netdev - Re: [tbench regression fixes]: digging out smelly deadmen.

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Message-ID: <490AE1CD.9040207@cosmosbay.com>
Date:	Fri, 31 Oct 2008 11:45:33 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	ilpo.jarvinen@...sinki.fi, shemminger@...tta.com, zbr@...emap.net,
	rjw@...k.pl, mingo@...e.hu, s0mbre@...rvice.net.ru,
	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, efault@....de, akpm@...ux-foundation.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

David Miller a écrit :
> From: "Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
> Date: Fri, 31 Oct 2008 11:40:16 +0200 (EET)
> 
>> Let me remind that it is just a single process, so no ping-pong & other 
>> lock related cache effects should play any significant role here, no? (I'm 
>> no expert though :-)).
> 
> Not locks or ping-pongs perhaps, I guess.  So it just sends and
> receives over a socket, implementing both ends of the communication
> in the same process?
> 
> If hash chain conflicts do happen for those 2 sockets, just traversing
> the chain 2 entries deep could show up.

tbench is very sensible to cache line ping-pongs (on SMP machines of course)

Just to prove my point, I coded the following patch and tried it
on a HP BL460c G1. This machine has 2 quad cores cpu 
(Intel(R) Xeon(R) CPU E5450  @3.00GHz)

tbench 8 went from 2240 MB/s to 2310 MB/s after this patch applied

[PATCH] net: Introduce netif_set_last_rx() helper

On SMP machine, loopback device (and possibly others net device)
should try to avoid dirty the memory cache line containing "last_rx"
field. Got 3% increase on tbench on a 8 cpus machine.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
---
 drivers/net/loopback.c    |    2 +-
 include/linux/netdevice.h |   16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)


View attachment "netif_set_last_rx.patch" of type "text/plain" (1233 bytes)