Siemens SM2 monitoring system, U3585-J-Z125-8-76 1 CPU utilization, System resource utilization

System resource utilization Notes on SM2 operation

6.1.2 CPU utilization

To facilitate understanding of the factors affecting CPU utilization, readers should be familiar with the monitoring methods (see section “Acquisition of monitored data” on page 15).

Method based on monitoring cycle

At the end of the monitoring cycle, the measured values of the previous monitoring cycle are calculated for all monitoring programs (with the exception of DISK and TASK). In contrast to the sample cycle, the monitoring cycle is so much longer that system utilization by this method based can be disregarded.

Method based on samples

A monitoring task routine is activated at specific intervals (see the SAMPLING-PERIOD operand of the MODIFY-MEASUREMENT-PERIODS statement) to take samples.

For each activation of the sampling routine, a basic load has to be processed, regardless of the variables (devices and tasks) to be monitored.

In addition to this basic load, further instructions are processed which depend on the number of objects to be monitored (devices, channels, tasks).

If this number remains constant in the monitoring cycle, CPU utilization is almost directly proportional to the sampling rate (i.e. halving the sampling cycle, for example, causes the induced system load to be doubled).

Lengthening the sampling cycle should be balanced by lengthening the monitoring cycle to prevent a deterioration in sampling precision.

The sample-driven method is used for device and channel utilization, the length of queues, and the monitoring programs CMS and TLM.

Method based on events

When this method is used, the monitor is the “passive” component in contrast to the other system components, which are “active”. When specific events occur in the system (e.g. starting of an I/O operation), specific monitoring programs are activated which collect the relevant data (e.g. which device, which task, etc.).

While the monitor is inactive, no system utilization is induced.

If, however, events occur which are to be monitored, utilization increases in proportion to the load (i.e. to the number of calls).

When this monitoring method is used, the system load can be reduced only by reducing the number of objects to be monitored.

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