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block: reorder bio::__bi_remaining for better packing

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Simple reordering of __bi_remaining can reduce bio size by 8 bytes that
are now wasted on padding (measured on x86_64):

struct bio {
        struct bio *               bi_next;              /*     0     8 */
        struct gendisk *           bi_disk;              /*     8     8 */
        unsigned int               bi_opf;               /*    16     4 */
        short unsigned int         bi_flags;             /*    20     2 */
        short unsigned int         bi_ioprio;            /*    22     2 */
        short unsigned int         bi_write_hint;        /*    24     2 */
        blk_status_t               bi_status;            /*    26     1 */
        u8                         bi_partno;            /*    27     1 */

        /* XXX 4 bytes hole, try to pack */

        struct bvec_iter   bi_iter;                      /*    32    24 */

        /* XXX last struct has 4 bytes of padding */

        atomic_t                   __bi_remaining;       /*    56     4 */

        /* XXX 4 bytes hole, try to pack */
[...]
        /* size: 104, cachelines: 2, members: 19 */
        /* sum members: 96, holes: 2, sum holes: 8 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 40 bytes */
};

Now becomes:

struct bio {
        struct bio *               bi_next;              /*     0     8 */
        struct gendisk *           bi_disk;              /*     8     8 */
        unsigned int               bi_opf;               /*    16     4 */
        short unsigned int         bi_flags;             /*    20     2 */
        short unsigned int         bi_ioprio;            /*    22     2 */
        short unsigned int         bi_write_hint;        /*    24     2 */
        blk_status_t               bi_status;            /*    26     1 */
        u8                         bi_partno;            /*    27     1 */
        atomic_t                   __bi_remaining;       /*    28     4 */
        struct bvec_iter   bi_iter;                      /*    32    24 */

        /* XXX last struct has 4 bytes of padding */
[...]
        /* size: 96, cachelines: 2, members: 19 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 32 bytes */
};

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
David Sterba 2019-10-24 19:31:10 +02:00 • committed by Jens Axboe
parent f7e76dbc24
commit 993e4cdebb
1 changed files with 1 additions and 1 deletions
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2
include/linux/blk_types.h
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@ -153,10 +153,10 @@ struct bio {
unsigned short bi_write_hint;
blk_status_t bi_status;
u8 bi_partno;
atomic_t __bi_remaining;
struct bvec_iter bi_iter;
atomic_t __bi_remaining;
bio_end_io_t *bi_end_io;
void *bi_private;