[NBLUG/talk] Extremely poor network performance

Eric T. Landerville eric at reltechs.com
Sun Mar 26 15:38:51 PST 2006


Lincoln Peters wrote:

> I have one box running Debian/testing on my network that has been 
> demonstrating hideously poor performance on my Ethernet LAN.  I tried 
> uploading a large directory to it via scp, and the throughput for each 
> file rarely exceeds 10KB/s.  There is also a delay of several seconds 
> between when one file finishes uploading and another begins.  At this 
> rate, it will take several weeks to upload a 35GB directory on a 
> 100BaseTX LAN!
>
>
> Two days ago, I tried pinging the misbehaving Linux box (60 pings 
> total, at 1 ping per second) from the machine I was trying to upload 
> from (a Macintosh running MacOS 10.4.5).  The results were:
>
> round-trip min/avg/max/stdev = 0.507/285.772/911.228/233.467 ms
>
>
> I had heard that this particular type of Macintosh had been reported 
> to have issues with network performance, so at first I suspected it 
> was having problems.  So I tried pinging two other Linux boxes from 
> the Macintosh:
>
> round-trip min/avg/max/stdev = 0.377/0.456/0.671/0.046 ms
> round-trip min/avg/max/stdev = 0.451/0.624/0.934/0.060 ms
>
> So apparently, the problem was NOT on the Mac's end.
>
>
> I didn't have time to invesigate further that day, so the next day, I 
> went to the misbehaving box and tried pinging two other Linux boxes on 
> the LAN:
>
> rtt min/avg/max/mdev = 0.240/548.421/1817.558/499.307 ms, pipe 2
> rtt min/avg/max/mdev = 0.146/408.204/1478.301/358.105 ms, pipe 2
>
>
> Then I tried pinging the misbehaving box from the Macintosh again, and 
> the results were even worse:
>
> round-trip min/avg/max/stdev = 0.433/1512.158/5762.936/1319.562 ms
>
>
> As far as I could tell, there is virtually no other traffic going in 
> or out of the computer that would explain these wildly varying 
> response times.  But just to be sure, I disabled every server program 
> that might have beeen generating network traffic.  This morning, I 
> tried using the misbehaving box to ping another Linux box:
>
> rtt min/avg/max/mdev = 0.151/4980.819/21034.876/4896.826 ms, pipe 10
>
>
> Then I tried pinging my mail server (which is on the Internet, and 
> therefore accessed via my router), and the results were almost shocking:
>
> 60 packets transmitted, 53 received, 11% packet loss, time 1058068ms
> rtt min/avg/max/mdev = 60.530/4647.363/22086.280/4880.662 ms, pipe 13
>
> (I indicate packet loss here because this is the only test I ran that 
> experienced any packet loss.)
>
> To make things even stranger, there was a huge lag between the 
> transmission of each ping.  Although I had indicated that a total of 
> 60 pings should be sent out at 1-second intervals, the entire 
> operation (according to the "time" command) took 18 minutes, 37.984 
> seconds!
>
>
> To be thorough, I tried using the Macintosh to ping the same mail server:
>
> 60 packets transmitted, 60 packets received, 0% packet loss
> round-trip min/avg/max/stdev = 60.243/62.168/65.857/1.105 ms
>
>
> Finally, I tried restarting the network connection on this computer 
> (as per "ifdown" and "ifup").  Now it seems to be fine, but I can't 
> figure out why.  Seeing as how this computer is running Linux and 
> therefore should not be subject to random failures, I worry that the 
> problem will come up again, that and it might even get worse over time.
>
> Any ideas what might be going on?

Have you tried looking at layer 1 issues, aka the physical medium.  I 
would try changing the cable to the switch router/first, and it you get 
nothing from that try swapping out the NIC in the debian box.  If you 
are not seeing any problems with the software check the hardware.  I 
have experience the same type of problems with CAT 6 cables that had to 
been pulled too hard.  The twists become stretched and will no longer 
work as expected.  If you have access to a cable tester you might want 
to check for NEXT (Near End Cross Talk) or more rarely FEXT (Far End 
Cross Talk).  It's usually easier to just swap the cables out and toss 
the old one if it is bad.
Eric




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