2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.3 MVS Real Storage Control Mechanisms › 2.2.4.3.2 UIC Update Process

2.2.4.3.2 UIC Update Process

Maintaining a record of the time since each page was last
referenced is central to the implementation of a LRU page
replacement algorithm.  This function, called the UIC
(unreferenced interval count) update process, is actually
implemented by the same routine that performs the page
stealing function.  We will discuss the two functions
separately in order to make it easier for you to follow the
developmental changes to each of the processes.

The UIC records the number of seconds since the page has been
referenced.  The system UIC is thus the age in seconds of the
oldest page in storage.  In Figure 2-30, the rates for UIC
updating for various values of the system UIC are presented
for both swappable and non-swappable address spaces.

------------------------------------------------------------
| System UIC      |0-2| 3-5| 6-7| 8 | 9 | 10-13 | 14 | 15+ |
|-----------------+---+----+----+---+---+-------+----+-----|
| Swappable (sec) | 1 |  1 |  1 | 1 | 2 |   2   |  2 |  2  |
|                 |   |    |    |   |   |       |    |     |
| Non-swap (sec)  | 1 |  2 |  3 | 3 | 3 |   4   |  5 |  6  |
------------------------------------------------------------


 Figure 2-30.  UIC Update Rates


For example, if the system UIC is six, then the UIC for
non-swappable address spaces is updated every three seconds.

The UIC for swappable address spaces needs to be maintained
more closely in order to effectively trim to the working set
on a swap out.  For non-swappable address spaces, it is not
necessary to keep close track of the working set if the
demand for real storage is light.  This also gives some page
stealing protection for non-swappable address spaces.