① Explainer · R4HA Tuning
Sub-capacity MLC bills on the single highest Rolling 4-Hour Average in the month. When a batch job overlaps the online day, it lifts that peak and sets the charge for every product. Shift it, spread it, or cap it, and the billable MSU figure falls while the work still runs.
Sub-capacity Monthly License Charge software is billed on the Rolling 4-Hour Average (R4HA). Each hour the Sub-Capacity Reporting Tool (SCRT) records the average MSU utilization over the preceding four hours, and the single highest of those averages across the whole month becomes the billable figure for each eligible product. A product peak is capped at the z/OS peak, so the operating system peak sets the ceiling for everything beneath it.
The consequence is blunt: one four hour window out of roughly seven hundred and twenty in the month decides the charge. If that window is lifted by batch work that overlaps the online day, the rest of the month, however efficient, does not lower the bill. This is why peak shaping, not average reduction, is the lever that pays.
Take an estate where the online day runs near 600 MSU and a heavy reporting batch overlaps the morning online ramp, stacking on top of it. The R4HA at that overlap is what bills. The table shows the billable peak before tuning, then after the reporting batch is shifted two hours later into a quieter window.
| Window | Before, R4HA MSU | After, R4HA MSU |
|---|---|---|
| Overnight batch | 520 | 520 |
| Online ramp + reporting | 760 | 610 |
| Midday online | 600 | 600 |
| Shifted reporting window | n/a | 640 |
| Billable peak | 760 | 640 |
The reporting batch did not disappear; it moved off the online ramp into a window that still had headroom. The monthly billable peak fell from 760 to 640 MSU, about 16 percent, and that reduction applies to every sub-capacity product capped by the z/OS peak. The work completed on the same day. Figures illustrate the mechanic, not any specific estate.
| Lever | What it does | Best when |
|---|---|---|
| Shift | Moves a job out of the peak window | The peak is one identifiable job |
| Spread | Stretches stacked jobs across more hours | Many jobs pile into one window |
| Soft cap | Holds the R4HA under an MSU ceiling | The workload tolerates brief throttling |
| Offload | Moves eligible work to zIIP engines | The workload is zIIP eligible |
The four levers combine. Shifting and spreading reshape when work runs; soft capping puts a ceiling on the result; zIIP offload removes eligible cycles from the general purpose processor count that SCRT measures. The right mix depends on which window sets your peak and how much the workload can flex.
The trap is blanket rescheduling without first finding the peak. Moving batch that was never near the peak saves nothing, and moving it into a window that already carries a high point can create a new peak and raise the bill. Tuning is a targeted exercise: identify the window that sets the monthly charge, confirm what drives it, then act only on that.
Buyer side levers
Shape the peak, not the average.
Related explainers: EWLC and entry workload license charges, the MIPS to MSU conversion question, and sysplex vs standalone pricing differences. Long tail product: CICS Transaction Server licensing, a frequent peak driver. Commercial: MSU optimization and cost optimization. Hub: the IBM buyer side guide.
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