linux-kernel - [PATCH] ceph: Reorder fields in 'struct ceph_snapid

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Message-Id: <559c9a70419846e0cfc319505d3d5fffd45b3358.1682618727.git.christophe.jaillet@wanadoo.fr>
Date:   Thu, 27 Apr 2023 20:05:42 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        ceph-devel@...r.kernel.org
Subject: [PATCH] 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@...adoo.fr>
---
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