Re: Q: explicit alignment control for the slab allocator

[Manfred Spraul]

Mark Hemment wrote:
> 
>   The original idea behind offset was for objects with a "hot" area
> greater than a single L1 cache line.  By using offset correctly (and to my
> knowledge it has never been used anywhere in the Linux kernel), a SLAB
> cache creator (caller of kmem_cache_create()) could ask the SLAB for more
> than one colour (space/L1 cache lines) offset between objects.
>

What's the difference between this definition of 'offset' and alignment?

alignment means that (addr%alignment==0)
offset means that (addr1-addr2 == n*offset)

Isn't the only difference the alignment of the first object in a slab?

>   As no one uses the feature it could well be broken, but is that a reason
> to change its meaning?
>

Some hardware drivers use HW_CACHEALIGN and assume certain byte
alignments, and arm needs 1024 byte aligned blocks.

--
	Manfred