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    SubjectRe: hackbench regression due to commit 9dfc6e68bfe6e
    On Wed, 7 Apr 2010, Pekka Enberg wrote:

    > Christoph Lameter wrote:
    > > I wonder if this is not related to the kmem_cache_cpu structure straggling
    > > cache line boundaries under some conditions. On 2.6.33 the kmem_cache_cpu
    > > structure was larger and therefore tight packing resulted in different
    > > alignment.
    > >
    > > Could you see how the following patch affects the results. It attempts to
    > > increase the size of kmem_cache_cpu to a power of 2 bytes. There is also
    > > the potential that other per cpu fetches to neighboring objects affect the
    > > situation. We could cacheline align the whole thing.
    > >
    > > ---
    > > include/linux/slub_def.h | 5 +++++
    > > 1 file changed, 5 insertions(+)
    > >
    > > Index: linux-2.6/include/linux/slub_def.h
    > > ===================================================================
    > > --- linux-2.6.orig/include/linux/slub_def.h 2010-04-07 11:33:50.000000000
    > > -0500
    > > +++ linux-2.6/include/linux/slub_def.h 2010-04-07 11:35:18.000000000
    > > -0500
    > > @@ -38,6 +38,11 @@ struct kmem_cache_cpu {
    > > void **freelist; /* Pointer to first free per cpu object */
    > > struct page *page; /* The slab from which we are allocating */
    > > int node; /* The node of the page (or -1 for debug) */
    > > +#ifndef CONFIG_64BIT
    > > + int dummy1;
    > > +#endif
    > > + unsigned long dummy2;
    > > +
    > > #ifdef CONFIG_SLUB_STATS
    > > unsigned stat[NR_SLUB_STAT_ITEMS];
    > > #endif
    >
    > Would __cacheline_aligned_in_smp do the trick here?

    This is allocated via the percpu allocator. We could specify cacheline
    alignment there but that would reduce the density. You basically need 4
    words for a kmem_cache_cpu structure. A number of those fit into one 64
    byte cacheline.



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