From 5b85575ad4280171c382e888b006cb8d12c35bde Mon Sep 17 00:00:00 2001
From: Anton Protopopov <aspsk@isovalent.com>
Date: Sat, 1 Apr 2023 20:06:02 +0000
Subject: [PATCH] bpf: optimize hashmap lookups when key_size is divisible by 4
The BPF hashmap uses the jhash() hash function. There is an optimized version
of this hash function which may be used if hash size is a multiple of 4. Apply
this optimization to the hashmap in a similar way as it is done in the bloom
filter map.
On practice the optimization is only noticeable for smaller key sizes, which,
however, is sufficient for many applications. An example is listed in the
following table of measurements (a hashmap of 65536 elements was used):
--------------------------------------------------------------------
| key_size | fullness | lookups /sec | lookups (opt) /sec | gain |
--------------------------------------------------------------------
| 4 | 25% | 42.990M | 46.000M | 7.0% |
| 4 | 50% | 37.910M | 39.094M | 3.1% |
| 4 | 75% | 34.486M | 36.124M | 4.7% |
| 4 | 100% | 31.760M | 32.719M | 3.0% |
--------------------------------------------------------------------
| 8 | 25% | 43.855M | 49.626M | 13.2% |
| 8 | 50% | 38.328M | 42.152M | 10.0% |
| 8 | 75% | 34.483M | 38.088M | 10.5% |
| 8 | 100% | 31.306M | 34.686M | 10.8% |
--------------------------------------------------------------------
| 12 | 25% | 38.398M | 43.770M | 14.0% |
| 12 | 50% | 33.336M | 37.712M | 13.1% |
| 12 | 75% | 29.917M | 34.440M | 15.1% |
| 12 | 100% | 27.322M | 30.480M | 11.6% |
--------------------------------------------------------------------
| 16 | 25% | 41.491M | 41.921M | 1.0% |
| 16 | 50% | 36.206M | 36.474M | 0.7% |
| 16 | 75% | 32.529M | 33.027M | 1.5% |
| 16 | 100% | 29.581M | 30.325M | 2.5% |
--------------------------------------------------------------------
| 20 | 25% | 34.240M | 36.787M | 7.4% |
| 20 | 50% | 30.328M | 32.663M | 7.7% |
| 20 | 75% | 27.536M | 29.354M | 6.6% |
| 20 | 100% | 24.847M | 26.505M | 6.7% |
--------------------------------------------------------------------
| 24 | 25% | 36.329M | 40.608M | 11.8% |
| 24 | 50% | 31.444M | 35.059M | 11.5% |
| 24 | 75% | 28.426M | 31.452M | 10.6% |
| 24 | 100% | 26.278M | 28.741M | 9.4% |
--------------------------------------------------------------------
| 28 | 25% | 31.540M | 31.944M | 1.3% |
| 28 | 50% | 27.739M | 28.063M | 1.2% |
| 28 | 75% | 24.993M | 25.814M | 3.3% |
| 28 | 100% | 23.513M | 23.500M | -0.1% |
--------------------------------------------------------------------
| 32 | 25% | 32.116M | 33.953M | 5.7% |
| 32 | 50% | 28.879M | 29.859M | 3.4% |
| 32 | 75% | 26.227M | 26.948M | 2.7% |
| 32 | 100% | 23.829M | 24.613M | 3.3% |
--------------------------------------------------------------------
| 64 | 25% | 22.535M | 22.554M | 0.1% |
| 64 | 50% | 20.471M | 20.675M | 1.0% |
| 64 | 75% | 19.077M | 19.146M | 0.4% |
| 64 | 100% | 17.710M | 18.131M | 2.4% |
--------------------------------------------------------------------
The following script was used to gather the results (SMT & frequency off):
cd tools/testing/selftests/bpf
for key_size in 4 8 12 16 20 24 28 32 64; do
for nr_entries in `seq 16384 16384 65536`; do
fullness=$(printf '%3s' $((nr_entries*100/65536)))
echo -n "key_size=$key_size: $fullness% full: "
sudo ./bench -d2 -a bpf-hashmap-lookup --key_size=$key_size --nr_entries=$nr_entries --max_entries=65536 --nr_loops=2000000 --map_flags=0x40 | grep cpu
done
echo
done
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230401200602.3275-1-aspsk@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
kernel/bpf/hashtab.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 96b645bba3a4d..00c253b84bf53 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -607,6 +607,8 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
static inline u32 htab_map_hash(const void *key, u32 key_len, u32 hashrnd)
{
+ if (likely(key_len % 4 == 0))
+ return jhash2(key, key_len / 4, hashrnd);
return jhash(key, key_len, hashrnd);
}
--
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