Tune TCP/IP buffer sizes
WAS 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.
Overview
Flow control can consume a significant amount of CPU time and result in additional network latency as a result of data transfer interruptions. IBM recommends that you increase buffer sizes 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 that the buffer accumulates more data than the system can process.
The default buffer size is 8 KB. The 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 you use the IBM Toolbox for Java JDBC driver, you can use custom properties to configure the buffer sizes for each data source. IBM recommends 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 exmaple, 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 your system. To change the system wide value, perform the following steps:
Procedure
Tune the TCP/IP buffer sizes.
- Change the TCP/IP configuration.
- Run the Change TCP/IP Attribute, CHGTCPA command.
- View and change the buffer sizes by pressing F4 on the Change TCP/IP Attributes window. The buffer sizes are displayed as the TCP receive and send buffer sizes. Type new values and save your changes.
- Recycle TCP/IP, then monitor CPU and paging rates to determine if they are within recommended system guidelines.
Results
Repeat this process until you determine the ideal buffer size.
What to do next
The TCP/IP buffer sizes are changed. Repeat this process until you determine the ideal buffer size.
Related tasks
Tuning the application serving environment