linux-ext4 - [PATCH V2] naturally align struct ext4_allocation

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Message-ID: <4A441FD4.9020403@redhat.com>
Date:	Thu, 25 Jun 2009 20:09:40 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Mike Snitzer <snitzer@...il.com>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: [PATCH V2] naturally align struct ext4_allocation_request

As Ted noted, the ext4_allocation_request isn't well aligned.

Looking at it with pahole we're wasting space on 64-bit
arches:

struct ext4_allocation_request {
        struct inode *             inode;              /*     0     8 */
        ext4_lblk_t                logical;            /*     8     4 */

        /* XXX 4 bytes hole, try to pack */

        ext4_fsblk_t               goal;               /*    16     8 */
        ext4_lblk_t                lleft;              /*    24     4 */

        /* XXX 4 bytes hole, try to pack */

        ext4_fsblk_t               pleft;              /*    32     8 */
        ext4_lblk_t                lright;             /*    40     4 */

        /* XXX 4 bytes hole, try to pack */

        ext4_fsblk_t               pright;             /*    48     8 */
        unsigned int               len;                /*    56     4 */
        unsigned int               flags;              /*    60     4 */
        /* --- cacheline 1 boundary (64 bytes) --- */

        /* size: 64, cachelines: 1, members: 9 */
        /* sum members: 52, holes: 3, sum holes: 12 */
};

grouping 32-bit members together closes these holes and shrinks
the structure by 12 bytes.

Signed-off-by: Eric Sandeen <sandeen@...hat.com>
---

V2: fix comments, thanks to Mike Snitzer

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index cc7d5ed..cefd94a 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -93,20 +93,20 @@ typedef unsigned int ext4_group_t;
 struct ext4_allocation_request {
 	/* target inode for block we're allocating */
 	struct inode *inode;
+	/* how many blocks we want to allocate */
+	unsigned int len;
 	/* logical block in target inode */
 	ext4_lblk_t logical;
-	/* phys. target (a hint) */
-	ext4_fsblk_t goal;
 	/* the closest logical allocated block to the left */
 	ext4_lblk_t lleft;
-	/* phys. block for ^^^ */
-	ext4_fsblk_t pleft;
 	/* the closest logical allocated block to the right */
 	ext4_lblk_t lright;
-	/* phys. block for ^^^ */
+	/* phys. target (a hint) */
+	ext4_fsblk_t goal;
+	/* phys. block for the closest logical allocated block to the left */
+	ext4_fsblk_t pleft;
+	/* phys. block for the closest logical allocated block to the right */
 	ext4_fsblk_t pright;
-	/* how many blocks we want to allocate */
-	unsigned int len;
 	/* flags. see above EXT4_MB_HINT_* */
 	unsigned int flags;
 };

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