CP6100 I/O Process Programming Manual

Using CP6100: Managing Lines
has overhead for new messages and message blocking; this
overhead includes processing time in the LIU and extra bytes
or bits in the data stream. The effects of changing specific
parameters are described later in this section.
• Line quality. Poor line quality results in frequent retries
of operations. Thus processing increases without a
corresponding increase in throughput.
• Protocol overhead and line delays. There is normally little
you can do about protocol overhead. Presumably you use a
protocol because your device requires it. In some cases, you
can make adjustments in the configuration; for example, if a
protocol polls its devices, you can change the poll interval
or the composition of the poll list. Line delays are an issue
if you use half-duplex lines (2-wire circuits); unless you
upgrade to full-duplex lines (4-wire circuits), there is
nothing you can do. There are also delays associated with
some media, notably satellite.
• The number of processes running in a CPU, and process
priorities. In addition to the fact that more processes do
more work--and that each process has to wait its turn while
others are working--there is overhead associated with
dispatching and swapping. Moving processes to different CPUs
and changing process priorities can often help performance.
• The number of lines on a path in a 6100 subsystem. It is
best to spread the work between controllers in a 6100
subsystem; it is usually best to spread the work, also,
between the CPUs. If the rest of the configuration leaves
the CPUs unbalanced, however, you might want to use one
CPU to control a whole subsystem (provided that this
strategy balances the total load on the CPUs).
October 1985
3-21