43 lines
4.3 KiB
HTML
43 lines
4.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<META http-equiv="Content-Type" content="text/html; charset=utf-8">
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<LINK rel="stylesheet" type="text/css" href="../../../rzahg/ic.css">
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<title>TCP/IP buffer sizes</title>
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</head>
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<BODY>
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<h4><a name="prftunetcp"></a>TCP/IP buffer sizes</h4>
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<p>WebSphere Application Server - Express 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.</p>
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<p>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 that the buffer accumulates more data than the system can process.</p>
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<p>The default buffer size is 8KB. The maximum size is 8MB. The optimal buffer size depends on several network environment factors including types of switches and systems, acknowledgment timing, error rates, 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.</p>
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<p>Buffer sizes for the socket connections between the Web server and WebSphere Application Server - Express are set at 64KB. In most cases this value is adequate.</p>
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<p>Flow control can be an issue when an application uses either the IBM Developer Kit for Java<sup>(TM)</sup> 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. It is recommended that you specify large buffer sizes, such as 1MB.</p>
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<p>Some system-wide settings can override the default 8KB buffer size for sockets. With some applications, such as WebSphere Commerce Suite, a buffer size of 180KB reduces flow control and typically does not increase 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 follow these steps:</p>
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<ol>
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<li>Run the Change TCP/IP Attributes (CHGTCPA) command on the CL command line.</li>
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<li>On the <strong>Change TCP/IP Attributes</strong> display, press <strong>F4</strong>. The buffer sizes are displayed as the TCP receive and send buffer size.</li>
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<li>Specify new values.</li>
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<li>Save your changes.</li>
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<li>Recycle TCP/IP.</li>
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<li>Monitor CPU and paging rates to determine if they are within recommended system guidelines.</li>
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</ol>
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<p>Repeat this process until you determine the ideal buffer size.</p>
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<p>For more information about TCP/IP performance, see Chapter 5 of the Performance Capabilities Reference. Links to several editions of the Performance Capabilities Reference are listed in the <a href="http://www.ibm.com/servers/eserver/iseries/perfmgmt/resource.htm" target="_blank">Performance Management Resource Library</a>. <img src="www.gif" width="18" height="15" align="absbottom" alt="Link outside Information Center" border="0"></p>
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</body>
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</html>
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