The mapped ratio value is the percentage of pages in all memory zones that belong to User Mode address spaces (sc->nr_mapped) with respect to the total number of allocatable page frames. A high value of mapped_ratio means that the dynamic memory is mostly used by User Mode processes, while a low value means that it is mostly used by the page cache.
The distress value is a measure of how effectively the PFRA is reclaiming page frames in this zone; it is based on the scanning priority of the zone in the previous run of the PFRA, which is stored in the prev_priority field of the zone descriptor. The distress value depends on the zone's previous priority as follows:
Zone prev. priority
12...7
6
5
4
3
2
1
0
Distress value
0
1
3
6
12
25
50
100
Finally, the swappiness value is a user-defined constant, which is usually set to 60. The system administrator may tune this value by writing in the /proc/sys/vm/swappiness file or by issuing the proper sysctl( ) system call.
Pages will be reclaimed from the address spaces of processes only if the zone's swap tendency is greater than or equal to 100. Thus, if the system administrator sets swappiness to 0, then the PFRA never reclaims pages in the User Mode address spaces unless the zone's previous priority is zero (an unlikely event); if the administrator sets swappiness to 100, then the PFRA reclaims pages in the User Mode address spaces at every invocation.
