Chapter3:MemoryManagement
if (cachep->colour_off < align)
cachep->colour_off = align;
cachep->colour = left_over/cachep->colour_off;
...The kernel uses the size of an L1 cache that can be determined using the architecture-specific
cache_line_sizefunction as an offset. It must also be ensured that the offset is a multiple of the
alignment used — otherwise, the alignment effect would be lost.
Table 3-9: Examplary Calculations of theAlignment on 4- and 8-Byte Boundaries
Object sizex Alignmenty ALIGN(x,y)148
448
588
888
91216
12 12 16
13 16 16
16 16 16
17 20 24
19 20 24
The color of the slab (i.e., the number of potential offset values) is calculated by dividing the
free space on the slab (known as theleft_over) by the color offset (colour_off)withouta
remainder.
For example, on an older IA-32 machine, the kernel produces the following results for
a cache that manages 256-byte objects aligned on the hardware cache withSLABHWCACHE
ALIGN:
❑ 15 objects are managed on a slab (num = 15).
❑ Onepageisused(gfp_order = 0).
❑ There are five possible colors (colour = 5),andanoffsetof32bytesisusedforeachcolor
(colour_off = 32).
❑ Theslabheadisstoredon-slab.