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dc51616486
After the rwsem replaced the tree lock implementation, the extent buffer
got smaller but leaving some holes behind. By changing log_index type
and reordering, we can squeeze the size further to 240 bytes, measured on
release config on x86_64. Log_index spans only 3 values and needs to be
signed.
Before:
struct extent_buffer {
u64 start; /* 0 8 */
long unsigned int len; /* 8 8 */
long unsigned int bflags; /* 16 8 */
struct btrfs_fs_info * fs_info; /* 24 8 */
spinlock_t refs_lock; /* 32 4 */
atomic_t refs; /* 36 4 */
atomic_t io_pages; /* 40 4 */
int read_mirror; /* 44 4 */
struct callback_head callback_head __attribute__((__aligned__(8))); /* 48 16 */
/* --- cacheline 1 boundary (64 bytes) --- */
pid_t lock_owner; /* 64 4 */
bool lock_recursed; /* 68 1 */
/* XXX 3 bytes hole, try to pack */
struct rw_semaphore lock; /* 72 40 */
short int log_index; /* 112 2 */
/* XXX 6 bytes hole, try to pack */
struct page * pages[16]; /* 120 128 */
/* size: 248, cachelines: 4, members: 14 */
/* sum members: 239, holes: 2, sum holes: 9 */
/* forced alignments: 1 */
/* last cacheline: 56 bytes */
} __attribute__((__aligned__(8)));
After:
struct extent_buffer {
u64 start; /* 0 8 */
long unsigned int len; /* 8 8 */
long unsigned int bflags; /* 16 8 */
struct btrfs_fs_info * fs_info; /* 24 8 */
spinlock_t refs_lock; /* 32 4 */
atomic_t refs; /* 36 4 */
atomic_t io_pages; /* 40 4 */
int read_mirror; /* 44 4 */
struct callback_head callback_head __attribute__((__aligned__(8))); /* 48 16 */
/* --- cacheline 1 boundary (64 bytes) --- */
pid_t lock_owner; /* 64 4 */
bool lock_recursed; /* 68 1 */
s8 log_index; /* 69 1 */
/* XXX 2 bytes hole, try to pack */
struct rw_semaphore lock; /* 72 40 */
struct page * pages[16]; /* 112 128 */
/* size: 240, cachelines: 4, members: 14 */
/* sum members: 238, holes: 1, sum holes: 2 */
/* forced alignments: 1 */
/* last cacheline: 48 bytes */
} __attribute__((__aligned__(8)));
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
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.