Thrashing
When an application constantly swapping pages in and out
- Very slow
Cause
- Process requires more memory than system has
- Each time a page is brought in, a page that will need to used later is evicted
- Memory access pattern does not exhibit temporal locality
- 80⧸20 Rule does not apply
Approach 1: Working Set Model
According to 80⧸20 Rule, some portion of the address space will be heavily used and the remainder will not.
- Call the heavily used portion the working set
- Changes across phases of using a program
- Set of pages in currently in primary memory is resident set
Given locality of reference, how big a cache does the process need?
Only run processes whose memory requirement can be satisfied
Approach 2: Page Fault Frequency
PFF = page faults / instructions executed
- If PFF rises above high threshold → needs more memory
- If PFF sinks below low threshold → can take away memory
