Christoph Lameter wrote:

>On Wed, 16 Aug 2006, Andi Kleen wrote:
>
>  
>
>>>2. use fls to calculate array position.
>>>      
>>>
>>I'm not sure that's a good idea. I always had my doubts that power of twos
>>are a good size distribution. I remember the original slab paper from Bonwick
>>also discouraged them. With fls you would hard code it.
>>    
>>
>
>The original paper from Bonwick is pretty dated now. There are some 
>interesting ideas in there and we have mostly followed them. But it is an 
>academic paper after all.
>
It's not just an academic paper, it's implemented in Solaris. Check the 
opensolaris sources.

> So not all ideas may be relevant in practice. 
>Support for non power of two sizes could lead to general slabs that do not 
>exactly fit into one page.
>
That's a good thing.
I'm not sure that the current approach with 
virt_to_page()/vmalloc_to_page() is the right thing(tm): Both functions 
are slow.
If you have non-power-of-two caches, you could store the control data at 
(addr&(~PAGE_SIZE)) - the lookup would be much faster. I wrote a patch a 
few weeks ago, it's attached.

Right now we have a few slab users that perform kmalloc(PAGE_SIZE). But 
that's a mostly for historic reasons: kmalloc was significantly (IIRC up 
to factor 10) faster than get_free_pages(). Now get_free_pages() also 
contains per-cpu structures, so we could convert __get_name or the pipe 
code back to get_free_pages().

--
    Manfred