Load on first use
Executable and data pages remain on secondary storage until referenced, reducing initial load and allowing more processes to coexist.
PART 3 • MEMORY & STORAGE
Give Every Process a Larger, Protected View of Memory
Follow a nonresident page from CPU reference to disk and back, compare replacement decisions, and diagnose when excessive paging destroys useful execution.
BY THE END, YOU CAN
Reason about residency
- Explain demand paging and valid bits.
- Trace a page fault safely.
- Run FIFO, LRU and Optimal.
- Identify Belady’s anomaly.
- Explain working sets and thrashing.
Virtual Memory Loads Only the Pages Needed Now
A process sees a private virtual address space. Page-table entries map resident pages to frames; a valid/present bit tells hardware whether translation can continue.
Delay private copying
Processes initially share read-only frames. A write fault creates a private copy, making fork() cheaper.
File bytes become pages
Reads and writes use ordinary memory references while the OS handles caching, loading and eventual persistence.
page = ⌊virtual address / page size⌋
offset = virtual address mod page size
EAT depends on TLB hit and fault rate
A TLB miss and a page fault are different. On a TLB miss the mapping may already exist in the page table; on a page fault the page is not resident and OS intervention is required.
A Page Fault Is a Controlled Exception
Trap and validate
Hardware traps to the kernel. The OS checks whether the address is legal; illegal references terminate or signal the process.
Obtain a frame
Use a free frame or select a victim. If the victim is dirty, schedule its write-back before reuse.
Read and block
Issue storage I/O and block the faulting process so another ready process can run.
Update translation
Record frame number and protection bits; invalidate stale TLB state.
Restart instruction
Return to user mode and repeat the faulting instruction as though the page had been present.
Run Every Reference against the Same Frames
FIFO removes the oldest loaded page; LRU removes the least recently used; Optimal removes the page used farthest in the known future and is an offline benchmark.
READY
Choose policy and run
The complete hit/fault and victim trace will appear here.
| Policy | Information | Strength | Limitation |
|---|---|---|---|
| FIFO | Load order | Small metadata | Belady anomaly possible |
| LRU | Past recency | Uses temporal locality | Exact tracking costly |
| Optimal | Future references | Minimum faults for trace | Not online implementable |
Thrashing Means the System Is Moving Pages More Than Doing Work
Working set
The pages referenced in a recent window approximate the process’s active locality. Enough frames keep that locality resident.
High faults, low CPU use
Processes block for paging; a naive scheduler may admit more processes and make pressure worse.
Reduce degree
Suspend processes, use working-set or page-fault-frequency control, and allocate frames according to locality.
Global replacement may take frames from another process; local replacement restricts victims to the faulting process. Global policies can improve utilization but couple processes and weaken performance isolation.
Trace One Fault from Instruction to Restart
Ready to trace
Press Next step to begin.
Ten Misconception-Specific Checks
Answered correctly: 0 of 10
Explain Mechanism and Trade-off
- Define demand paging.
- TLB miss versus page fault?
- What is a dirty bit?
- Define thrashing.
- What is locality?
- Trace page-fault service.
- Compare FIFO, LRU and Optimal.
- Explain Belady’s anomaly.
- Explain working-set control.
- Calculate effective access time.
- Why can more frames hurt FIFO?
- Why is Optimal useful?
- How is LRU approximated?
- Global versus local replacement?
- What triggers copy-on-write?
Strong answer: “Explain a page fault”
Start with present-bit failure, validate the address, obtain a frame, write dirty victim if necessary, read the page, update page table/TLB, unblock and restart. Mention that disk latency dominates and another process can run meanwhile.
LEVEL 9 SUMMARY
You Can Explain Why a Page Is Present—or Missing
- Demand paging separates virtual size from current residency.
- A legal nonresident reference causes a recoverable page fault.
- Replacement policy selects a victim only when no free frame exists.
- Optimal is a benchmark; practical systems approximate locality.
- Working-set pressure can cause thrashing.
COURSE CHECKPOINT
Mark complete after you can trace one fault and manually run LRU.
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