Re: Poor thunderbolt-net interface performance when bridged

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 26 May 2025 16:28:23 +0200
Andrew Lunn <andrew@lunn.ch> wrote:

> > > Simple peer-to-peer, no routing nothing. Anything else is making things
> > > hard to debug. Also note that this whole thing is supposed to be used as
> > > peer-to-peer not some full fledged networking solution.  
> >   
> > > Let's forget bridges for now and anything else than this:
> > >  Host A <- Thunderbolt Cable -> Host B  
> > 
> > Right, but that's precisely what I'm digging into: red->blue runs at line speed,
> > and so does blue->purple. From what I understand about drivers and networking,
> > it doesn't make sense then that the red->blue->purple path drops down so much in
> > performance (9Gbps to 5Mbps)  
> 
> Agreed.
> 
> Do the interfaces provide statistics? ethtool -S. Where is the packet
> loss happening?
> 
> Is your iperf testing with TCP or UDP?  A small amount of packet loss
> will cause TCP to back off a lot. Also, if the reverse direction is
> getting messed up, ACKs are getting lost, TCP will also stall.
> 
> Maybe try a UDP stream, say 500Mbs. What is the packet loss? Try the
> reverse direction, what is the packet loss. Then try --bidir, so you
> get both directions at the same time.
> 
> 	Andrew
> 

What is MTU? Bridging is L2 and if MTU does not match packets are
silently dropped.