linux-ext4 - Re: [PATCH] naturally align struct ext4_allocation

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Date:	Thu, 25 Jun 2009 20:50:16 -0400
From:	Mike Snitzer <snitzer@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] naturally align struct ext4_allocation_request

On Wed, Jun 24, 2009 at 12:30 PM, Eric Sandeen<sandeen@...hat.com> wrote:
> 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>
> ---
>
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index cc7d5ed..9a438cf 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. target (a hint) */
> +       ext4_fsblk_t goal;
> +       /* phys. block for ^^^ */
> +       ext4_fsblk_t pleft;
>        /* phys. block for ^^^ */
>        ext4_fsblk_t pright;

Hey Eric,

Looks like the comments for pleft and pright should be updated to
reference lleft and lright respectively (rather than ^^^).

Mike
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