I-stream controls

The z/TPF system provides I-stream controls that you can use to manage the number of I-streams that are used by the system, including the I-stream cap, HiperDispatch, and the I-stream floor.

I-stream cap

A means to restrict the use of I-streams in a logical partition (LPAR) or z/VM® guest where the z/TPF system is running.

An LPAR can be configured to have more I-streams than are immediately needed. When the z/TPF system is restarted, all I-streams that are configured to the LPAR are available to be used. You can set the I-stream cap to the number of I-streams that are currently needed. Any I-stream that is greater than the I-stream cap is collapsed immediately and application work is not distributed to the collapsed I-stream any more. Any I-stream that has an I-stream number greater than the I-stream cap is called a fenced I-stream.

When you need more capacity, you can change the I-stream cap when the z/TPF system is running in NORM state to add capacity without an outage.

You can use the ZMISS command to set the I-stream cap. If the I-stream cap is not set by the ZMISS command, the z/TPF system uses the I-stream cap that is defined in the utilization configuration file. If the I-stream cap is not set by the ZMISS command or in the utilization configuration file, the z/TPF system uses the number of active I-streams as the I-stream cap.

HiperDispatch

A means to improve efficiency of a z/TPF LPAR when it is running as a shared LPAR. Efficiency is improved by using a minimal number of I-streams. The HiperDispatch setting is not available when the z/TPF system is running as a dedicated LPAR or when the z/TPF system is running as a guest under z/VM.

When HiperDispatch is not used, the z/TPF system distributes work equally to all I-streams with numbers that are less than or equal to the I-stream cap.

When you enable HiperDispatch and the z/TPF system cycles to NORM state, all I-streams that are less than or equal to the I-stream cap are in use. After a wait time that you can specify on the ZMISS command, the z/TPF system checks the utilization of in-use I-streams every second to determine whether to collapse or expand the number of in-use I-streams.

To ensure that a minimal number of I-streams are in use, the z/TPF system uses an average of 80% utilization as a goal. If average utilization across all in-use I-streams is greater than 80%, the z/TPF system expands the number of in-use I-stream by one if the next I-stream is less than or equal to the I-stream cap. If average utilization across in-use I-streams minus one is less than 80% for the last 10 seconds, the z/TPF system collapses the in-use I-stream with the highest number if it is greater than the I-stream floor. A collapsed in-use I-stream is defined as a parked I-stream. The 80% utilization goal increases when the utilization for all shared LPARs on the machine approaches 100% busy.

You can use the ZMISS command to manage the HiperDispatch setting. If you do not enable the HiperDispatch setting with the ZMISS command, the initial setting is NO.

I-stream floor

A means to prevent the z/TPF system from collapsing in-use I-streams below a specified I-stream number when HiperDispatch is in use.

You can use the ZMISS command to set the I-stream floor. If the I-stream floor is not set by the ZMISS command, the z/TPF system uses the I-stream floor that is defined in the utilization configuration file. If the I-stream floor is not set by the ZMISS command or in the utilization configuration file, the default I-stream floor is used.