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    SubjectRe: [patch 3/4] slab reclaim balancing
    Manfred Spraul wrote:
    > Andrew Morton wrote:
    > >
    > > Was the microbenchmark actually touching the memory which it was
    > > allocating from slab? If so then yes, we'd expect to see cache
    > > misses against those cold pages coming out of the buddy.
    > >
    > No, it was just measuring the cost of the kmem_cache_grow/shrink.
    > Btw, 140 cycles for kmem_cache_alloc+free is inflated - someone enabled
    > kmem_cache_alloc_head() even in the no-debugging version.
    > As expected, done by Andrea, who neither bothered to cc me, nor actually
    > understood the code.

    hm, OK. Sorry, I did not realise that you were this closely
    interested/involved with slab, so things have been sort of
    going on behind your back :(

    > >
    > >>For SMP and slabs that are per-cpu cached, the change could be right,
    > >>because the arrays should absorb bursts. But I do not think that the
    > >>change is the right approach for UP.
    > >
    > >
    > > I'd suggest that we wait until we have slab freeing its pages into
    > > the hotlists, and allocating from them. That should pull things back.
    > >
    > You are asking a interesting question:
    > The slab is by design far from LIFO - it tries to find pages with no
    > allocated objects, that are possible to return to the page allocator. It
    > doesn't try to optimize for cache hit rates.
    > Is that actually the right approach? For large objects, it would be
    > possible to cripple the freeable slabs list, and to perform the cache
    > hit optimization (i.e. per-cpu LIFO) in page_alloc.c, but that doesn't
    > work with small objects.

    Well with a, what? 100:1 speed ratio, we'll generally get best results
    from optimising for locality/recency of reference.

    > On SMP, the per-cpu arrays are the LIFO and should give good cache hit
    > rates. On UP, I haven't enabled them, because they could increase the
    > internal fragmentation of the slabs.
    > Perhaps we should enable the arrays on UP, too, and thus improve the
    > cache hit rates? If there is no increase in fragmentation, we could
    > ignore it. Otherwise we could replace the 3-list Bonwick slab with
    > another backend, something that's stronger at reducing the internal
    > fragmentation.

    Definitely worthy of investigation. Memory sizes are increasing,
    and the cached-versus-noncached latencies are increasing. Both
    these say "optimise for cache hits".

    Plus we'd lose a ton of ifdefs if we enabled it on UP as well...

    Bill wrote a couple of handy slab-monitoring tools, btw. - I use bloatmeter.
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