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Date: Thu, 16 Apr 2015 06:49:05 +0100
From: Patrick McHardy <kaber@...sh.net>
To: Jesper Dangaard Brouer <netdev@...uer.com>
Cc: Daniel Borkmann <daniel@...earbox.net>,
Pablo Neira Ayuso <pablo@...filter.org>,
netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
davem@...emloft.net
Subject: Re: [PATCH 1/7] net: refactor __netif_receive_skb_core
On 15.04, Jesper Dangaard Brouer wrote:
> > Out of curiosity, what is actually the performance impact on all
> > of this? We were just arguing on a different matter on two more
> > instructions in the fast-path, here it's refactoring the whole
> > function into several ones, I presume gcc won't inline it.
>
> Pablo asked me to performance test this change. Full test report below.
>
> The performance effect (of this patch) depend on the Gcc compiler
> version.
>
> Two tests:
> 1. IP-forwarding (unloaded netfilter modules)
> 2. Early drop in iptables "raw" table
>
> With GCC 4.4.7, which does not inline the new functions
> (__netif_receive_skb_ingress and __netif_receive_skb_finish) the
> performance impact/regression is definitly measurable.
>
> With GCC 4.4.7:
> 1. IP-forwarding: +25.18 ns (slower) (-27776 pps)
> 2. Early-drop : +7.55 ns (slower) (-66577 pps)
>
> With GCC 4.9.1, the new functions gets inlined, thus the refactor
> splitup of __netif_receive_skb_core() is basically "cancled".
> Strangly there is a small improvement for forwarding, likely due to
> some lucky assember reordering that give less icache/fetch-misses.
> The early-drop improvement is below accuracy levels, can cannot be
> trusted.
>
> With GCC 4.9.1:
> 1. IP-forwarding: -10.05ns (faster) (+17532 pps)
> 2. Early-drop : -1.54ns (faster) (+16216 pps) below accuracy levels
>
> I don't know what to conclude, as the result depend on the compiler
> version... but these kind of change do affect performance, and should
> be tested/measured.
Thanks Jesper. This effect without inlinging was to be expected I guess.
The interesting question would be a patch that uses nf_hook() without okfn
callback, moves the ingress qdisc to register with the netfilter ingress
hook and moves the TTL tracking of ing_filter() to the ingress qdisc,
where it belongs.
My expectation would be that this would result in an overall performance
gain.
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