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bpf: lru: Lower the PERCPU_NR_SCANS from 16 to 4

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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>
This commit is contained in:
Martin KaFai Lau 2017-04-14 10:30:29 -07:00 committed by David S. Miller
parent 9fd63d05f3
commit 695ba2651a
2 changed files with 2 additions and 2 deletions
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2
kernel/bpf/bpf_lru_list.c
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@ -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 */

2
tools/testing/selftests/bpf/test_lru_map.c
View File

@ -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;