netdev - Re: [bug] stuck localhost TCP connections, v2.6.26-rc3+

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Date:	Mon, 26 May 2008 15:59:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [bug] stuck localhost TCP connections, v2.6.26-rc3+


* Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:

> > in terms of debugging there's not much i can do i'm afraid. It's not 
> > possible to get a tcpdump of this incident, given the extreme amount 
> > of load these testboxes handle.
> 
> ...but you can still tcpdump that particular flow once the situation 
> is discovered to see if TCP still tries to do something, no? One needs 
> to tcpdump couple of minutes at minimum. Also please get /proc/net/tcp 
> for that flow around the same time.

ok, will try those.

> > One clue (which might or might not matter) is that distcc is one of 
> > the very few applications that makes use of sendfile().
> 
> Can you please try with /proc/sys/net/ipv4/tcp_frto set to zero though 
> recv-q symptom seems weird would it be related to that (but there were 
> some recent fixes to FRTO and retrans_stamp change could have some 
> significance here)?
> 
> Other than that, nothing since -rc1 seems suspicious to me (though I 
> hardly understand every part of networking).

ok, i will first wait for it to trigger on a box and will do the tcpdump 
session (and /proc/net/tcp output), then i'll continue the tests with 
this done in the rc.local:

  echo 0 > /proc/sys/net/ipv4/tcp_frto

and will see whether the hung connections still occur. The cycle of 
testing will be very slow i suspect.

	Ingo
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