Use sync.Pool for small pages in db.allocate

Benchmark results:
benchmark                          old ns/op     new ns/op     delta
BenchmarkDBBatchAutomatic-4        2552625       2485200       -2.64%
BenchmarkDBBatchSingle-4           59632698      50757603      -14.88%
BenchmarkDBBatchManual10x100-4     2564789       2452735       -4.37%

benchmark                          old allocs     new allocs     delta
BenchmarkDBBatchAutomatic-4        10199          10202          +0.03%
BenchmarkDBBatchSingle-4           56642          56653          +0.02%
BenchmarkDBBatchManual10x100-4     5986           5995           +0.15%

benchmark                          old bytes     new bytes     delta
BenchmarkDBBatchAutomatic-4        433587        382462        -11.79%
BenchmarkDBBatchSingle-4           32504533      16308931      -49.83%
BenchmarkDBBatchManual10x100-4     1362370       881765        -35.28%

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

db.go

@@ -36,6 +36,9 @@ const (
 	DefaultAllocSize         = 16 * 1024 * 1024
 )
 
+// default page size for db is set to the OS page size.
+var defaultPageSize = os.Getpagesize()
+
 // DB represents a collection of buckets persisted to a file on disk.
 // All data access is performed through transactions which can be obtained through the DB.
 // All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.
@@ -320,7 +323,7 @@ func (db *DB) mmapSize(size int) (int, error) {
 // init creates a new database file and initializes its meta pages.
 func (db *DB) init() error {
 	// Set the page size to the OS page size.
-	db.pageSize = os.Getpagesize()
+	db.pageSize = defaultPageSize
 
 	// Create two meta pages on a buffer.
 	buf := make([]byte, db.pageSize*4)
@@ -779,10 +782,21 @@ func (db *DB) meta() *meta {
 	return db.meta1
 }
 
+var pagePool = sync.Pool{
+	New: func() interface{} {
+		return make([]byte, defaultPageSize)
+	},
+}
+
 // allocate returns a contiguous block of memory starting at a given page.
 func (db *DB) allocate(count int) (*page, error) {
 	// Allocate a temporary buffer for the page.
-	buf := make([]byte, count*db.pageSize)
+	var buf []byte
+	if count == 1 && db.pageSize == defaultPageSize {
+		buf = pagePool.Get().([]byte)
+	} else {
+		buf = make([]byte, count*db.pageSize)
+	}
 	p := (*page)(unsafe.Pointer(&buf[0]))
 	p.overflow = uint32(count - 1)
 

tx.go

@@ -441,6 +441,8 @@ func (tx *Tx) write() error {
 	for _, p := range tx.pages {
 		pages = append(pages, p)
 	}
+	// Clear out page cache early.
+	tx.pages = make(map[pgid]*page)
 	sort.Sort(pages)
 
 	// Write pages to disk in order.
@@ -485,8 +487,18 @@ func (tx *Tx) write() error {
 		}
 	}
 
-	// Clear out page cache.
-	tx.pages = make(map[pgid]*page)
+	// put small pages back to sync.Pool
+	for _, p := range pages {
+		if int(p.overflow) != 0 || tx.db.pageSize != defaultPageSize {
+			continue
+		}
+		buf := (*[maxAllocSize]byte)(unsafe.Pointer(p))[:defaultPageSize]
+		// See https://go.googlesource.com/go/+/f03c9202c43e0abb130669852082117ca50aa9b1
+		for i := range buf {
+			buf[i] = 0
+		}
+		pagePool.Put(buf)
+	}
 
 	return nil
 }