Virtual Memory: Understanding Page Faults and Replacement Algorithms, Study notes of Computer Science

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Operating SystemsOperating Systems

CMPSC 473 CMPSC 473

Virtual Memory Virtual Memory

March 18, 2008 - Lecture March 18, 2008 - Lecture 1616

Instructor: Trent Jaeger Instructor: Trent Jaeger

• Last class:
  • Paging
• Today:
  • Virtual Memory

Example

Physical

Memory

(16 KB)

Virtual Address Space-1 (1 MB) Virtual Address Space-2 (1 MB) Virtual Address Space-n (1 MB)

Page Faults

• If a Page-table mapping indicates an absence of the

page in physical memory, hardware raises a “Page-

Fault”.

• OS traps this fault and the interrupt handler services

the fault by initiating a disk-read request.

• Once page is brought in from disk to main memory,

page-table entry is updated and the process which

faulted is restarted.

• May involve replacing another page and invalidating the

corresponding page-table entry.

Page Fault

• If there is a reference to a page, first reference to that page will trap to operating system: - page fault
• Operating system looks at another table to decide:
  • Invalid reference -- abort
  • Just not in memory
• Get empty frame
• Swap page into frame
• Reset tables
• Set validation bit = v
• Restart the instruction that caused the page fault

Steps in Handling a Page Fault

Demand Paging Example

• Memory access time = 200 nanoseconds
• Average page-fault service time = 8 milliseconds
• EAT = (1 – p) x 200 + p (8 milliseconds) = (1 – p x 200 + p x 8,000, = 200 + p x 7,999,
• If one access out of 1,000 causes a page fault, then EAT = 8.2 microseconds. This is a slowdown by a factor of 40!!

Putting it all together!

int A[2K];

int B[2k];

main() {

int i, j, p;

p = malloc(16K);

VAS before execution

0x0… 0 0xf…f Page Table

PC

Heap Pointer Stack Pointer All Point To Null (i.e. fault the first time)

4K page size

No Page table Entries here

Page Replacement

• When bringing in a page, something has to

be evicted.

• What should we evict? – page replacement

algorithm.

Optimal Page Replacement Algorithm

• Why optimal?
  • No other algorithm can have # of page faults lower than this, for a given page reference stream.
• Algorithm:
  • At any point, amongst the given pages in memory, evict the one whose first reference from now is the furthest.

Problem with OPT

• Not implementable!
• Requires us to know the future.
• But it has the best page fault behavior
• How do we approach OPT?

1. First-in First-out

• Maintain a linked list of pages in the order they

were brought into PM.

• On a page fault, evict the one at the head.

• Put the newly brought in page (from disk) at tail of

this list.

• Problems:

• Reference String: 1,2,3,4,1,1,5,1,1,…
• Page fault at (5) would replace (1)!
• Need to know what is in recent use!

3. Second Chance Replacement or Clock Algorithm

• Same as FIFO, except you skip over the pages whose reference bit is set, resetting this bit, and moving those pages to end of list.
• Implementation: 1

Evict

4. Least Recently Used

• Order the list of physical memory pages in decreasing order

of recency of usage.

• Replace the page at the tail.
• Problem:
  • This list will need to be updated on each memory reference.
  • Asking the h/w to do this is ridiculous!
• Solution: Approximate LRU