[PATCH 18/18] xfs: pack xfs_buf structure more tightly

Dave Chinner
2010-09-24T12:30:54Z

pahole reports the struct xfs_buf has quite a few holes in it, so
packing the structure better will reduce the size of it by 16 bytes.
Also, move all the fields used in cache lookups into the first
cacheline.

Before on x86_64:

/* size: 320, cachelines: 5 */
/* sum members: 298, holes: 6, sum holes: 22 */

After on x86_64:

/* size: 304, cachelines: 5 */
/* padding: 6 */
/* last cacheline: 48 bytes */

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
---
fs/xfs/linux-2.6/xfs_buf.h | 30 +++++++++++++++++++-----------
1 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.h b/fs/xfs/linux-2.6/xfs_buf.h
index c79882e..1613337 100644
--- a/fs/xfs/linux-2.6/xfs_buf.h
+++ b/fs/xfs/linux-2.6/xfs_buf.h
@@ -162,33 +162,41 @@ typedef int (*xfs_buf_bdstrat_t)(struct xfs_buf *);
#define XB_PAGES 2

typedef struct xfs_buf {
+ /*
+ * first cacheline holds all the fields needed for an uncontended cache
+ * hit to be fully processed. The semaphore straddles the cacheline
+ * boundary, but the counter and lock sits on the first cacheline,
+ * which is the only bit that is touched if we hit the semaphore
+ * fast-path on locking.
+ */
+ struct rb_node b_rbnode; /* rbtree node */
+ xfs_off_t b_file_offset; /* offset in file */
+ size_t b_buffer_length;/* size of buffer in bytes */
+ atomic_t b_hold; /* reference count */
+ xfs_buf_flags_t b_flags; /* status flags */
struct semaphore b_sema; /* semaphore for lockables */
- unsigned long b_queuetime; /* time buffer was queued */
- atomic_t b_pin_count; /* pin count */
+
wait_queue_head_t b_waiters; /* unpin waiters */
struct list_head b_list;
- xfs_buf_flags_t b_flags; /* status flags */
- struct rb_node b_rbnode; /* rbtree node */
struct xfs_perag *b_pag; /* contains rbtree root */
xfs_buftarg_t *b_target; /* buffer target (device) */
- atomic_t b_hold; /* reference count */
xfs_daddr_t b_bn; /* block number for I/O */
- xfs_off_t b_file_offset; /* offset in file */
- size_t b_buffer_length;/* size of buffer in bytes */
size_t b_count_desired;/* desired transfer size */
void *b_addr; /* virtual address of buffer */
struct work_struct b_iodone_work;
- atomic_t b_io_remaining; /* #outstanding I/O requests */
xfs_buf_iodone_t b_iodone; /* I/O completion function */
xfs_buf_relse_t b_relse; /* releasing function */
struct completion b_iowait; /* queue for I/O waiters */
void *b_fspriv;
void *b_fspriv2;
- unsigned short b_error; /* error code on I/O */
- unsigned int b_page_count; /* size of page array */
- unsigned int b_offset; /* page offset in first page */
struct page **b_pages; /* array of page pointers */
struct page *b_page_array[XB_PAGES]; /* inline pages */
+ unsigned long b_queuetime; /* time buffer was queued */
+ atomic_t b_pin_count; /* pin count */
+ atomic_t b_io_remaining; /* #outstanding I/O requests */
+ unsigned int b_page_count; /* size of page array */
+ unsigned int b_offset; /* page offset in first page */
+ unsigned short b_error; /* error code on I/O */
#ifdef XFS_BUF_LOCK_TRACKING
int b_last_holder;
#endif