Resident Set Management

operating-systems

Definition

Resident Set Management

Resident set management determines how many frames of physical memory to allocate to a process at any time. The resident set is the collection of pages currently in RAM.

The working set $W(D,t)$ at time $t$ is the set of pages referenced during the last $D$ time units (working set window). If the resident set contains the working set, the process has very few page faults.

Allocation Strategies

Fixed Allocation

Each process gets a fixed number of frames. Simple, but may cause excessive page faults (too small) or wasted memory (too large).

Variable Allocation

Number of frames changes based on process behaviour (e.g., page fault rate).

Management Rules

Frame Adjustment

• Resident Set $\subset$ Working Set → Allocate more frames
• Resident Set $\supset$ Working Set → Reclaim unused frames
• Sum of all working sets exceeds RAM → Suspend processes (Swapping)

Performance

Thrashing Prevention

Effective management prevents thrashing — where the OS spends more time handling page faults than executing user code. Systems often monitor Page Fault Frequency (PFF) as a proxy for working set size.