netdev - Re: UDP multicast packet loss not reported if TX ring overrun?

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Message-ID: <4A9410E9.6090602@gmail.com>
Date:	Tue, 25 Aug 2009 18:27:21 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Sridhar Samudrala <sri@...ibm.com>,
	Nivedita Singhvi <niv@...ibm.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: UDP multicast packet loss not reported if TX ring overrun?

Christoph Lameter a écrit :
> On Tue, 25 Aug 2009, Eric Dumazet wrote:
> 
>> It wont be very nice, because it'll add yet another 32bits counter in each socket
>> structure, for a unlikely use. While rx_drops can happen if application is slow.
> 
> tx_drops happen if the application sends too fast.
> 
> TX drop tracking is important due to the braindamaged throttling logic
> during send. If SO_SNDBUF is less than what happens to fit in the TX ring then the
> application will be throttled and no packet loss happens. If SO_SNDBUF is
> set high then the TX ring will overflow and packets are dropped.
> 
> We need some way to diagnose TX drops per socket as long as we have
> that mind boggling issue. TX drops means that one should reduce the size
> of the sendbuffer in order to get better throttling which reduces packet
> loss.
> 
>> Also, tx_drops might be done later and not noticed.
>>
>> Please read this old (and usefull) thread, with Alexey words...
>>
>> http://oss.sgi.com/archives/netdev/2002-10/msg00612.html
>>
>> http://oss.sgi.com/archives/netdev/2002-10/msg00617.html
>>
>>
>> So I bet your best choice is to set IP_RECVERR, as mentioned in 2002 by Jamal and Alexey :)
> 
> I read this just yesterday. IP_RECVERR means that the application wants to
> see details on each loss. We just want some counters that give us accurate
> statistics to gauge where packet loss is occurring. Applications are
> usually not interested in tracking the fate of each packet.

Yep,  but IP_RECVERR also has the side effect of letting kernel returns -ENOBUFS error
in sending and congestion, which was your initial point :)

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