Re: [MODSLAB 3/7] A Kmalloc subsystem

[Manfred Spraul <manfred@colorfullife.com>]

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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