netdev - Re: [TCP bug] stuck distcc connections in latest -git

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Date:	Wed, 23 Jul 2008 10:26:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Willy Tarreau <w@....eu>
Cc:	David Newall <davidn@...idnewall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>
Subject: Re: [TCP bug] stuck distcc connections in latest -git


* Willy Tarreau <w@....eu> wrote:

> On Tue, Jul 22, 2008 at 05:34:43PM +0200, Ingo Molnar wrote:
> > * David Newall <davidn@...idnewall.com> wrote:
> > 
> > > Ingo Molnar wrote:
> > > > * David Newall <davidn@...idnewall.com> wrote:
> > > >   
> > > >> You really should start that capture, and on both client and server. 
> > > >> You don't need to dump everything, only traffic to or from 
> > > >> server:distcc.
> > > >>     
> > > >
> > > > It's not feasible. That box did in excess of 200 GB of network traffic 
> > > > in the past 7 hours alone.
> > > 
> > > You only need distcc traffic, and perhaps only after it's hung.  With 
> > > 250k outstanding per socket, are you certain that no traffic was sent? 
> > > Is it certain that one packet wasn't being sent each three minutes?  I 
> > > suppose you're right and the stack really is stuck, but this is such 
> > > an easy thing to check and eliminate that you should do so.  I 
> > > suppose, too, that you should trace the server-side processes and 
> > > confirm that they are waiting for socket input.  You should dump tcp 
> > > (for the distcc port) next time the problem recurs and also check that 
> > > the server processes are waiting for socket input.
> > 
> > ok, will do that if it happens again.
> 
> Ingo,
> 
> if it can help, I have a "capture" script which allows you to define a 
> size and will rotate captures within that size. That's what I'm using 
> to troubleshoot rarely occuring problems in datacenters, so it's 
> horrible but efficient :-)
> 
> You just have to stop it once the problem has happened again. Ping me 
> if you're interested (I'm lazy to start my laptop right just for it 
> now in fact).

yeah, that would be handy, thanks.

Alas, the problem has not reoccured since then - more than a thousand 
kernel builds down the line. Yesterday it triggered so quickly when i 
updated the buildbox to the new kernel, and happened repeatedly when i 
tried to build a new kernel, that i didnt assume it was something hard 
to reproduce - but it went poof after i restarted distccd on the server.

So i'd suggest we do not count this as a regression, i've got no way at 
the moment of reproducing it reliably.

	Ingo
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