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: <bf2686f5-21a5-4654-9825-c883a626baad@blackwall.org>
Date: Sun, 17 Aug 2025 12:40:08 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: cpaasch@...nai.com, David Ahern <dsahern@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Ido Schimmel <idosch@...sch.org>
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/2] net: Make nexthop-dumps scale linearly
 with the number of nexthops

On 8/17/25 02:12, Christoph Paasch via B4 Relay 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>
> ---
>  net/ipv4/nexthop.c | 36 +++++++++++++++++++++++++++++++++---
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 

Very nice,
Reviewed-by: Nikolay Aleksandrov <razor@...ckwall.org>