IBM REDP-4285-00 User Manual

4285ch04.fm

Draft Document for Review May 4, 2007 11:35 am

126

Linux Performance and Tuning Guidelines

netstat

, tcpdump and ethereal are useful tools to get more accurate characteristics (Refer to

2.3.11, “netstat” on page 53 and 2.3.13, “tcpdump / ethereal” on page 55).

4.7.2 Speed and duplexing

It may sound trivial but one of the easiest ways to improve network performance is by
checking the actual speed of the network interface because there can be issues between
network components (such as switches or hubs) and the network interface cards. The
mismatch can have a large performance impact as shown in Example 4-17.

Example 4-17 Using ethtool to check the actual speed an duplex settings

[root@linux ~]# ethtool eth0
Settings for eth0:
      Supported ports: [ MII ]
      Supported link modes:   10baseT/Half 10baseT/Full
                              100baseT/Half 100baseT/Full
                              1000baseT/Half 1000baseT/Full
      Supports auto-negotiation: Yes
      Advertised link modes: 10baseT/Half 10baseT/Full
                              100baseT/Half 100baseT/Full
                              1000baseT/Half 1000baseT/Full
      Advertised auto-negotiation: Yes
      Speed: 100Mb/s
      Duplex: Full

From the benchmark results shown in Figure 4-15, note that a small data transfer is less
impacted than a larger data transfer when network speeds are incorrectly negotiated.
Especially data transfers larger than 1KB shows the drastic performance impact (throughput
declines 50-90%). Make sure that the speed and duplex are correctly set.

Figure 4-15 Performance degradation caused by auto negotiation failure

Numerous network devices default to 100 Mb half-duplex in case of a minor mismatch during
the auto negotiation process. To check for the actual line speed and duplex setting of a
network connection, use the ethtool command.

Note that most network administrators believe that the best way to attach a network interface
to the network is by specifying static speeds at both the NIC and the switch or hub port. To

100Mbsp half duplex

0.01

0.10

1.00

10.00

100.00

1,000.00

1024

2048

4096

8192

16384 32768 65536 131070 262144

socket sizes

es/

sec

128

16K

32K

64K

128K

Response
data size

1Gbps full duplex