Performance Considerations
These items warrant consideration since they have potential for a negative impact to performance.
- MDC Tuning with Large Real Memory
- Large V=R Area
MDC Tuning with Large Real Memory
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.
Large V=R Area
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.