These items warrant consideration since they have potential for a negative impact to performance.
- MDC Tuning with Large Real Memory
- Large V=R Area
When running in large real memory sizes and particularly in real memory sizes larger than 2 Gigabytes, it is important to review the current minidisk cache tuning settings for possible changes. The most likely tuning action needed will be to ensure that the real MDC does not get too large. See MDC Tuning Recommendations for further information.
The V=R area, if present, is used to accommodate preferred guest (V=R and V=F) virtual machines. Be careful not to define a V=R area that is too large. This can cause a performance problem even when running in a large real storage size (greater than 2G). The reason for this is that most real storage frames referenced by CP and the real storage frames occupied by the V=R area itself must reside in the first 2 Gigabytes of real storage. As a result, if the V=R area takes up too many of these real storage frames, the number of remaining available frames below the 2G line may be insufficient to meet the needs of the rest of the system, resulting in a thrashing situation. See The 2G Line for further discussion and measurement results. Those results suggest, as a rough rule-of-thumb, that there should be at least 15-20 4K page frames available below the 2G line per CMS user.