IBM 170 Servers Manual page 75

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.
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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.
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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.
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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.
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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.
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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).
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ICF and CPI-C have very similar performance for small data transfers.
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ICF allows for locate mode which means one less move of the data. This makes a significant
difference when using larger records.
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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.
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Having ANYNET = *YES causes extra CPU processing. Only have it set to *YES if it is needed
functionally; otherwise, leave it set to *NO.
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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.
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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.
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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
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Copyright IBM Corp. 2008 Chapter 5 - Communications Performance 75