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    SubjectRe: [RFC] slub: ARCH_SLAB_MINALIGN defaults to 8 on x86_32. is this too big?
    On Wed, 3 Feb 2010, Richard Kennedy wrote:

    > slub.c sets the default value of ARCH_SLAB_MINALIGN to sizeof(unsigned
    > long long) if the architecture didn't already override it.
    > And as x86_32 doesn't set a value this means that slab objects get
    > aligned to 8 bytes, potentially wasting 4 bytes per object. Slub forces
    > objects to be aligned to sizeof(void *) anyway, but I don't see that
    > there is any need for it to be 8 on 32bits.

    Note that 64 bit entities may exist even under 32 bit (llong) that need
    to be aligned properly.

    struct buffer_head contains a sector_t which is 64 bit so you should align
    to an 8 byte boundary.

    > I'm working on a patch to pack more buffer_heads into each kmem_cache
    > slab page.
    > On 32 bits the structure size is 52 bytes and with the alignment applied
    > I end up with a slab of 73 x 56 byte objects. However, if the minimum
    > alignment was sizeof(void *) then I'd get 78 x 52 byte objects. So there
    > is quite a memory saving to be had in changing this.

    SLUB is not restricted to order 0 pages and can use order 1 or 2 pages as
    long as this reduces the memory footprint (byte wastage in a slab page is
    reduced) and as long as the kernel has contiguous memory available. It
    will use order 0 when memory is fragmented.

    > Can I define a ARCH_SLAB_MINALIGN in x86_64 to sizeof(void *) ?
    > or would it be ok to change the default in slub.c to sizeof(void *) ?
    > Or am I missing something ?

    I'd say leave it alone.

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      Last update: 2010-02-03 16:45    [W:0.020 / U:2.756 seconds]
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