netdev - Re: loaded router, excessive getnstimeofday in oprofile

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Message-ID: <20080826205158.GA15266@2ka.mipt.ru>
Date:	Wed, 27 Aug 2008 00:51:58 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Denys Fedoryshchenko <denys@...p.net.lb>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: loaded router, excessive getnstimeofday in oprofile

On Tue, Aug 26, 2008 at 10:44:56PM +0200, Eric Dumazet (dada1@...mosbay.com) wrote:
> >Do you have any packet sockets in this system? Like running dhcp daemon?
> >
> 
> Another way to see this problem can be to start a sniffer on the machine, 
> even with a restrictive pcap filter, to check if performance change or not. 
> (It should decrease)

Or just check /proc/net/packet iirc.
Anyway, having at least one packet socket ends up with timestamping of
each packet, so you will get fair load of getnstimeofday() in that case.

> For example, I believe that running "ping" could have the same effect
> (increasing netstamp_needed variable : every incoming packet has to be 
> timestamped)
> 
> So beware of pings, traceroute and other networking tools...

Yup, this innocent toys can end up with this such behaviour on modern
highly loaded machines.

-- 
	Evgeniy Polyakov
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