Re: [PATCH v5 bpf-next] selftests/bpf: Add benchmark for local_storage get

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Dave Marchevsky <davemarchevsky@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>, bpf <bpf@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH v5 bpf-next] selftests/bpf: Add benchmark for local_storage get
Date: Thu, 9 Jun 2022 08:49:15 -0700	[thread overview]
Message-ID: <CAADnVQJ+q6gmSt6E7YmhrdbNbW53tAwC3JKHEF1ts7VBU=x5GQ@mail.gmail.com> (raw)
In-Reply-To: <79b4e95c-437d-45c5-c7a8-c077f692c18a@fb.com>

On Thu, Jun 9, 2022 at 7:27 AM Dave Marchevsky <davemarchevsky@fb.com> wrote:
> >> +
> >> +    if (use_hashmap) {
> >> +            idx = bpf_get_prandom_u32() % hashmap_num_keys;
> >> +            bpf_map_lookup_elem(inner_map, &idx);
> > Is the hashmap populated ?
> >
>
> Nope. Do you expect this to make a difference? Will try when confirming key /
> val size above.

Martin brought up an important point.
The map should be populated.
If the map is empty lookup_nulls_elem_raw() will select a bucket,
it will be empty and it will return NULL.
Whereas the more accurates apples to apples comparison
would be to find a task in a map, since bpf_task_storage_get(,F_CREATE);
will certainly find it.
Then if (l->hash == hash && !memcmp ... will be triggered.
When we're counting nsecs that should be noticeable.