WAS v8.5 > Tune performance > Tune the application serving environment
Tune TCP/IP buffer sizes
WebSphere Application Server uses the TCP/IP sockets communication mechanism extensively. For a TCP/IP socket connection, the send and receive buffer sizes define the receive window. The receive window specifies the amount of data that can be sent and not received before the send is interrupted. If too much data is sent, it overruns the buffer and interrupts the transfer. The mechanism that controls data transfer interruptions is referred to as flow control. If the receive window size for TCP/IP buffers is too small, the receive window buffer is frequently overrun, and the flow control mechanism stops the data transfer until the receive buffer is empty. Flow control can consume a significant amount of CPU time and result in additional network latency as a result of data transfer interruptions. It is recommended that you increase buffer sizes to avoid flow control under normal operating conditions. A larger buffer size reduces the potential for flow control to occur, and results in improved CPU utilization. However, a large buffer size can have a negative effect on performance in some cases. If the TCP/IP buffers are too large and applications are not processing data fast enough, paging can increase. The goal is to specify a value large enough to avoid flow control, but not so large the buffer accumulates more data than the system can process.
The default buffer size is 8 KB. Maximum size is 8 MB (8096 KB). The optimal buffer size depends on several network environment factors including types of switches and systems, acknowledgment timing, error rates and network topology, memory size, and data transfer size. When data transfer size is extremely large, you might want to set the buffer sizes up to the maximum value to improve throughput, reduce the occurrence of flow control, and reduce CPU cost.
Buffer sizes for the socket connections between the web server and WAS are set at 64KB. In most cases this value is adequate.
Flow control can be an issue when an application uses either the IBM Developer Kit for Java(TM) JDBC driver or the IBM Toolbox for Java JDBC driver to access a remote database. If the data transfers are large, flow control can consume a large amount of CPU time. If we use the IBM Toolbox for Java JDBC driver, we can use custom properties to configure the buffer sizes for each data source. It is recommended specified large buffer sizes, for example, 1 MB.
Some system-wide settings can override the default 8 KB buffer size for sockets. With some applications, for example, WebSphere Commerce Suite, a buffer size of 180 KB reduces flow control and typically does not adversely affect paging. The optimal value is dependent on specific system characteristics. You might need to try several values before you determine the ideal buffer size for the system.
For more information, see "TCP/IP network settings "in Run IBM WAS on System p and AIX: Optimization and Best Practices . In addition, see TCP streaming workload tuning.
For more information, see the following documents:
For more information, see "Network
configuration for WAS communication "in IBM WAS V6.1 on the Solaris
10 Operating System
For information on tuning TCP/IP buffer sizes, see the Windows
2000 and Windows Server 2003 TCP Featuresdocument. Consider
setting the TcpWindowSize value to either 8388608 or 16777216.