On Fri, 2023-04-28 at 08:53 +0800, Xiubo Li wrote:
> On 4/28/23 02:05, Christophe JAILLET wrote:
> > 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;
> > };
> >
>
> This looks good to me. Thanks.
>
> Will apply it to the testing branch.
>
> - Xiubo
>
>
>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
