Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

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