netdev - Re: scp stalls mysteriously

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Message-ID: <alpine.DEB.2.00.0912031630390.7024@wel-95.cs.helsinki.fi>
Date:	Thu, 3 Dec 2009 16:36:18 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
cc:	Frederic Leroy <fredo@...rox.org>,
	Damian Lukowski <damian@....rwth-aachen.de>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Greg KH <gregkh@...e.de>
Subject: Re: scp stalls mysteriously

On Thu, 3 Dec 2009, Arnd Hannemann wrote:

> Ilpo Järvinen wrote:
> 
> [snipped]
> 
> > Also, we have the another mystery to be solved, the fast retransmission is 
> > not triggered for some reason (or alternatively not captured in to a 
> > log), even in the working .9. case. It would be easy to see whether it 
> > works at all from TCP point of view by looking into mibs once you have 
> > have some transfers in a working configuration:
> > 
> > grep -A1 TCP /proc/net/netstat
> > 
> > ...luckily this fast retransmit issue is less crucial as almost all people 
> > are pretty happy already if their RTO-based recovery works even if the 
> > fast recovery would not. So figuring it out can be postponed (if one has 
> > to prioritize) until the silent death issue is out of the way.
> > 
> > 
> 
> I looked at the working .9 case stream from 192.168.1.15 to 192.168.1.19.
> I don't think it is a mystery that fast retransmit does not trigger.
> The condition SACKED_DATA > 3* SMSS is simply not fulfilled.
> Neither are there 3 non-continuous SACK sequences.
> The segments sent are too small :-(
> Interesting though, seems to me in this case non-SACK would be better than SACK.
> Or did I miss something?

Yes, a particularly big one, linux does not count SACKs bytes but packets. 
In the first recovery, plenty of packets are SACKed:

    135 sack 1 {2598:2646}>
    108 sack 1 {2598:2694}>
    121 sack 1 {2598:2742}>
     95 sack 1 {2598:2790}>
    426 sack 1 {2598:2838}>

fackets_out should be 6 now which is way more than 3 which is the 
default tp->reordering.

> Hey we could cook up a draft for this problem ;-)
>
> Anyway, real problem is, RTO does not trigger...

There are two problems. ...Both are real. ;-) But significance of the 
other is much worse than the other.

-- 
 i.