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@xxxxxxxxxx>
---
Using pahole
Before:
======
struct ceph_snapid_map {
struct rb_node node __attribute__((__aligned__(8))); /* 0 24 */
struct list_head lru; /* 24 16 */
atomic_t ref; /* 40 4 */
/* XXX 4 bytes hole, try to pack */
u64 snap; /* 48 8 */
dev_t dev; /* 56 4 */
/* XXX 4 bytes hole, try to pack */
/* --- cacheline 1 boundary (64 bytes) --- */
long unsigned int last_used; /* 64 8 */
/* size: 72, cachelines: 2, members: 6 */
/* sum members: 64, holes: 2, sum holes: 8 */
/* forced alignments: 1 */
/* last cacheline: 8 bytes */
} __attribute__((__aligned__(8)));
---
fs/ceph/mds_client.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index 0598faa50e2e..2328dbda5ab6 100644
--- 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;
};
--
2.34.1
