ndt-users list created

[Richard Carlson <>]

Hi Dale;

At 02:07 PM 2/16/2006, Dale Blount wrote:
> On Wed, 2006-02-15 at 10:40 -0500, Richard Carlson wrote:
> > Hi Dale;
> >
> > I don't recall if I replied to this earlier but here it is again.
> >
> > I'm seeing this problem more and more and I am currently trying to find a
> > real fix for this problem.  Here's what's happening.
> >
> > The NDT server starts sending data to the remote client.  It enters a
> > simple send loop and pumps data into the network for 10 seconds.  It then
> > exits the loop and sends the final results over to the client.
> >
> > The problem is, that when in this loop it is possible for the OS to
> > transfer data to the TCP stack, faster than the TCP stack pump it out into
> > the network.  This results in a large standing queue, visible with the
> > netstat -nat command).  So at the end of 10 seconds the code stops pumping
> > more data but the client keeps reading until the queue empties.  Note the
> > Web100 Duration variable has a value of 34,893,925 microseconds, or almost
> > 35 seconds.
> >
> > One temporary step is to limit the max buffer space (tcp_wmax) to 
> something
> > in the 512 KB to 1MB range.  This will keep the queue from building up too
> > much, but it's really just a band-aid until I can figure out how to 
> monitor
> > the queue length to prevent such large queues in the first place.
> >
>
> Rich,
>
> I can't find a tcp_wmax setting, but here is what I have set:

Sorry that was a typo on my part.  It should have been tcp_wmem not _wmax..

> net.core.wmem_max = 131072
> net.core.rmem_max = 131072
> net.ipv4.tcp_wmem = 4096 16384 131072
> net.ipv4.tcp_rmem = 4096 16384 131072
>
> Upload always works OK, but on anything but the lan, download is right
> around 75k.  It doesn't really matter if it's set to 128kb/512kb/2Mb,
> it's always 70-80kb (both on a 5Mbps upload cable modem and a 768kbps
> upload dsl link, both 3 hops from the ndt server).
>
> The old server that this is replacing is still around, and speedtests to
> it work just fine.  Could the newer hardware alone be causing this whole
> problem?  I've tried the sysctl settings from the old server with the
> same results.

It could be a hardware issue or an OS issue.  Did you change/upgrade the OS 
level too?  I noticed a problem with my server when I went from Linux 
2.6.12 to 2.6 13.  I finally tracked it down to a change in the Intel 
FastEthernet (e100) NIC driver that came with the new OS.  I replaced the 
new e100.c file with the one from the .12 kernel and everything started 
working again.  I also have a report of a problem with a built in NIC, and 
the problem was resolved when a PCI bus based NIC was installed.  Perhaps 
this is a bigger problem than I realize.

> > If anyone has any suggestions on how to do this, please let me know.
> >
>
> Couldn't the client be adjusted to stop reading after 10 seconds?  It
> could then report the data transferred so far.

There is a timer that runs in the client to clean things up, but I either 
there's a bug in my code or something else is wrong and the timer isn't 
working.  I am currently testing a patch on my server at 
http://web100.internet2.edu:7123  It would help if you could try this 
server and let me know if the tests run long or what happens.

Please let me know what you find.

Regards;
Rich

> Thanks,
>
> Dale

------------------------------------



Richard A. Carlson                              e-mail: 

Network Engineer                                phone:  (734) 352-7043
Internet2                                       fax:    (734) 913-4255
1000 Oakbrook Dr; Suite 300
Ann Arbor,  MI  48104