On Wednesday 04 January 2006 12:41, Eric Dumazet wrote:
> > The overhead of the kmem_cache_t by itself is negligible.
>
> This seems a common misconception among kernel devs (even the best ones Andi :) )
It used to be true at least at some point :/
>
> On SMP (and/or NUMA) machines : overhead of kmem_cache_t is *big*
>
> See enable_cpucache in mm/slab.c for 'limit' determination :
>
> if (cachep->objsize > 131072)
> limit = 1;
> else if (cachep->objsize > PAGE_SIZE)
> limit = 8;
> else if (cachep->objsize > 1024)
> limit = 24;
> else if (cachep->objsize > 256)
> limit = 54;
> else
> limit = 120;
>
> On a 64 bits machines, 120*sizeof(void*) = 120*8 = 960
>
> So for small objects (<= 256 bytes), you end with a sizeof(array_cache) = 1024
> bytes per cpu
Hmm - in theory it could be tuned down for SMT siblings which really don't
care about sharing because they have shared caches. But I don't know
how many complications that would add to the slab code.
>
> If 16 CPUS : 16*1024 = 16 Kbytes + all other kmem_cache structures : (If you
> have a lot of Memory Nodes, then it can be *very* big too).
>
> If you know that no more than 100 objects are used in 99% of setups, then a
> dedicated cache is overkill, even locking 100 pages because of extreme
> fragmentation is better.
A system with 16 memory nodes should have more than 100 processes, but ok.
>
> Maybe we can introduce an ultra basic memory allocator for such objects
> (without CPU caches, node caches), so that the memory overhead is small.
> Hitting a spinlock at thread creation/deletion time is not that time critical.
Might be a good idea yes. There used to a "simp" allocator long ago for this,
but it was removed because it had other issues. But this was before even slab
got the per cpu/node support.
-Andi
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