Technical data

Tuning Techniques
2.1 Subsystem Attributes
2.1.5 Modifying Socket Subsystem Attributes
The socket subsystem attributes control the maximum number of pending
connection attempts per server socket (that is, the maximum depth of the listen or
SYN queue) and other behavior. You may be able to improve server performance
by modifying the socket subsystem attributes described in Table 2–2.
Table 2–2 socket Subsystem Attributes
Attribute Description
somaxconn Controls the maximum number of pending TCP
connections.
sominconn Controls the minimum number of pending TCP
connections.
sb_max Controls the maximum size of a socket buffer.
In addition, the socket subsystem attributes sobacklog_hiwat , sobacklog_drops ,
and somaxconn_drops track events related to socket listen queues. By monitoring
these attributes, you can determine whether the queues are overflowing.
2.1.5.1 Increasing the Maximum Number of Pending TCP Connections
The socket subsystem attribute somaxconn specifies the maximum number of
pending TCP connections (the socket listen queue limit) for each server socket
(for example, for the HTTP server socket). Busy servers often experience large
numbers of pending connections. If the listen queue connection limit is too small,
incoming connection requests may be dropped. Pending TCP connections can be
caused by lost packets in the internet or denial of service (DOS) attacks.
The default value for
somaxconn is 1024.
HP recommends increasing the somaxconn attribute to the maximum value,
except on low-memory systems. The maximum value is 65535. Specifying a value
that is higher than the maximum value can cause unpredictable behavior.
2.1.5.2 Increasing the Minimum Number of Pending TCP Connections
The
socket subsystem attribute sominconn specifies the minimum number of
pending TCP connections (backlog) for each server socket. This attribute controls
how many SYN packets can be handled simultaneously before additional requests
are discarded. Network performance can degrade if a client saturates a socket
listen queue with erroneous TCP SYN packets, effectively blocking other users
from the queue.
The value of the sominconn attribute overrides the application-specific backlog
value, which may be set too low for some server software. If you do not have your
application source code, you can use the
sominconn attribute to set a sufficient
pending-connection quota.
The default value is 0.
HP recommends increasing the value of the sominconn attribute to the maximum
value of 65535. The value of the sominconn attribute should be the same as the
value of the somaxconn attribute (see Section 2.1.5.1).
2–8 Tuning Techniques