On 10/24/19 11:31 AM, David Sterba wrote:
> 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 */
> };
This is great, obviously been too long since this kind of thing was
looked at by myself. Thanks, applied.
--
Jens Axboe
