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[PATCH 16/16] xfs; pack xfs_buf structure more tightly

To: xfs@xxxxxxxxxxx
Subject: [PATCH 16/16] xfs; pack xfs_buf structure more tightly
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 22 Sep 2010 16:44:29 +1000
In-reply-to: <1285137869-10310-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1285137869-10310-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Alex Elder <aelder@xxxxxxx>
---
 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 4f3c845..c2e2b50 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
-- 
1.7.1

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