Re: slab-alignment-rework.patch in -mc

[Manfred Spraul]

Andrea Arcangeli wrote:

> /mirror/kernel/people/akpm/patches/2.6/2.6.5/2.6.5-mc4/broken-out/slab-alignment-rework.patch
>
> I don't think this is right:
>
>  
>
> > -	if (flags & SLAB_HWCACHE_ALIGN) {
> > -		/* Need to adjust size so that objs are cache aligned. */
> > -		/* Small obj size, can get at least two per cache line. */
> > +	if (!align) {
> > +		/* Default alignment: compile time specified l1 cache size.
> > +		 * Except if an object is really small, then squeeze multiple
> > +		 * into one cacheline.
> > +		 */
> > +		align = cache_line_size();
> > 		while (size <= align/2)
> > 			align /= 2;
> > -		size = (size+align-1)&(~(align-1));
> > 	}
> > +	size = ALIGN(size, align);
> >
> >    
>
> I want anon-vma to really use only 12 bytes, period.
Then pass "4" as the align parameter to kmem_cache_create. That's the 
main point of the patch: it's now possible to explicitely specify the 
requested alignment. 32 for the 3rd level page tables, the optimal 
number for the pte_chains, etc.

> No best-guess must
> be made automatically by the slab code, rounding it to 16 bytes.
If you pass 0 as align to kmem_cache_create, then it's rounded to L2 
size. It's questionable if that's really the best thing - on 
uniprocessor, 16-byte might result is better performance - there is no 
risk of false sharing.

--
   Manfred

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/