Re: GSO (was: Please enter issues into the issue tracker - Issue system organisation needed)

[Jesper Dangaard Brouer <hawk@comx.dk>]

On Fri, 2011-02-25 at 17:33 +0100, Eric Dumazet wrote:
> Le vendredi 25 février 2011 à 16:48 +0100, Jesper Dangaard Brouer a
> écrit :
> 
> > Yes, both servers (/proc/sys/net/ipv4/tcp_sack = 1).
> > 
> > I think that the bufferbloat theory is that SACKs will not work, due to
> > the long delays introduced by buffers(bloat).   In this case, you can
> > see on the graph, a max RTT around 150 ms and an average of 20 ms.  
> > 
> > While another, more well behaved path in the network to the speed
> > server, I would only see a max RTT around 25 ms and an average of 15 ms,
> > see:
> > http://people.netfilter.org/hawk/dropbox/bloat_vs_GSO/speed-to-pc314a-1.png
> > 
> > You can also see this path had an ave of 90Mbit/s, but with significant
> > throughput drops (the 92Mbit/s line is an artificial line on the graph).
> > This behavior is probaly caused by the GSO effect.
> > 
> > Disabling GSO on speed server fixed the problem as can be seen on graph:
> > http://people.netfilter.org/hawk/dropbox/bloat_vs_GSO/speed-to-grantoften-solved.png
> > 
> > The really strange part when troubleshooting this issue was that the
> > throughput as fine between the two customer end-boxes ("grantoften" and
> > "pc314a") as can be see here:
> > http://people.netfilter.org/hawk/dropbox/bloat_vs_GSO/pc314a-to-grantoften-1.png
> > 
> > 
> 
> Its a bit hard to interpret these graphs, I am a bit lost...
> What exactly is sampled ? Is it from tcpdump analysis or output from
> HTB/SFQ stats ?

The graph is generated (with GNUplot) with data from the
throughput-latency tool called "thrulay".  Its created by Stanislav
Shalunov, and its homepage is here: http://shlang.com/thrulay/

I really love this "thrulay" tool, as it measure both the throughput and
records the TCP sessions experienced delay.  And the output can be used
directly by GNUplot. Nice! :-)

-- 
Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network Kernel Developer
  Cand. Scient Datalog / MSc.CS
  Author of http://adsl-optimizer.dk
  LinkedIn: http://www.linkedin.com/in/brouer