linux-kernel - Re: [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call

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Message-ID: <CAPhsuW7PU1PP91e8vD2diwhBAwGJHWu6wAKOoBThT86f4r5OJw@mail.gmail.com>
Date:   Wed, 24 Jul 2019 12:20:04 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Brian Vazquez <brianvv@...gle.com>
Cc:     Brian Vazquez <brianvv.kernel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>,
        Stanislav Fomichev <sdf@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 0/6] bpf: add BPF_MAP_DUMP command to dump more
 than one entry per call

On Wed, Jul 24, 2019 at 10:09 AM Brian Vazquez <brianvv@...gle.com> wrote:
>
> This introduces a new command to retrieve multiple number of entries
> from a bpf map.
>
> This new command can be executed from the existing BPF syscall as
> follows:
>
> err =  bpf(BPF_MAP_DUMP, union bpf_attr *attr, u32 size)
> using attr->dump.map_fd, attr->dump.prev_key, attr->dump.buf,
> attr->dump.buf_len
> returns zero or negative error, and populates buf and buf_len on
> succees
>
> This implementation is wrapping the existing bpf methods:
> map_get_next_key and map_lookup_elem
>
> Note that this implementation can be extended later to do dump and
> delete by extending map_lookup_and_delete_elem (currently it only works
> for bpf queue/stack maps) and either use a new flag in map_dump or a new
> command map_dump_and_delete.
>
> Results show that even with a 1-elem_size buffer, it runs ~40 faster

Why is the new command 40% faster with 1-elem_size buffer?

> than the current implementation, improvements of ~85% are reported when
> the buffer size is increased, although, after the buffer size is around
> 5% of the total number of entries there's no huge difference in
> increasing it.
>
> Tested:
> Tried different size buffers to handle case where the bulk is bigger, or
> the elements to retrieve are less than the existing ones, all runs read
> a map of 100K entries. Below are the results(in ns) from the different
> runs:
>
> buf_len_1:       69038725 entry-by-entry: 112384424 improvement
> 38.569134
> buf_len_2:       40897447 entry-by-entry: 111030546 improvement
> 63.165590
> buf_len_230:     13652714 entry-by-entry: 111694058 improvement
> 87.776687
> buf_len_5000:    13576271 entry-by-entry: 111101169 improvement
> 87.780263
> buf_len_73000:   14694343 entry-by-entry: 111740162 improvement
> 86.849542
> buf_len_100000:  13745969 entry-by-entry: 114151991 improvement
> 87.958187
> buf_len_234567:  14329834 entry-by-entry: 114427589 improvement
> 87.476941

It took me a while to figure out the meaning of 87.476941. It is probably
a good idea to say 87.5% instead.

Thanks,
Song