CP6100 I/O Process Programming Manual

Using CP6100: Managing Lines
Monitoring Line Performance
To evaluate the performance of a line, you consider three major
factors:
• The throughput of the line: the amount of data an application
can transfer in a given time. (Because control characters and
other protocols are transparent and uninteresting to business
applications, they are best excluded from this figure.) Every
line has a theoretical limit, the data rate of the slowest
piece of hardware--a modem, a multiplexer, a terminal, a
controller, or the transport medium itself. The real
throughput is always somewhat lower than the theoretical
maximum. Some factors that influence throughput are
enumerated below.
• The overhead for controlling the line: the CPU utilization by
the I/O process and application processes.
• The efficiency of the configuration: the cost in CPU usage
for every byte transferred. Efficiency is the relationship
between throughput and overhead.
When you tune a line, you normally try to improve its efficiency.
You want to move as much data as you can on the smallest possible
number of lines and with the least possible overhead. (Of
course, there should be enough capacity to handle peak traffic
and path switches.)
The following factors influence the efficiency of a line:
• The number of lines served by an I/O process. If the increase
in CPU usage for each new line is non-linear, i.e., if with
each additional line the increase in CPU usage grows larger,
you should think about moving some lines to other I/O
processes. The same consideration applies to the number of
devices on a line.
• The number of lines with which an application communicates.
An application process that handles multiple lines must have
logic to treat the activity on those lines independently; Such
a process probably also maintains contextual data for each
line. As you add lines to the workload of the process, you
increase not only the CPU usage, but also the instances of
swapping the context data in and out. Balance the application
workload by creating copies of the application process and
dividing the lines among the copies so the overhead is equal.
• Message, frame, and buffer sizes. These parameters affect the
use of pool space by the I/O process. (See, for instance, the
discussion of error 33 in Appendix A.) Also, each protocol
October 1985
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