netdev - Re: [PATCH net-next V2 1/3] net: Add GRO support for UDP encapsulating protocols

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Date:	Tue, 07 Jan 2014 12:32:15 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>, Jerry Chu <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Yan Burman <yanb@...lanox.com>,
	Shlomo Pongratz <shlomop@...lanox.com>
Subject: Re: [PATCH net-next V2 1/3] net: Add GRO support for UDP
 encapsulating protocols

On Tue, 2014-01-07 at 22:19 +0200, Or Gerlitz wrote:
> On Tue, Jan 7, 2014 at 6:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Tue, 2014-01-07 at 17:29 +0200, Or Gerlitz wrote:
> >>
> >> +
> >> +#define MAX_UDP_PORT (1 << 16)
> >> +extern const struct net_offload __rcu *udp_offloads[MAX_UDP_PORT];
> >
> > Thats 512 KB of memory.
> > This will greatly impact forwarding performance of UDP packets with
> > random ports, and will increase kernel memory size for embedded devices.
> 
> Re forwarding, are you referring to the case where the forwarded
> packets are encapsulated? packets which are not encapusalted will be
> flushed in the gro receive handler (this went out by mistake in V2 but
> exists in V1)  if skb->encapsulation isn't set.
> 

How do you know encapsulation must be tried for a given incoming
packet ? NIC do not magically sets skb->encapsulation I think...

You access udp_offloads[XXX], with XXX being in 0..65535 range, right ?


> As for encapsulated packets, when you say random ports, are you
> referring to a router which has multiple udp encapsulating protocols
> where each uses different udp port? for this case and also to reduce
> the memory footprint, we can use lookup in a list as done for the L2
> protocols gro handlers in the list_for_each loop of dev_gro_receive(),
> makes sense?

I am speaking of a normal router, running linux kernel, and having
GRO/TSO enabled.

If each incoming UDP packet has to access one extra cache line in a
512KB array, its likely to be an extra cache line miss, if UDP dest
port is mostly random (compared to ports used by very recent UDP
packets)



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