Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

[Stephen Hemminger <shemminger@osdl.org>]

Andrew Morton wrote:
> On Sat, 17 Jun 2006 16:23:34 -0700
> Harry Edmon <harry@atmos.washington.edu> wrote:
>
>   
> > Andrew Morton wrote:
> >     
> > > On Fri, 16 Jun 2006 09:01:23 -0700
> > > Harry Edmon <harry@atmos.washington.edu> wrote:
> > >
> > >       
> > > > I have a system with a strange network performance degradation from 
> > > > 2.6.11.12 to most recent kernels including 2.6.16.20 and 2.6.17-rc6.   
> > > > The system is has Dual single core Xeons with hyperthreading on.   The 
> > > > application is the LDM system from UCAR/Unidata 
> > > > (http://www.unidata.ucar.edu/software/ldm).   This system requests 
> > > > weather data from a variety of systems using RPC calls over a reserved 
> > > > TCP port (388), puts them into a memory mapped queue file, and then 
> > > > sends the data out to a variety of downstream requesting systems, again 
> > > > using RPC calls.  When the load is heavy, the 2.6.16.20 kernel falls way 
> > > > behind with the data ingestion.  The 2.6.11.12 kernel does not.   I have 
> > > > tried an experiment with a 2.6.17-rc6 system where it just does the 
> > > > ingestion, and not the downstream distribution, and it is able to keep 
> > > > up.   I would really appreciate any pointers as to where the problem may 
> > > > be and how to diagnose it.  I have attached the config files from both 
> > > > kernels and the sysctl.conf file I am using.   I have also included the 
> > > > output from "netstat -s" on the 2.6.16.20 system during a time when it 
> > > > was having problems.
> > > >
> > > >         
> > > (added netdev)
> > >
> > > A quick grep indicates that it isn't using TCP_NODELAY - we've had problems
> > > with that in the past.
> > >
> > > Perhaps a tcpdump of the net traffic will help to determine what's going on.
> > >       
>
> [ edit, edit - please don't top-post ]
>
>   
> > I assume you are talking about using TCP_NODELAY as a socket option within the 
> > LDM software.  I could give that a try.
> >     
>
> The use of TCP_NODELAY caused problems with the JVM debugger.  I'm not
> suggesting that enabling it will fix anything here.
>
>   
> > There is a lot of traffic on this node, on the order of 2000 packets in and out 
> > per second, so the tcpdump output will grow pretty fast.  How long a tcpdump 
> > would be useful, and what options would you suggest?
> >     
>
> I don't know, frankly - first one needs to develop some sort of theory,
> then use the diagnostic tools to prove or disprove that theory.  And I
> don't have a theory.
>
> I guess a simple one-second bare `tcpdump -i eth0' would be a starting
> point.  Perhaps compare the output of that with the output from a
> correctly-operating kernel, see if anything suggests itself.  That might
> also give us something which the networking developers can use.
>
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>   
Does this fix it?
   # sysctl -w net.ipv4.tcp_abc=0

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