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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>
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| Documentation | ||
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.