1) Add special case for len == 40 as that is the hottest value. The
nets a ~8-9% latency improvement and a ~30% throughput improvement
in the len == 40 case.
2) Use multiple accumulators in the 64-byte loop. This dramatically
improves ILP and results in up to a 40% latency/throughput
improvement (better for more iterations).
Results from benchmarking on Icelake. Times measured with rdtsc()
len lat_new lat_old r tput_new tput_old r
8 3.58 3.47 1.032 3.58 3.51 1.021
16 4.14 4.02 1.028 3.96 3.78 1.046
24 4.99 5.03 0.992 4.23 4.03 1.050
32 5.09 5.08 1.001 4.68 4.47 1.048
40 5.57 6.08 0.916 3.05 4.43 0.690
48 6.65 6.63 1.003 4.97 4.69 1.059
56 7.74 7.72 1.003 5.22 4.95 1.055
64 6.65 7.22 0.921 6.38 6.42 0.994
96 9.43 9.96 0.946 7.46 7.54 0.990
128 9.39 12.15 0.773 8.90 8.79 1.012
200 12.65 18.08 0.699 11.63 11.60 1.002
272 15.82 23.37 0.677 14.43 14.35 1.005
440 24.12 36.43 0.662 21.57 22.69 0.951
952 46.20 74.01 0.624 42.98 53.12 0.809
1024 47.12 78.24 0.602 46.36 58.83 0.788
1552 72.01 117.30 0.614 71.92 96.78 0.743
2048 93.07 153.25 0.607 93.28 137.20 0.680
2600 114.73 194.30 0.590 114.28 179.32 0.637
3608 156.34 268.41 0.582 154.97 254.02 0.610
4096 175.01 304.03 0.576 175.89 292.08 0.602
There is no such thing as a free lunch, however, and the special case
for len == 40 does add overhead to the len != 40 cases. This seems to
amount to be ~5% throughput and slightly less in terms of latency.
Testing:
Part of this change is a new kunit test. The tests check all
alignment X length pairs in [0, 64) X [0, 512).
There are three cases.
1) Precomputed random inputs/seed. The expected results where
generated use the generic implementation (which is assumed to be
non-buggy).
2) An input of all 1s. The goal of this test is to catch any case
a carry is missing.
3) An input that never carries. The goal of this test si to catch
any case of incorrectly carrying.
More exhaustive tests that test all alignment X length pairs in
[0, 8192) X [0, 8192] on random data are also available here:
https://github.com/goldsteinn/csum-reproduction
The reposity also has the code for reproducing the above benchmark
numbers.
Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230511011002.935690-1-goldstein.w.n%40gmail.com
