netdev - Re: scp stalls mysteriously

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Date:	Thu, 03 Dec 2009 16:34:12 +0100
From:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
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

Ilpo Järvinen wrote:
> 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.

Ok, you probable know better than me.
But, aren't the SKBs collapsed to SMSS size segments and then
counted? I thought so.
The 3*SMSS restriction is from RFC 3517, but of course you know.

> 
>> 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 agree.
I'm already trying to get scp stalling, but no luck so far. Neither with
artificially dropping packets, nor using WLAN :-(


Best regards,
Arnd
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