8.1 General performance considerations When discussing performance and response times, keep in mind that performance can be influenced by many factors: Application programs Performance is influenced by obvious elements, such as application path lengths, large memory moves, and I/O operations. I/O response is subject to considerable tuning in a variety of ways. Furthermore, the structure of the application itself must be considered, for example, the choice of single-thread or multiple thread design is important. An application design that permits multiple instances of itself to be active has obvious advantages. This chapter concentrates on network aspects of performance; application design is largely outside the scope of the current discussion. TCP window size The TCP window governs the amount of data that the sender can transmit to the receiver before an acknowledgement is returned. A larger window can optimize normal traffic, but has a higher processing impact if frame retransmission is needed. The size of the TCP window is adjustable. The maximum standard window size is 65,535 bytes. Optionally, RFC 1323 (window scaling) can be employed. The maximum window with RFC 1323 is 1,073,725,440 bytes. With z/OS, the maximum allowable window and the use of window scaling is determined by the TCP send and receive buffer settings. Window scaling is automatically turned on when the TCP send buffer is set above 64 KB. HiperSockets MTU size HiperSockets TCP/IP devices are configured similar to OSA-Express QDIO devices. Each