Siemens U3585-J-Z125-8-76 1 manual CPU utilization, Device utilization, Channel utilization

Notes on SM2 operation Accuracy of the SM2 data

6.2.2 Notes on particular variables

CPU utilization

The term “percentage deviation” used in the following always denotes the absolute deviation, in other words:

X and Y have a deviation of P if abs (X-Y) ≤ P.

TU/TPR/SIH time, IDLE time, or /390 time with SR2000.

The operating system uses timers to determine three time components: the total for TU and TPR times, the SIH time and the IDLE time. In the case of SR2000, the /390 time is also determined as a fourth time component. Consequently, falsifications due to the non-inter- ruptible SIH code cannot occur. Investigations have shown that this timer is sufficiently precise for realistic monitoring cycles. As a rule, comparative measurements made with hardware monitors under typical load conditions have shown deviations of less than 1%.

SM2 divides the total for TU and TPR times into separate values for the TU time and the TPR time on the basis of samples taken during the sampling period. The accuracy of this procedure depends on the number of samples, which means that large deviations are possible if few samples are taken. The /390 time is also affected by this inaccuracy, because it is output as a relative proportion of TU.

Device utilization

As regards the hardware, the device is busy from the time an I/O request is received until data transfer terminates. As regards the software, the device is busy from the time an I/O request arrives in the I/O system module until channel termination.

The values determined by the software monitor should therefore always be higher than the actual device utilization. However, this systematic monitoring error is virtually balanced out by the sampling delays caused primarily by the non-interruptible system states.

Comparative measurements with hardware monitors for typical load conditions and about 6000 samples have shown that the maximum deviation is ± 2%.

Channel utilization

SM2 uses a special machine instruction to determine whether a channel is active or not. Thus there are no deviations between hardware and software monitors in the interpretation of this condition (see also restrictions on the ability of monitoring the channel load, description of the CHANNEL report). The sampling delays caused primarily by non-inter- ruptible system states result in the values determined by SM2 always being smaller than the actual values. Comparative measurements with a hardware monitor for typical load conditions and about 6000 samples have shown that the deviation is less than 2%.

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