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 */ };
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| @@ -153,10 +153,10 @@ struct bio { · 152 unmodified lines | |||
| 153 | unsigned short bi_write_hint; | 153 | unsigned short bi_write_hint; |
| 154 | blk_status_t bi_status; | 154 | blk_status_t bi_status; |
| 155 | u8 bi_partno; | 155 | u8 bi_partno; |
| 156 | atomic_t __bi_remaining; | ||
| 156 | 157 | ||
| 157 | struct bvec_iter bi_iter; | 158 | struct bvec_iter bi_iter; |
| 158 | 159 | ||
| 159 | atomic_t __bi_remaining; | ||
| 160 | bio_end_io_t *bi_end_io; | 160 | bio_end_io_t *bi_end_io; |
| 161 | 161 | ||
| 162 | void *bi_private; | 162 | void *bi_private; |