Adjust the WAN line speed for optimum iSeries server performance
In many cases, the communications line is the largest contributor to overall
response time in the wide area network (WAN). Therefore, you should closely
plan and manage its performance. In general, having the appropriate line speed
is the key consideration for gaining the best performance.
To adjust the line speed for your wide area network, perform these tasks:
- Check the difference in performance between half-duplex utilization and
full-duplex utilization on the line description.
- For interactive environments, keep line use below 30% to maintain predictable
and consistent response times. Exceeding 50% line use typically slows down
response time. The line use can be measured with the iSeries™ system performance
tools.
- 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.
- 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 might change as follows:
- With interactive transactions, performance might be slightly faster.
- With a large transfer, performance might be significantly faster.
- With a single job, performance might 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:
- A common misconception about the line speed of each attached communications
line is that the central processing unit (CPU) resource is used in a uniform
fashion. Exact statements cannot be made about the number of lines that any
given iSeries server model can support.
- 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.
- 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.
- 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.
book.