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SubjectRe: [MODSLAB 3/7] A Kmalloc subsystem
Christoph Lameter wrote:

>On Sat, 19 Aug 2006, Manfred Spraul wrote:
>
>
>>What about:
>>
>>if (unlikely(addr & (~(PAGE_SIZE-1))))
>> slabp=virt_to_page(addr)->pagefield;
>>else
>> slabp=addr & (~(PAGE_SIZE-1));
>>
>>
>
>Well this would not be working with the simple slab design that puts the
>first element at the page border to simplify alignment.
>
>And as we have just seen virt to page is mostly an address
>calculation in many configurations. I doubt that there would be a great
>advantage. Todays processors biggest cause for latencies are
>cacheline misses..
>
It involves table walking on discontigmem archs. "slabp=addr &
(~(PAGE_SIZE-1));" means no pointer chasing, and the access touches the
same page, i.e. definitively no TLB miss.

> Some arithmetic with addresses does not seem to
>be that important. Misaligning data in order to not put objects on such
>boundaries could be an issue.
>
> > Modify the kmalloc caches slightly and use non-power-of-2 cache sizes. Move
>
>
>>the kmalloc(PAGE_SIZE) users to gfp.
>>
>>
>
>Power of 2 cache sizes make the object align neatly to cacheline
>boundaries and make them fit tightly into a page.
>
>
IMHO not really an issue. 2kb-cache_line_size() also aligns perfectly.

--
Manfred
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