VMM thresholds

Several numerical thresholds define the objectives of the VMM. When one of these thresholds is breached, the VMM takes appropriate action to bring the state of memory back within bounds. This section discusses the thresholds that the system administrator can alter through the vmo command.

The number of page frames on the free list is controlled by the following parameters:

minfree

Minimum acceptable number of real-memory page frames in the free list. When the size of the free list falls below this number, the VMM begins stealing pages. It continues stealing pages until the size of the free list reaches maxfree.

maxfree

Maximum size to which the free list will grow by VMM page-stealing. The size of the free list may exceed this number as a result of processes terminating and freeing their working-segment pages or the deletion of files that have pages in memory.

The VMM attempts to keep the size of the free list greater than or equal to minfree. When page faults or system demands cause the free list size to fall below minfree, the page-replacement algorithm runs. The size of the free list must be kept above a certain level (the default value of minfree) for several reasons. For example, the operating system's sequential-prefetch algorithm requires several frames at a time for each process that is doing sequential reads. Also, the VMM must avoid deadlocks within the operating system itself, which could occur if there were not enough space to read in a page that was required to free a page frame.

The following thresholds are expressed as percentages. They represent the fraction of the total real memory of the machine that is occupied by file pages (pages of noncomputational segments).

minperm

If the percentage of real memory occupied by file pages falls below this level, the page-replacement algorithm steals both file and computational pages, regardless of repage rates.

maxperm

If the percentage of real memory occupied by file pages rises above this level, the page-replacement algorithm steals only file pages.

maxclient

If the percentage of real memory occupied by file pages is above this level, the page-replacement algorithm steals only client pages.

When the percentage of real memory occupied by file pages is between the minperm and maxperm parameter values, the VMM normally steals only file pages, but if the repaging rate for file pages is higher than the repaging rate for computational pages, computational pages are stolen as well.

The main intent of the page-replacement algorithm is to ensure that computational pages are given fair treatment. For example, the sequential reading of a long data file into memory should not cause the loss of program text pages that are likely to be used again soon. The page-replacement algorithm's use of the thresholds and repaging rates ensures that both types of pages get treated fairly, with a slight bias in favor of computational pages.