Intel 170 Servers, AS/400 RISC Server, 7xx Servers manual

there is network congestion or overruns to certain target system adapters, then increasing the value from the default=*NONE to 2 or something larger may improve performance. MAXLENRU for APPC on the mode description (MODD): If a value of *CALC is selected for the maximum SNA request/response unit (RU) the system will select an efficient size that is compatible with the frame size (on the LIND) that you choose. The newer LAN IOPs support IOP assist. Changing the RU size to a value other than *CALC may negate this performance feature.

•Some APPC APIs provide blocking (e.g., ICF and CPI-C), therefore scenarios that include repetitive small puts (that may be blocked) may achieve much better performance.

•A large transfer with the System i sending each record repetitively using the default blocking provided by OS/400 to the System i client provides the best level of performance.

•A large transfer with the System i flushing the communications buffer after each record (FRCDTA keyword for ICF) to the System i client consumes more CPU time and reduces the potential data rate. That is, each record will be forced out of the server system to the client system without waiting to be blocked with any subsequent data. Note that ICF and CPI-C support blocking, Sockets does not.

•A large transfer with the System i sending each record requiring a synchronous confirm (e.g., CONFIRM keyword for ICF) to the System is client uses even more CPU and places a high level of serialization reducing the data rate. That is, each record is forced out of the server system to the client system. The server system program then waits for the client system to respond with a confirm (acknowledgment). The server application cannot send the next record until the confirm has been received.

•Compression with APPC should be used with caution and only for slower speed WAN environments. Many suggest that compression should be used with speeds 19.2 kbps and slower and is dependent on the data being transmitted (# of blanks, # and type of repetitions, etc.). Compression is very CPU-intensive. For the CPB benchmark, compression increases the CPU time by up to 9 times. RLE compression uses less CPU time than LZ9 compression (MODD parameters).

•ICF and CPI-C have very similar performance for small data transfers.

•ICF allows for locate mode which means one less move of the data. This makes a significant difference when using larger records.

•The best case data rate is to use the normal blocking that OS/400 provides. For best performance, the use of the ICF keywords force data and confirm should be minimized. An application's use of these keywords has its place, but the tradeoff with performance should be considered. Any deviation from using the normal blocking that OS/400 provides may cause additional trips through the communications software and hardware; therefore, it increases both the overall delay and the amount of resources consumed.

•Having ANYNET = *YES causes extra CPU processing. Only have it set to *YES if it is needed functionally; otherwise, leave it set to *NO.

•For send and receive pairs, the most efficient use of an interface is with it's "native" protocol stack. That is, ICF and CPI-C perform the best with APPC, and Sockets performs best with TCP/IP. There is CPU time overhead when the "cross over" is processed. Each interface/stack may perform differently depending on the scenario.

•Copyfile with DDM provides an efficient way to transfer files between System i systems. DDM provides large blocking which limits the number of times the communications support is invoked. It also maximizes efficiencies with the data base by doing fewer larger I/Os. Generally, a higher data rate can be achieved with DDM compared with user-written APPC programs (doing data base accesses) or with ODF.

•When ODF is used with the SNDNETF command, it must first copy the data to the distribution queue on the sending system. This activity is highly CPU-intensive and takes a considerable amount of time. This time is dependent on the number and size of the records in the file. Sending an object to more than one target System i server only requires one copy to the distribution queue. Therefore, the realized data rate may appear higher for the subsequent transfers.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 5 - Communications Performance	75