If performance problems arise without a correspondingly heavy CPU workload (<90%), one of the following causes may be involved:
Conflict situation on the DMS I/O side
(see section "I/O load")Overloading of volumes containing files which are required equally by a large number of users (e.g. particular user control files or the system file TSOSCAT or SYSEAM) has a particularly unpleasant effect.
The SM2 monitoring program FILE can be used to calculate the number of I/O operations on these files.
Paging rate too high (see section "Paging")
In the case of severe main memory shortage (i.e. when the paging rate far exceeds the standard values given), bottlenecks result on the public volumes with regard to the number of paging I/O operations to be executed per second. This leads to a forced CPU wait state (IDLE).
Delays caused by PASS/VPASS calls
If the SVCs PASS/VPASS are called too often, tasks are placed in a wait state too frequently and cannot utilize the CPU.
Bottlenecks caused by server tasks
Some applications have large portions of their workload handled by specialized server tasks. If the progress of these tasks is impeded by paging, deadlocks, resource overloads, unfavorable priorities, etc., this can detrimentally affect the entire application. The server task itself acts as a resource.
Insufficient parallel processing because number of tasks too small
Not only do the service times of the relevant hardware resources affect performance, but also the amount of time during which a task is occupied with CPU and I/O activity in order to perform a desired user function.
Example
If the average processing time per transaction is 0.5 seconds (0.2 seconds CPU time + 0.3 seconds I/O time), it is possible to obtain the following maximum transaction rate (transactions/s) for a task (not including the wait time for the relevant hardware resources):
1 / 0.5 sec. per transaction = 2 transactions per second
This yields a CPU workload of 0.2 * 2 = 0.4 or 40%.
A higher transaction rate or more favorable utilization of CPU capacity can only be obtained by raising the number of tasks (for details, see section "Task occupancy time").