netdev - Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif

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Date:	Mon, 31 Mar 2008 12:44:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl
Subject: Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of
	netif_rx()

* David Miller <davem@...emloft.net> wrote:

> I don't think it's safe.
> 
> Every packet you receive can result in a sent packet, which in turn 
> can result in a full packet receive path being taken, and yet again 
> another sent packet.
> 
> And so on and so forth.
> 
> Some cases like this would be stack bugs, but wouldn't you like that 
> bug to be a very busy cpu instead of a crash from overrunning the 
> current stack?

sure.

But the core problem remains: our loopback networking scalability is 
poor. For plain localhost<->localhost connected sockets we hit the 
loopback device lock for every packet, and this very much shows up on 
real workloads on a quad already: the lock instruction in netif_rx is 
the most expensive instruction in a sysbench DB workload.

and it's not just about scalability, the plain algorithmic overhead is 
way too high as well:

 $ taskset 1 ./bw_tcp -s
 $ taskset 1 ./bw_tcp localhost
 Socket bandwidth using localhost: 2607.09 MB/sec
 $ taskset 1 ./bw_pipe
 Pipe bandwidth: 3680.44 MB/sec

i dont think this is acceptable. Either we should fix loopback TCP 
performance or we should transparently switch to VFS pipes as a 
transport method when an app establishes a plain loopback connection (as 
long as there are no frills like content-modifying component in the 
delivery path of packets after a connection has been established - which 
covers 99.9% of the real-life loopback cases).

I'm not suggesting we shouldnt use TCP for connection establishing - but 
if the TCP loopback packet transport is too slow we should use the VFS 
transport which is both more scalable, less cache-intense and has lower 
straight overhead as well.

	Ingo
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