Messages in this thread |  | | | Date | Wed, 14 Dec 2005 09:06:45 +0100 | | From | Eric Dumazet <> | | Subject | Re: [PATCH] Cpuset: rcu optimization of page alloc hook |
Paul Jackson a écrit : > Eric wrote: > >>struct kmem_cache itself will be about 512*8 + some bytes >>then for each cpu a 'struct array_cache' will be allocated (count 128 bytes > > > Hmmm ... 'struct array_cache' looks to be about 6 integer words, > so if that is the main per-CPU cost, the minimal cost of a slab > cache (once created, before use) is about 24 bytes per cpu.
Nope, because struct array_cache includes a variable length table of pointers to hold a cache of available objects per cpu. The 'limit' (the number of pointer in this cache) depends on the object size.
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;
Let's take an example :
grep dentry /proc/slabinfo
dentry_cache 157113 425289 224 17 1 : tunables 120 60 8 : slabdata 25017 25017 0
'limit' is the number following 'tunable' : 120
On a 64 bits machines, 120*sizeof(void*) = 120*8 = 960
So for small objects (<= 256 bytes), you end with a sizeof(arracy_cache) = 1024 bytes per cpu
If 512 CPUS : 512*1024 = 512 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.
> > But whether its 24 or 128 bytes per cpu, that's a heavier weight > hammer than is needed here. > > Time for me to learn more about rcu. > > Thanks for raising this issue. >
You are welcome.
Eric - 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/
|  |