Messages in this thread Patch in this message | | | Date | Fri, 25 May 2007 12:19:33 +0200 | From | Hagen Paul Pfeifer <> | Subject | INTERNODE_CACHE_SHIFT redefinition for hot spot data alignment |
| |
I read code in include/linux/mmzone.h and attract attention to the padding construct for struct zone:
struct zone_padding { char x[0]; } ____cacheline_internodealigned_in_smp;
The definition for ____cacheline_internodealigned_in_smp is definded as __attribute__((__aligned__(1 << (INTERNODE_CACHE_SHIFT))))
INTERNODE_CACHE_SHIFT in turn is defined as L1_CACHE_SHIFT for all architectures (expect x86_64) as L1_CACHE_SHIFT - which is not that what everybody should expected if they use ____cacheline_internodealigned_in_smp! The comment stated out: "The maximum alignment needed for some critical structures. These could be inter-node cacheline sizes/L3 cacheline size etc. Define this in asm/cache.h for your arch"
BTW: the superior solution is to utilize the gcc 4.2 alternative to use the cpuid instruction (which is new in gcc release 4.2) to gather cpu attributes at compile time and replace the static #define L1_CACHE_BYTES (1 << L1_CACHE_SHIFT) macros
My suggestion is added as an patch -- any superior suggestions
Shalom Hagen Paul Pfeifer
Increase (double) the definition for INTERNODE_CACHE_SHIFT to accommodate a higher cache line size for l2 and l3 cacheline sizes - compared to l1 cache line size. The introduced overhead for architectures where the l1 cache line size correspond to the l2 cache line size is evanescent small, cause INTERNODE_CACHE_SHIFT is used only for hot spots. --- include/linux/cache.h | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/include/linux/cache.h b/include/linux/cache.h index 4552504..ebc366c 100644 --- a/include/linux/cache.h +++ b/include/linux/cache.h @@ -48,7 +48,7 @@ * size etc. Define this in asm/cache.h for your arch */ #ifndef INTERNODE_CACHE_SHIFT -#define INTERNODE_CACHE_SHIFT L1_CACHE_SHIFT +#define INTERNODE_CACHE_SHIFT (L1_CACHE_SHIFT + 1) #endif #if !defined(____cacheline_internodealigned_in_smp) -- 1.4.4.4 - 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/
| |