When I increase NPAGEVIR, where does the memory come from?

There are several areas of memory: FREE PAGE LIST, MODIFIED PAGE LIST, NON-PAGED DYNAMIC and PAGED DYNAMIC. If you add memory to one area, it must come from another area. So, when you add memory to NPAGEVIR, it comes from the FREE PAGE LIST.

Paged Dynamic Memory is used for shared logical names, known file list entries and VMS RMS file sharing structures.

Non-Paged Dynamic Memory is used for a variety of special things, such as data structures that I/O Express uses to manage the data in cache. VMS also uses lots of non-paged pool.

The Modified Page List is used as a temporary storage location for processes, when their working sets are found to be too small for the work they are doing. Pages in the working sets are moved to the Modified Page List to make room for other pages that are to be moved in from disk. Occasionally, when the Free Page List drops below a certain value, the entire Modified Page List is written to disk, so that the system will not starve for memory, thereby keeping the swapper from swapping out processes. The Free Page List is used as a temporary storage place, to house data from disk, while work is being done to it.

There are SYSGEN parameters that set limits on the size of each of these segments of memory. The Free Page List uses FREELIM as the lower limit that must be maintained all the time. Once a system memory shortage has occurred, the swapper starts to release pages until the Free Page List has reached the value of FREEGOAL. NPAGEDYN and NPAGEVIR set the initial value and the extent level of non-paged pool. PAGEDYN sets the size in pages for paged dynamic pool. MPW-LOLIMIT and MPW-HILIMIT are the lower and upper limits for the Modified Page List.

If you increase the value of NPAGEVIR and reboot, the configuration of memory will change. Pages are deallocated from the FREE PAGE LIST and given to NPAGEVIR, but because the FREE PAGE LIST has lower and upper limits, the swapper will always make sure that there are plenty of pages on the FREE PAGE LIST.

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