diff --git a/events.go b/events.go
index 53e8c028f..28d324984 100644
--- a/events.go
+++ b/events.go
@@ -199,7 +199,7 @@ func convertMemoryEntry(c cgroups.MemoryData) types.MemoryEntry {
 }
 
 func convertBlkioEntry(c []cgroups.BlkioStatEntry) []types.BlkioEntry {
-	var out []types.BlkioEntry
+	out := make([]types.BlkioEntry, 0, len(c))
 	for _, e := range c {
 		out = append(out, types.BlkioEntry(e))
 	}
diff --git a/libcontainer/internal/userns/usernsfd_linux.go b/libcontainer/internal/userns/usernsfd_linux.go
index a1151a3fb..35ac373cd 100644
--- a/libcontainer/internal/userns/usernsfd_linux.go
+++ b/libcontainer/internal/userns/usernsfd_linux.go
@@ -41,10 +41,11 @@ func (m Mapping) toSys() (uids, gids []syscall.SysProcIDMap) {
 // the uid and gid mappings (because the order doesn't matter to the kernel).
 // The set of userns handles is indexed using this ID.
 func (m Mapping) id() string {
-	var uids, gids []string
+	uids := make([]string, 0, len(m.UIDMappings))
 	for _, idmap := range m.UIDMappings {
 		uids = append(uids, fmt.Sprintf("%d:%d:%d", idmap.ContainerID, idmap.HostID, idmap.Size))
 	}
+	gids := make([]string, 0, len(m.GIDMappings))
 	for _, idmap := range m.GIDMappings {
 		gids = append(gids, fmt.Sprintf("%d:%d:%d", idmap.ContainerID, idmap.HostID, idmap.Size))
 	}