ceph: reorder fields in 'struct ceph_snapid_map'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct ceph_snapid_map' from 72 to 64
bytes.

When such a structure is allocated, because of the way memory allocation
works, when 72 bytes were requested, 96 bytes were allocated.

So, on x86_64, this change saves 32 bytes per allocation and has the
structure fit in a single cacheline.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index 18b026b..724307f 100644 (file)
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -355,8 +355,8 @@ struct ceph_snapid_map {
        struct rb_node node;
        struct list_head lru;
        atomic_t ref;
-       u64 snap;
        dev_t dev;
+       u64 snap;
        unsigned long last_used;
 };