Why is this network connection so slow?

Question

I am having some problems with network performance speed on a Linux server running Ubuntu 9.10. Transfer speeds on all types of traffic are around 1.5MB/s on a 1000mbit/s wired ethernet connection. This server has achieved 55MB/s over samba in the recent past. I have not changed the hardware or network set-up. I do run updates on a regular basis and the latest and greatest from Ubuntu's repositories is running on this machine.

Hardware set-up

Desktop Windows PC - 1000 switch - 1000 switch - Linux server

All switches are netgear, and they all show a green light for their connections which means the connection is 1000mbit/s. The lights are yellow when the connection is only 100mbit/s. Other diagnostic information:

root@server:~# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0c:6e:3e:ae:36
          inet addr:192.168.1.30  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:6eff:fe3e:ae36/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:28678 errors:0 dropped:0 overruns:0 frame:0
          TX packets:73531 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:2109780 (2.1 MB)  TX bytes:111039729 (111.0 MB)
          Interrupt:22

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:113 errors:0 dropped:0 overruns:0 frame:0
          TX packets:113 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:23469 (23.4 KB)  TX bytes:23469 (23.4 KB)


root@server:~# ethtool eth0
Settings for eth0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised auto-negotiation: Yes
        Speed: 1000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pg
        Wake-on: g
        Current message level: 0x00000037 (55)
        Link detected: yes

root@server:~# mii-tool
eth0: negotiated 1000baseT-FD flow-control, link ok

The server thinks its got a 1000mbit/s connection. I have tested the speed of transfer by copying files using Samba. I have also used netcat (nc target 10000 < aBigFile) on the server to transfer to Windows (nc -l -p 10000) and saw similar levels of poor performance.

I tested the speed of the hard drives using hdparm and got:

root@server:~# hdparm -tT /dev/md0
/dev/md0:
 Timing cached reads:   1436 MB in  2.00 seconds = 718.01 MB/sec
 Timing buffered disk reads:  444 MB in  3.02 seconds = 147.24 MB/sec

Reading the same file for transfer using DD produced the following:

paul@server:/home/share/Series/New$ dd if=aBigFile of=/dev/null
3200369+1 records in
3200369+1 records out
1638589012 bytes (1.6 GB) copied, 12.7091 s, 129 MB/s

I am stumped. What could be causing the poor network performance which is 2 orders of magnitude lower than what the network is capable of?

Answer 1

Some things you should consider checking:

1. Duplex -- if one side thinks the link is full duplex and the other side thinks the link is half duplex, expect badness.
2. Defective switch? Bypass it/them.
3. Jumbo frames. 9000 byte MTU decreases overhead, which should increase throughput (forfeiting a little latency). It sounds like your problem is so bad that this won't help, though.
4. TCP features: ECN, SACK, congestion control alg
5. TCP Send/Receive window sizes (details for linux)

netperf is great at troubleshooting network performance. But netcat's not bad in a pinch.

Answer 2

In my professional experience, I've struggled to get good solid network performance with Samba on GNU/Linux. You mentioned you have achieved speeds of 55 MBps with it, which I believe, so I'm guessing something else is definitely at play.

However, have you tried NFS, FTP and SCP? Are the bandwidth issues consistent across the different protocols? If so, it's likely narrowed down to the physical connection. If you get inconsistent results, then it's likely a software problem.

Aside from testing the other protocols, are you using encryption on the transfer? For example, using rsync -z is sweet for enabling compression, but it's comes at a CPU cost, which severely impacts overall speed of the transfer. If using SSH with rsync, then you have encryption on top of compression, and your CPU will be under a bit of stress, causing severe speed penalties.

Answer 3

1. Try netstat -i and look for rx/tx errors.
2. Try netstat -s and look for tcp issues - compare values before and after the file copy and look for large spikes in resets or retransmits.

Answer 4

You might check the congestion of your network; perhaps some other devices are consuming all of your bandwidth?

Beyond that, maybe something is wrong with your network interface and/or its driver. Pretty weird.

Answer 5

If at all possible, to remove most doubt that it is indeed an OS/driver/card issue, connect the computers together using a cross over cable. This will remove the switch and other possible networking issues from your equation.