HP-UX TCP/IP Performance White Paper, March 2008

4.1.1.6 tcphashsz

tcphashsz controls the size of several hash tables maintained within the kernel. For better performance it

is better to have larger tables at the expense of more memory being used when there is a large number of

concurrent connections in the system. On modern-day servers memory may not be a major constraint.

When Web server and application servers are run on the same physical machine, the suggested value for

this tunable parameter is 32768. If Web server and application servers are running on different machines,

then the number of concurrent connections on the application server may not be very large. In this case the

default value (number of CPUs * 1024) should suffice. This parameter is set using the following command:

# kctune tcphashsz=32768

Please note that system has to be rebooted for the new value to take effect. Otherwise system will continue

to use the current value of the tcphashsz parameter.

Refer to section 4.2.3 on page 21 for more discussion on tuning tcphashsz.

4.1.2 Tuning Database Servers

There are several different database systems that are deployed today on HP-UX. Typically networking may

not be a bottleneck on a database server as compared to I/O. Nevertheless, the following tuning may help

improve the overall efficiency from a networking perspective.

4.1.2.1 tcp_xmit_hiwater_def

This parameter controls the amount of unsent data that can be queued to the connection before subsequent

attempts by the application to send data will cause the call to block or return EWOULDBLOCK/EAGAIN if

the socket is marked non-blocking. For typical OLTP types of transactions (short requests and short

responses) this parameter needs no tuning. However, you may consider increasing it to 65536 from the

default value 32768 for DSS (Decision Support System) or BI (Business Intelligence) workloads that require

a large amount of data to be transferred from the database server. Furthermore, this may help in data

backups from database servers to an external storage device over a network-attached storage (NAS).

4.1.2.2 socket_udp_rcvbuf_default

Cluster based database technologies are becoming popular. Typically nodes of such database clusters

communicate among themselves using UDP. A large amount of data may get exchanged between server

nodes in a database cluster connected through an interconnect. In this case you may want to consider

increasing the tunable parameter socket_udp_rcvbuf_default which defines the default receive

buffer size for UDP sockets. If the command netstat –p udp shows socket overflows, it might be

desirable to increase this tunable parameter. It should be noted that increasing the size of the socket buffer

only helps if the length of the overload condition is short and the burst of traffic is less than the size of the

socket buffer. Increasing the socket buffer size will not help if the overload is sustained.

4.1.2.3 socket_udp_sndbuf_default

As mentioned above, cluster based database technologies may use UDP to communicate between nodes in

the cluster. This tunable parameter sets the default send buffer size for UDP sockets. The default value for

this tunable parameter is 65536, which is optimal for cluster based database technologies used on HP-UX.