IBM REDP-4285-00 4.7 Tuning the network subsystem

Chapter 4. Tuning the operating system 125

Draft Document for Review May 4, 2007 11:35 am 4285ch04.fm

Streaming and sequential content usually benefits from large stripe sizes by reducing disk

head seek time and improving throughput, but the more random type of activity, such as that

found in databases, performs better with a stripe size that is equivalent to the record size.

4.7 Tuning the network subsystem

The network subsystem should be tuned when the OS is first installed as well as when there

is a perceived bottleneck in the network subsystem. A problem here can affect other

subsystems: for example, CPU utilization can be affected significantly, especially when packet

sizes are too small, and memory use can increase if there is an excessive number of TCP

connections.

4.7.1 Considerations of traffic characteristics

One of the most important considerations for network performance tuning is to understand

network traffic patterns as accurately as possible. Keep in mind that performance greatly

varies depending on the network traffic characteristics.

For example, the following two figures shows the result of throughput performance using

netperf and they illustrate quite different performance characteristics. The only difference is

traffic type. Figure 4-14 shows the result of TCP_RR type traffic and TCP_CRR type traffic

(refer to 2.4.3, “netperf” on page 73). This performance difference is mainly caused by the

TCP session connect and close operations overhead and the major factor is Netfilter

connection tracking (refer to 4.7.6, “Performance impact of Netfilter” on page 133).

Figure 4-14 An example result of netperf TCP_RR and TCP_CRR benchmark

As we have shown here, even in exactly the same configuration, performance varies greatly

depending on even slight traffic characteristics differences. You should take much care of

network traffic characteristics and requirements. At least be familiar or have a reasonable

guess about the followings.

򐂰Transaction throughput requirements (peak, average)

򐂰Data transfer throughput requirements (peak, average)

򐂰Latency requirements

򐂰Transfer data size

򐂰Proportion of send and receive

򐂰Frequency of connection establishment and close or number of concurrent connections.

򐂰Protocol (TCP, UDP and application protocol such as HTTP, SMTP, LDAP etc.)

TCP_CRR benchmark

500

1000

1500

2000

2500

3000

3500

4000

1024 2048 4096 8192 16384 32768 65536 131070 262144

remote send socket size

trans per sec

128

1024

1460

4096

16384

32768

65536

131072

Data size

(bytes)

TCP_RR benchmark

2000

4000

6000

8000

10000

12000