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btrfs: reduce compressed_bio members' types

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Several members of compressed_bio are of type that's unnecessarily big
for the values that they'd hold:

- the size of the uncompressed and compressed data is 128K now, we can
  keep is as int
- same for number of pages
- the compress type fits to a byte
- the errors is 0/1

The size of the unpatched structure is 80 bytes with several holes.
Reordering nr_pages next to the pages the hole after pending_bios is
filled and the resulting size is 56 bytes. This keeps the csums array
aligned to 8 bytes, which is nice. Further size optimizations may be
possible but right now it looks good to me:

struct compressed_bio {
        refcount_t                 pending_bios;         /*     0     4 */
        unsigned int               nr_pages;             /*     4     4 */
        struct page * *            compressed_pages;     /*     8     8 */
        struct inode *             inode;                /*    16     8 */
        u64                        start;                /*    24     8 */
        unsigned int               len;                  /*    32     4 */
        unsigned int               compressed_len;       /*    36     4 */
        u8                         compress_type;        /*    40     1 */
        u8                         errors;               /*    41     1 */

        /* XXX 2 bytes hole, try to pack */

        int                        mirror_num;           /*    44     4 */
        struct bio *               orig_bio;             /*    48     8 */
        u8                         sums[];               /*    56     0 */

        /* size: 56, cachelines: 1, members: 12 */
        /* sum members: 54, holes: 1, sum holes: 2 */
        /* last cacheline: 56 bytes */
};

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2019-10-14 14:38:33 +02:00
parent 49547068f6
commit 282ab3ff16
2 changed files with 11 additions and 11 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
fs/btrfs/compression.c
View file

@ -507,7 +507,7 @@ blk_status_t btrfs_submit_compressed_write(struct btrfs_inode *inode, u64 start,
}
if (bytes_left < PAGE_SIZE) {
btrfs_info(fs_info,
"bytes left %lu compress len %lu nr %lu",
"bytes left %lu compress len %u nr %u",
bytes_left, cb->compressed_len, cb->nr_pages);
}
bytes_left -= PAGE_SIZE;

20
fs/btrfs/compression.h
View file

@ -31,6 +31,9 @@ struct compressed_bio {
/* number of bios pending for this compressed extent */
refcount_t pending_bios;
/* Number of compressed pages in the array */
unsigned int nr_pages;
/* the pages with the compressed data on them */
struct page **compressed_pages;
@ -40,20 +43,17 @@ struct compressed_bio {
/* starting offset in the inode for our pages */
u64 start;
/* number of bytes in the inode we're working on */
unsigned long len;
/* Number of bytes in the inode we're working on */
unsigned int len;
/* number of bytes on disk */
unsigned long compressed_len;
/* Number of bytes on disk */
unsigned int compressed_len;
/* the compression algorithm for this bio */
int compress_type;
/* number of compressed pages in the array */
unsigned long nr_pages;
/* The compression algorithm for this bio */
u8 compress_type;
/* IO errors */
int errors;
u8 errors;
int mirror_num;
/* for reads, this is the bio we are copying the data into */