IBM AS/400 - Manual (Page 14)

v For interactive environments, keep line use below 30% to maintain predictable and consistent response
times. Exceeding 50% line use usually slows down response time. The line use can be measured with
the AS/400 performance tools.
v For large transfer environments, or for environments in which only a small number of users are sharing
a line, increase line use to allow for acceptable response times.
v The CPU usage for fractional T1 support and other high-speed WAN connections is similar to any other
line that runs the same type of work. As the speed of a line increases from a traditional low speed to a
high-speed or full T1/E1/J1 speed, performance characteristics may change as follows:
– With interactive transactions, performance may be slightly faster.
– With a large transfer, performance may be significantly faster.
– With a single job, performance may be too serialized to use the entire bandwidth.
– With high throughput, performance is more sensitive to frame size.
– With high throughput, performance is more sensitive to application efficiency.
– With synchronous data link control (SDLC), the communications controller CPU usage increases
because of polling.
Additional considerations for adjusting the wide area network line speed are the following:
v A common misconception about the line speed of each attached communications line is that central
processing unit (CPU) resource is used in a uniform fashion. Exact statements cannot be made about
the number of lines that any given AS/400 model can support.
v Most communications applications use a lot of CPU resource (to process data, to support disk input and
output) and communications line resource (to send and receive data or display I/O). The amount of line
resource that is used is proportional to the total number of bytes that are sent or received on the line.
Some additional CPU resource is used to process the communications software to support the
individual sends (puts or writes) and receives (gets or reads). Communications input/output processor
resource is also used to support the line activity.
v When a single job is running disk operations or doing non-overlapped CPU processing, the
communications link is idle. If several sessions transfer concurrently, then the jobs are more interleaved
and make better use of the communications link.
v Polling is an important consideration for synchronous data link control (SDLC) environments. All SDLC
polling is handled by the communications controller and is governed by parameters in both the line and
controller descriptions.
v For information about AS/400 configuration, see the Communications Configuration
book.
v For more information about performance tools, see the Performance Tools for AS/400 book.
WAN line speed considerations for IOPs
When configuring a communications controller, you should consider both subsystem storage and
aggregate line speed. Subsystem storage is the amount of storage available on the communications
controller. Aggregate line speed is the sum of individual lines speeds that are attached to the
communications controller.
The following information can help you understand network line speed considerations for input/output
processors (IOPs).
v For interactive environments, you should not exceed 60% use on the communications IOP. Exceeding
this threshold in a large transfer environment or with a small number of concurrent users may still offer
acceptable performance. Use the AS/400 performance tools to get the utilization.
v You can attach multiple IOPs to an AS/400 system. The maximum number of IOPs that can be attached
is determined by the AS/400 model. It is important to distribute the work load across several IOPs if the
performance capabilities of a single IOP is exceeded.
v Even though an IOP can support certain configurations, a given AS/400 model may not have enough
system resource (for example, CPU processing capacity) to support the work load over the lines.
v The use of larger frames generally improves large transfer performance in terms of capacity for the
communications IOP and in terms of system response time. The amount of time that the IOP spends
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