Commit 282ab3ff authored by David Sterba's avatar David Sterba
Browse files

btrfs: reduce compressed_bio members' types 



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: default avatarAnand Jain <anand.jain@oracle.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 49547068

fs/btrfs/compression.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

fs/btrfs/compression.h

+10 −10
Original line number Diff line number Diff line
@@ -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 on disk */

	unsigned long compressed_len;

	/* Number of bytes in the inode we're working on */

	unsigned int len;



	/* the compression algorithm for this bio */

	int compress_type;

	/* Number of bytes on disk */

	unsigned int compressed_len;



	/* 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 */