netdev - Re: [PATCH net-next v2 1/2] net: Make nexthop-dumps scale linearly with the number of nexthops

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Message-ID: <CANn89iKBky8z-duf1ygOsqD1kWX-kTL3bXEmt7RYz8YAWD1P9w@mail.gmail.com>
Date: Mon, 18 Aug 2025 02:54:14 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: cpaasch@...nai.com
Cc: David Ahern <dsahern@...nel.org>, Nikolay Aleksandrov <razor@...ckwall.org>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Ido Schimmel <idosch@...sch.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/2] net: Make nexthop-dumps scale linearly
 with the number of nexthops

On Sat, Aug 16, 2025 at 4:13 PM Christoph Paasch via B4 Relay
<devnull+cpaasch.openai.com@...nel.org> wrote:
>
> From: Christoph Paasch <cpaasch@...nai.com>
>
> When we have a (very) large number of nexthops, they do not fit within a
> single message. rtm_dump_walk_nexthops() thus will be called repeatedly
> and ctx->idx is used to avoid dumping the same nexthops again.
>
> The approach in which we avoid dumping the same nexthops is by basically
> walking the entire nexthop rb-tree from the left-most node until we find
> a node whose id is >= s_idx. That does not scale well.
>
> Instead of this inefficient approach, rather go directly through the
> tree to the nexthop that should be dumped (the one whose nh_id >=
> s_idx). This allows us to find the relevant node in O(log(n)).
>
> We have quite a nice improvement with this:
>
> Before:
> =======
>
> --> ~1M nexthops:
> $ time ~/libnl/src/nl-nh-list | wc -l
> 1050624
>
> real    0m21.080s
> user    0m0.666s
> sys     0m20.384s
>
> --> ~2M nexthops:
> $ time ~/libnl/src/nl-nh-list | wc -l
> 2101248
>
> real    1m51.649s
> user    0m1.540s
> sys     1m49.908s
>
> After:
> ======
>
> --> ~1M nexthops:
> $ time ~/libnl/src/nl-nh-list | wc -l
> 1050624
>
> real    0m1.157s
> user    0m0.926s
> sys     0m0.259s
>
> --> ~2M nexthops:
> $ time ~/libnl/src/nl-nh-list | wc -l
> 2101248
>
> real    0m2.763s
> user    0m2.042s
> sys     0m0.776s
>
> Signed-off-by: Christoph Paasch <cpaasch@...nai.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>