Virtual Memory Management

Question 1

What is virtual memory used for?

Answer 1

Processes addresses a virtual address space. Virtual memory is larger than physical memory.

Question 2

What is swapping?

Answer 2

Swap complete processes in and out of physical memory. Save/load from disk on very context switch, and restore paging tables.

Question 3

What is demand paging?

Answer 3

Contents of virtual address space held on disk. Pages are copied to physical memory as needed by the Pager.

Question 4

What are the benefits of demand paging?

Answer 4

Programmer isn’t concerned with amount of physical memory, making programming simpler and portable.

More ps can be run concurrently.

Less IO needed for loading and swapping processes

Question 5

What are the drawbacks of demand paging?

Answer 5

Implementation is complex.

Question 6

How is demand paging implemented?

Answer 6

HW support need, valid and invalid bit for each page table entry.

Valid bit indicates page is in memory

Invalid bit indicates page isn’t in memory.

Question 7

What is a page fault?

Answer 7

Check the V/I bit on each memory reference, page as normal when valid and trap on invalid.

Question 8

What happens when a page fault occurs?

Answer 8

Check address is legal, find a free frame then schedule disk read operation. Set the frame number and valid bit in the page table and restart the instruction that caused the fault

Question 9

What is pure demand paging?

Answer 9

Need have no pages in memory to start execution of a process. PC initially points to invalid address. Page containing first instruction is faulted on PC dereference.

Question 10

Why do we need to be able to restart an instruction?

Answer 10

Any instruction that accesses memory can cause a page fault, must be able to restart. Can be expensive for complex instructions.

Question 11

How do we calculate demand paging performance?

Answer 11

EAT = (1-p) * ma * page fault time

where p = probability of page fault and ma is memory access time (10-200 ns)

Question 12

What is over-allocation?

Answer 12

Process may use less physical memory than size of VM.

Physical memory can be over-allocated.

Question 13

What are the benefits of over-allocation?

Answer 13

allows increased multi-programming and increases CPU utilisation and throughput.

Question 14

What are the drawbacks of over-allocation?

Answer 14

When 1 > processes have increased VM demands memory is also used to buffer IO.

May find no free frames when servicing page fault.

Question 15

How is over-allocation handles?

Answer 15

Terminate the process

Swap out a process completely (freeing its frames)

Page replacement

• local (affects current process)
• global (affect any process)

Question 16

How does Basic page replacement work?

Answer 16

Pick a page that isn’t being heavily used.
Free the frame (write page back to disk) and update page table.

Fault page into freed frame.

Question 17

What are the overheads of replacement?

Answer 17

2 page transfers are necessary, page fault service time is doubled.

Question 18

How can we reduce overheads of replacement?

Answer 18

Unmodified pages don’t have to be written to disk on replacement. Indice this with modify (dirty) bit.