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

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Date:	Mon, 17 Aug 2009 17:12:12 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Nivedita Singhvi <niv@...ibm.com>, netdev@...r.kernel.org,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: UDP multicast packet loss not reported if TX ring overrun?

On Mon, 2009-08-17 at 18:52 -0400, Christoph Lameter wrote:
> On Mon, 17 Aug 2009, Sridhar Samudrala wrote:
> 
> > On Mon, 2009-08-17 at 18:13 -0400, Christoph Lameter wrote:
> > > On Mon, 17 Aug 2009, Sridhar Samudrala wrote:
> > >
> > > > What about ethtool -S ? Does it report any errors?
> > >
> > > Neither. This is is a broadcom bnx2 NIC.
> >
> > Are you sure the packets are dropped at the sender?
> 
> Yes I am sending 400k messages from the app and the receiver only gets
> 341k @300 byte (which is the line rate). There is no way that the 400k get
> over the line. Also if I reduce SO_SNDBUF then both receiver and
> sender get down to 341k.
> 
> I added the output of ethtool -S at the end.
> 
> The mcast tool can be had from http://gentwo.org/ll or from my directory
> on www.kernel.org.
> 
> > Another place where packet drops/errors are counted is
> >     /proc/net/softnet_stat
> > It tracks some counters that could result in drops. I thought these are
> > all receive statistics. But looks like cpu_collision is a tx stat. The
> > name of the structure is netif_rx_stat and it includes cpu_collison
> > counter.
> 
> How do I decode that information?
  total dropped time_squeeze 0 0 0 0 0 cpu_collision

The first 3 are rx stats and the last one is a tx stat.
Anyway, only the first field(total packets received) seems to be non-zero 
in your softnet_stat output on both sender and receiver.
So it is possible that there is some other place in the stack where the packets 
are gettting dropped but not counted.

-Sridhar


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