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DateWed, 21 Mar 2007 07:57:26 +0100
FromEric Dumazet <>
Subject[PATCH] SLAB : Use num_possible_cpus() in enable_cpucache()
The existing comment in mm/slab.c is *perfect*, so I reproduce it :

/*
* CPU bound tasks (e.g. network routing) can exhibit cpu bound
* allocation behaviour: Most allocs on one cpu, most free operations
* on another cpu. For these cases, an efficient object passing between
* cpus is necessary. This is provided by a shared array. The array
* replaces Bonwick's magazine layer.
* On uniprocessor, it's functionally equivalent (but less efficient)
* to a larger limit. Thus disabled by default.
*/
As most shiped linux kernels are now compiled with CONFIG_SMP, there is no way
a preprocessor #if can detect if the machine is UP or SMP. Better to use
num_possible_cpus().

This means on UP we allocate a 'size=0 shared array', to be more efficient.

Another patch can later avoid the allocations of 'empty shared arrays', to
save some memory.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
diff --git a/mm/slab.c b/mm/slab.c
index 57f7aa4..a69d0a5 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3975,10 +3975,8 @@ static int enable_cpucache(struct kmem_c
* to a larger limit. Thus disabled by default.
*/
shared = 0;
-#ifdef CONFIG_SMP
- if (cachep->buffer_size <= PAGE_SIZE)
+ if (cachep->buffer_size <= PAGE_SIZE && num_possible_cpus() > 1)
shared = 8;
-#endif

#if DEBUG
/*
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