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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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