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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Date:   Wed, 24 Jul 2019 15:15:44 -0700
From:   Brian Vazquez <brianvv.kernel@...il.com>
To:     Song Liu <liu.song.a23@...il.com>
Cc:     Brian Vazquez <brianvv@...gle.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 12:20 PM Song Liu <liu.song.a23@...il.com> wrote:
>
> 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?

The test is using a really simple map structure: uint64_t key,val.
Which makes the lookup_elem logic faster since it doesn't spend too
much time copying the value. My conclusion with the 40% was that this
new implementation only needs 1 syscall per elem compare to the 2
syscalls that we needed with previous implementation so in this
particular case the number of ops that we are doing is almost halved.
I did one experiment increasing the value_size (448*64B) and it was
only 14% 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.

right, will change it in next version.