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

Matthew Dillon dillon at
Mon Oct 8 09:45:47 PDT 2012


    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.

					Matthew Dillon 
					<dillon at>

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