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SubjectRe: Network performance degradation from 2.6.11.12 to 2.6.16.20
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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