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
The operating system uses timers to determine three time components: the total for TU and TPR times, the SIH time and the IDLE time. Consequently, falsifications due to the noninterruptible 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.
Device utilization
All devices except disks which support RSC I/Os
The utilization is recorded by SM2 using the sample-driven monitoring method.
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.
DIsks which support RSC I/Os
The device utilization is calculated by SM2 from the average number of RSC I/Os per second, where: 4 RSC-IOs = 100% utilization.
Channel utilization for FC channels on /390 servers
The utilization is calculated by SM2 from the data rate and the blocking of the I/Os.
You will find the data on which the calculation is based in the Performance Handbook [5 (Related publications)].