From 695ba2651a2ecbb336145f9cc033c85b9c6a5cee Mon Sep 17 00:00:00 2001 From: Martin KaFai Lau Date: Fri, 14 Apr 2017 10:30:29 -0700 Subject: [PATCH] bpf: lru: Lower the PERCPU_NR_SCANS from 16 to 4 After doing map_perf_test with a much bigger BPF_F_NO_COMMON_LRU map, the perf report shows a lot of time spent in rotating the inactive list (i.e. __bpf_lru_list_rotate_inactive): > map_perf_test 32 8 10000 1000000 | awk '{sum += $3}END{print sum}' 19644783 (19M/s) > map_perf_test 32 8 10000000 10000000 | awk '{sum += $3}END{print sum}' 6283930 (6.28M/s) By inactive, it usually means the element is not in cache. Hence, there is a need to tune the PERCPU_NR_SCANS value. This patch finds a better number of elements to scan during each list rotation. The PERCPU_NR_SCANS (which is defined the same as PERCPU_FREE_TARGET) decreases from 16 elements to 4 elements. This change only affects the BPF_F_NO_COMMON_LRU map. The test_lru_dist does not show meaningful difference between 16 and 4. Our production L4 load balancer which uses the LRU map for conntrack-ing also shows little change in cache hit rate. Since both benchmark and production data show no cache-hit difference, PERCPU_NR_SCANS is lowered from 16 to 4. We can consider making it configurable if we find a usecase later that shows another value works better and/or use a different rotation strategy. After this change: > map_perf_test 32 8 10000000 10000000 | awk '{sum += $3}END{print sum}' 9240324 (9.2M/s) i.e. 6.28M/s -> 9.2M/s The test_lru_dist has not shown meaningful difference: > test_lru_dist zipf.100k.a1_01.out 4000 1: nr_misses: 31575 (Before) vs 31566 (After) > test_lru_dist zipf.100k.a0_01.out 40000 1 nr_misses: 67036 (Before) vs 67031 (After) Signed-off-by: Martin KaFai Lau Acked-by: Alexei Starovoitov Acked-by: Daniel Borkmann Signed-off-by: David S. Miller --- kernel/bpf/bpf_lru_list.c | 2 +- tools/testing/selftests/bpf/test_lru_map.c | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c index f62d1d56f41d..e6ef4401a138 100644 --- a/kernel/bpf/bpf_lru_list.c +++ b/kernel/bpf/bpf_lru_list.c @@ -13,7 +13,7 @@ #define LOCAL_FREE_TARGET (128) #define LOCAL_NR_SCANS LOCAL_FREE_TARGET -#define PERCPU_FREE_TARGET (16) +#define PERCPU_FREE_TARGET (4) #define PERCPU_NR_SCANS PERCPU_FREE_TARGET /* Helpers to get the local list index */ diff --git a/tools/testing/selftests/bpf/test_lru_map.c b/tools/testing/selftests/bpf/test_lru_map.c index 87c05e5bdfdd..8c10c9180c1a 100644 --- a/tools/testing/selftests/bpf/test_lru_map.c +++ b/tools/testing/selftests/bpf/test_lru_map.c @@ -22,7 +22,7 @@ #include "bpf_util.h" #define LOCAL_FREE_TARGET (128) -#define PERCPU_FREE_TARGET (16) +#define PERCPU_FREE_TARGET (4) static int nr_cpus;