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| author | Martin KaFai Lau <kafai@fb.com> | 2017-04-14 10:30:29 -0700 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2017-04-17 13:55:52 -0400 |
| commit | 695ba2651a2ecbb336145f9cc033c85b9c6a5cee (patch) | |
| tree | 8eece29f36fc15ed76516d54eb08368e3d34523f | |
| parent | 9fd63d05f3e8476282cd8c484eb34d3f6be54f40 (diff) | |
| download | linux-695ba2651a2ecbb336145f9cc033c85b9c6a5cee.tar.bz2 | |
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 <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
| -rw-r--r-- | kernel/bpf/bpf_lru_list.c | 2 | ||||
| -rw-r--r-- | 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; |