Re: [PATCH] Cpuset: rcu optimization of page alloc hook

From: Eric Dumazet
Date: Wed Dec 14 2005 - 03:05:57 EST


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@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/