git: kmalloc: Use 'fls' to round up the size to the nearest power of 2

[Sepherosa Ziehau]

On Tue, Oct 9, 2012 at 12:45 AM, Matthew Dillon
<dillon at apollo.backplane.com> wrote:
> :http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/1e57f8673c953ace8a25ada0a39ab83008646ce3
> :
>
>     For cache alignment any allocation that is already a power of 2 will be
>     at least that specific power-of-2 aligned.  Any allocation that is not
>     a power of 2 will be aligned to the slab chunking:
>
>  *      Alloc Size      Chunking        Number of zones
>  *      0-127           8               16
>  *      128-255         16              8
>  *      256-511         32              8
>  *      512-1023        64              8
>  ...
>
>     What this means is that in order to cache align an allocation you only
>     need to do it for allocations < 256 bytes (if the cache line size is
>     32 bytes).  Any allocation >= 256 bytes will be at least 32-byte aligned
>     even if it is not a power of 2.
>
>     So we don't necessarily need to enforce a power-of-2 size in all cases,
>     only in cases where the allocation size < 256 bytes.

Ah, ok, I see.  However, I think the cache line size is 64bytes on
x86, so as far as I understand from the above table, allocation >=512
will be at least 64bytes aligned?

Best Regards,
sephe

>
>                                         -Matt
>                                         Matthew Dillon
>                                         <dillon at backplane.com>



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