler), so we do a cheap if
+				// as well since this is a tight loop.
 				require.NoError(b, err)
 			}
 		}
@@ -268,6 +271,8 @@ func benchmarkRandomReadsWrites(b *testing.B, db DB) {
 			idxBytes := int642Bytes(idx)
 			valBytes, err := db.Get(idxBytes)
 			if err != nil {
+				// require.NoError() is very expensive (according to profiler), so we do a cheap if
+				// as well since this is a tight loop.
 				require.NoError(b, err)
 			}
 			//fmt.Printf("Get %X -> %X\n", idxBytes, valBytes)

mem_batch.go

@@ -3,7 +3,7 @@ package db
 import "sync"
 
 type atomicSetDeleter interface {
-	Mutex() *sync.RWMutex
+	Mutex() *sync.Mutex
 	SetNoLock(key, value []byte)
 	SetNoLockSync(key, value []byte)
 	DeleteNoLock(key []byte)

mem_db.go

@@ -9,6 +9,17 @@ import (
 	"github.com/google/btree"
 )
 
+const (
+	// The approximate number of items and children per B-tree node. Tuned with benchmarks.
+	bTreeDegree = 32
+
+	// Size of the channel buffer between traversal goroutine and iterator. Using an unbuffered
+	// channel causes two context switches per item sent, while buffering allows more work per
+	// context switch. Tuned with benchmarks.
+	chBufferSize = 64
+)
+
+// item is a btree.Item with byte slices as keys and values
 type item struct {
 	key   []byte
 	value []byte
@@ -40,26 +51,26 @@ func init() {
 var _ DB = (*MemDB)(nil)
 
 type MemDB struct {
-	mtx   sync.RWMutex
+	mtx   sync.Mutex
 	btree *btree.BTree
 }
 
 func NewMemDB() *MemDB {
 	database := &MemDB{
-		btree: btree.New(32),
+		btree: btree.New(bTreeDegree),
 	}
 	return database
 }
 
 // Implements atomicSetDeleter.
-func (db *MemDB) Mutex() *sync.RWMutex {
+func (db *MemDB) Mutex() *sync.Mutex {
 	return &(db.mtx)
 }
 
 // Implements DB.
 func (db *MemDB) Get(key []byte) ([]byte, error) {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
+	db.mtx.Lock()
+	defer db.mtx.Unlock()
 	key = nonNilBytes(key)
 
 	i := db.btree.Get(newKey(key))
@@ -71,8 +82,8 @@ func (db *MemDB) Get(key []byte) ([]byte, error) {
 
 // Implements DB.
 func (db *MemDB) Has(key []byte) (bool, error) {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
+	db.mtx.Lock()
+	defer db.mtx.Unlock()
 	key = nonNilBytes(key)
 
 	return db.btree.Has(newKey(key)), nil
@@ -164,8 +175,8 @@ func (db *MemDB) Print() error {
 
 // Implements DB.
 func (db *MemDB) Stats() map[string]string {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
+	db.mtx.Lock()
+	defer db.mtx.Unlock()
 
 	stats := make(map[string]string)
 	stats["database.type"] = "memDB"
@@ -183,16 +194,16 @@ func (db *MemDB) NewBatch() Batch {
 
 // Implements DB.
 func (db *MemDB) Iterator(start, end []byte) (Iterator, error) {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
+	db.mtx.Lock()
+	defer db.mtx.Unlock()
 
 	return newMemDBIterator(db.btree, start, end, false), nil
 }
 
 // Implements DB.
 func (db *MemDB) ReverseIterator(start, end []byte) (Iterator, error) {
-	db.mtx.RLock()
-	defer db.mtx.RUnlock()
+	db.mtx.Lock()
+	defer db.mtx.Unlock()
 
 	return newMemDBIterator(db.btree, start, end, true), nil
 }
@@ -209,7 +220,7 @@ var _ Iterator = (*memDBIterator)(nil)
 
 func newMemDBIterator(bt *btree.BTree, start []byte, end []byte, reverse bool) *memDBIterator {
 	ctx, cancel := context.WithCancel(context.Background())
-	ch := make(chan *item, 64)
+	ch := make(chan *item, chBufferSize)
 	iter := &memDBIterator{
 		ch:     ch,
 		cancel: cancel,