Merge pull request #197 from Shopify/freelist_stats

Add freelist stats to db stats

db.go

@@ -556,8 +556,12 @@ func (db *DB) allocate(count int) (*page, error) {
 
 // Stats represents statistics about the database.
 type Stats struct {
+	// Freelist stats
+	FreelistN     int // total number of pages on the freelist
+	FreelistAlloc int // total bytes used by the freelist and the pages on it
+
 	// Transaction stats
-	TxN     int // total number of completed read transactions
+	TxN     int // total number of started read transactions
 	OpenTxN int // number of currently open read transactions
 
 	TxStats TxStats // global, ongoing stats.

db_test.go

@@ -253,7 +253,8 @@ func TestDB_Stats(t *testing.T) {
 			return err
 		})
 		stats := db.Stats()
-		assert.Equal(t, 2, stats.TxStats.PageCount)
+		assert.Equal(t, 2, stats.TxStats.PageCount, "PageCount")
+		assert.Equal(t, 2, stats.FreelistN, "FreelistN %d", db.freelist.count())
 	})
 }
 

freelist.go

@@ -22,7 +22,16 @@ type freelist struct {
 
 // size returns the size of the page after serialization.
 func (f *freelist) size() int {
-	return pageHeaderSize + (int(unsafe.Sizeof(pgid(0))) * len(f.all()))
+	return pageHeaderSize + (int(unsafe.Sizeof(pgid(0))) * f.count())
+}
+
+// count returns count of pages on the freelist
+func (f *freelist) count() int {
+	var count = len(f.ids)
+	for _, list := range f.pending {
+		count += len(list)
+	}
+	return count
 }
 
 // all returns a list of all free ids and all pending ids in one sorted list.

tx.go

@@ -232,11 +232,17 @@ func (tx *Tx) rollback() {
 
 func (tx *Tx) close() {
 	if tx.writable {
+		// Grab freelist stats.
+		var freelistN = tx.db.freelist.count()
+		var freelistAlloc = tx.db.freelist.size()
+
 		// Remove writer lock.
 		tx.db.rwlock.Unlock()
 
 		// Merge statistics.
 		tx.db.statlock.Lock()
+		tx.db.stats.FreelistN = freelistN
+		tx.db.stats.FreelistAlloc = freelistAlloc
 		tx.db.stats.TxStats.add(&tx.stats)
 		tx.db.statlock.Unlock()
 	} else {