Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

Andrew Morton wrote:

On Sat, 17 Jun 2006 16:23:34 -0700
Harry Edmon [off-list ref] wrote:

  

quoted

Andrew Morton wrote:
    

quoted

On Fri, 16 Jun 2006 09:01:23 -0700
Harry Edmon [off-list ref] wrote:

      

quoted

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 ]

  

quoted

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.

  

quoted

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