Technical data

Tuning Techniques
2.1 Subsystem Attributes
When to Tune
Increase the number of hash buckets in the kernel TCP connection table if you
have an Internet server.
Recommended Values
The default value of the tcbhashsize attribute is 512. For Internet servers, set
the tcbhashsize attribute to 16384.
2.1.6.2 Increasing the Number of TCP Hash Tables
You can increase the number of hash tables the kernel uses to look up TCP
control blocks. Because the kernel must look up the connection block for
every Transmission Control Protocol (TCP) packet it receives, a bottleneck may
occur at the TCP hash table in SMP systems. Increasing the number of tables
distributes the load and may improve performance. The inet subsystem attribute
tcbhashnum specifies the number of TCP hash tables.
Performance Benefits and Tradeoffs
For SMP systems, you may be able to reduce hash table lock contention by
increasing the number of hash tables that the kernel uses to look up TCP control
blocks. This will slightly increase pooled memory.
You cannot modify the tcbhashnum attribute without rebooting the system.
When to Tune
Increase the number of TCP hash tables if you have an SMP system that is an
Internet server.
Recommended Values
The minimum value of the
tcbhashnum attribute is 1 (the default). The maximum
value is 64. For busy server SMP systems, you can increase the value of the
tcbhashnum attribute to 16. If you increase this attribute, you should also
increase the size of the hash table by a similar factor. See Section 2.1.6.1 for
more information.
2.1.6.3 Increasing the Size of the Kernel Interface Alias Table
The
inet subsystem attribute inifaddr_hsize specifies the number of hash
buckets in the kernel interface alias table ( in_ifaddr ).
If a system is used as a server for many different server domain names, each of
which is bound to a unique IP address, the code that matches arriving packets to
the right server address uses the hash table to speed lookup operations for the IP
addresses.
Performance Benefits and Tradeoffs
Increasing the number of hash buckets in the table can improve performance on
systems that use large numbers of aliases.
When to Tune
Increase the number of hash buckets in the kernel interface alias table if your
system uses large numbers of aliases.
You can modify the
inifaddr_hsize attribute without rebooting the system.
2–10 Tuning Techniques