Messages in this thread | | | Date | Thu, 26 Sep 2002 14:39:19 -0700 | From | Andrew Morton <> | Subject | Re: [patch 3/4] slab reclaim balancing |
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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. http://www.zip.com.au/~akpm/linux/patches/ - I use bloatmeter. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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