From 5307e3abcb6668ceb84114e1c62b635cb995f4c7 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Thu, 28 Mar 2024 14:30:01 -0400 Subject: [PATCH] ppc: Document cache line size choice Signed-off-by: Mathieu Desnoyers Change-Id: I63603aaf5529e9a7810cfeffe19f32aea0dd13df --- include/urcu/arch/ppc.h | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/include/urcu/arch/ppc.h b/include/urcu/arch/ppc.h index be9d158..1285b6d 100644 --- a/include/urcu/arch/ppc.h +++ b/include/urcu/arch/ppc.h @@ -19,7 +19,30 @@ extern "C" { #endif -/* Include size of POWER5+ L3 cache lines: 256 bytes */ +/* + * Most powerpc machines have 128 bytes cache lines, but to make sure + * there is no false sharing on all known Power hardware, use the + * largest known cache line size, which is the physical size of POWER5 + * L3 cache lines (256 bytes). + * + * "Each slice [of the L3] is 12-way set-associative, with 4,096 + * congruence classes of 256-byte lines managed as two 128-byte sectors + * to match the L2 line size." + * + * From: "POWER5 system microarchitecture", + * IBM Journal of Research & Development, + * vol. 49, no. 4/5, July/September 2005 + * https://www.eecg.utoronto.ca/~moshovos/ACA08/readings/power5.pdf + * + * This value is a compile-time constant, which prevents us from + * querying the processor for the cache line size at runtime. We + * therefore need to be pessimistic and assume the largest known cache + * line size. + * + * This value is exposed through public headers, so tuning it for + * specific environments is a concern for ABI compatibility between + * applications and liburcu. + */ #define CAA_CACHE_LINE_SIZE 256 #ifdef __NO_LWSYNC__ -- 2.39.5