netdev - Re: loaded router, excessive getnstimeofday in oprofile

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Date:	Wed, 27 Aug 2008 09:17:06 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: loaded router, excessive getnstimeofday in oprofile

On Wed, 27 Aug 2008 14:54:12 +0200
Andi Kleen <andi@...stfloor.org> wrote:

> Evgeniy Polyakov <johnpol@....mipt.ru> writes:
> >
> > Yup, this innocent toys can end up with this such behaviour on modern
> > highly loaded machines.
> 
> I and also other people had some patches to move the time stamp
> measuring into the socket. This way the time stamping didn't need to
> be enabled on all packets, only on those that actually end up at a
> socket that requires the time stamp.
> 
> Unfortunately DaveM didn't like it because some bank wanted
> different semantics, see the discussion in 
> http://thread.gmane.org/gmane.linux.network/91679
> 
> Perhaps you can find out which bank it was and send them a bill for
> your CPU time ;-)
> 
> -Andi
> 

Look at /proc/net/ptype to see if any AF_PACKET sockets are open.
There are several causes of this:
   * Applications like DHCP use AF_PACKET when they could use something else
   * AF_PACKET API was poorly designed and always has timestamps
   * The choice was made to get more accurate timestamps by stamping early in
     receive code. A better alternative would be to do it in protocol handler
     after the socket filter. Sorry, Andi socket layer is too late.
   * No driver is using hardware mechanisms to get accurate/free timestamps.
     I was working on sky2, but never was stable/complete.

Easist advice now is to fix userspace.
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