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SubjectRe: [PATCH][RFC] slub: increasing order reduces memory usage of some key caches
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On Wed, 2008-07-16 at 08:21 -0500, Christoph Lameter wrote:
> Richard Kennedy wrote:
>
>
> > on my amd64 3 gb ram desktop typical numbers :-
> >
> > [kernel,objects,pages/slab,slabs,total pages,diff]
> > radix_tree_node
> > 2.6.26 33922,2,2423 4846
> > +patch 33541,4,1165 4660,-186
> > dentry
> > 2.6.26 82136,1,4323 4323
> > +patch 79482,2,2038 4076,-247
> > the extra dentries would use 136 pages but that still leaves a saving of
> > 111 pages.
>
> Good numbers....
>
> > Can anyone suggest any other tests that would be useful to run?
> > & Is there any way to measure what impact this is having on
> > fragmentation?
>
> Mel would be able to tell you that but I think we better figure out what went wrong first.
>
>
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 315c392..c365b04 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2301,6 +2301,14 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> > if (order < 0)
> > return 0;
> >
> > + if (order < slub_max_order ) {
> > + unsigned long waste = (PAGE_SIZE << order) % size;
> > + if ( waste *2 >= size ) {
> > + order++;
> > + printk ( KERN_INFO "SLUB: increasing order %s->[%d] [%ld]\n",s->name,order,size);
> > + }
> > + }
> > +
> > s->allocflags = 0;
> > if (order)
> > s->allocflags |= __GFP_COMP;
>
> The order and waste calculation occurs in slab_order(). If modifications are needed then they need to occur in that function.

Definitely -- this was only intended demonstration code :)

> Looks like the existing code is not doing the best thing for dentries on your box?
>
> On my 64 bit box dentries are 208 bytes long, 39 objects per page and 84 bytes
> are lost per order 1 page. So this would not trigger your patch at all. There must be something special to your configuration.
>
It's a slightly modified fedora config -- I'm not aware of anything
particularly special. I'm setting the processor type to amd
athlon64/opteron (CONFIG_MK8)

>
> /linux-2.6$ slabinfo dentry
>
> Slabcache: dentry Aliases: 0 Order : 1 Objects: 554209
> ** Reclaim accounting active
>
> Sizes (bytes) Slabs Debug Memory
> ------------------------------------------------------------------------
> Object : 208 Total : 14215 Sanity Checks : Off Total: 116449280
> SlabObj: 208 Full : 14179 Redzoning : Off Used : 115275472
> SlabSiz: 8192 Partial: 32 Poisoning : Off Loss : 1173808
> Loss : 0 CpuSlab: 4 Tracking : Off Lalig: 0
> Align : 8 Objects: 39 Tracing : Off Lpadd: 1137200
>
>
> Can you post the slabinfo information about the caches that you are concerned with? Please a before and after state.
>
I don't have SYSFS slab info turned on right now, But I'll rebuild and get those for you.

but I get this from /proc/slabinfo

before
dentry 82136 82137 208 19 1 : tunables 0 0 0 : slabdata 4323 4323 0
after
dentry 79482 79482 208 39 2 : tunables 0 0 0 : slabdata 2038 2038 0

Richard



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