opt(restore): Sort the buffer before spinning the writeToDisk gorouti…

…ne (#7984)

Sort the buffer beforehand instead of sorting it in the goroutine used for
writing the buffer to disk. The writeToDisk goroutines are throttled and
making it expensive causes other goroutines to block.

This change significantly improves the restore map phase.

worker/restore_map.go

@@ -162,11 +162,6 @@ func (m *mapper) writeToDisk(buf *z.Buffer) error {
 	if buf.IsEmpty() {
 		return nil
 	}
-	buf.SortSlice(func(ls, rs []byte) bool {
-		lme := mapEntry(ls)
-		rme := mapEntry(rs)
-		return y.CompareKeys(lme.Key(), rme.Key()) < 0
-	})
 
 	f, err := m.newMapFile()
 	if err != nil {
@@ -239,6 +234,11 @@ func (mw *mapper) sendForWriting() error {
 	if mw.buf.IsEmpty() {
 		return nil
 	}
+	mw.buf.SortSlice(func(ls, rs []byte) bool {
+		lme := mapEntry(ls)
+		rme := mapEntry(rs)
+		return y.CompareKeys(lme.Key(), rme.Key()) < 0
+	})
 
 	if err := mw.thr.Do(); err != nil {
 		return err