netdev - Re: [RFC PATCH bpf-next 2/2] selftests/bpf: add benchmark bpf

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Date:   Sat, 6 Nov 2021 11:43:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hou Tao <houtao1@...wei.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 2/2] selftests/bpf: add benchmark bpf_strcmp

On Sat, Nov 06, 2021 at 09:28:22PM +0800, Hou Tao wrote:
> The benchmark runs a loop 5000 times. In the loop it reads the file name
> from kprobe argument into stack by using bpf_probe_read_kernel_str(),
> and compares the file name with a target character or string.
> 
> Three cases are compared: only compare one character, compare the whole
> string by a home-made strncmp() and compare the whole string by
> bpf_strcmp().
> 
> The following is the result:
> 
> x86-64 host:
> 
> one character: 2613499 ns
> whole str by strncmp: 2920348 ns
> whole str by helper: 2779332 ns
> 
> arm64 host:
> 
> one character: 3898867 ns
> whole str by strncmp: 4396787 ns
> whole str by helper: 3968113 ns
> 
> Compared with home-made strncmp, the performance of bpf_strncmp helper
> improves 80% under x86-64 and 600% under arm64. The big performance win
> on arm64 may comes from its arch-optimized strncmp().

80% and 600% improvement?!
I don't understand how this math works.

Why one char is barely different in total nsec than the whole string?
The string shouldn't miscompare on the first char as far as I understand the test.