Virtual Memory

Virtual Memory

Virtual memory is a technique that is used to execute processes that are not in main memory completely. The remaining part are placed on the backing storage.

Demand paging

It is a technique that is used to bring a page into memory only when it is needed.

Demand paging cycle Page is needed reference to it Referenced page not in real memory Page Fault Just not in memory Demand paging Find free frame Swap page into frame Reset tables to indicate page now in memory Restart the instruction that caused the page fault

Page Replacement

“Page replacement completes separation between logical memory and physical memory large virtual memory can be provided on a smaller physical memory”

How to solve page faults?

It terminates the program immediatelyOr

Swaps out program & frees all of its frames It reduces the degree of multiprogramming

Basic Page ReplacementSteps performed by OS while replacing a page upon a page fault:

1. Find the location of the desired page on disk

2. Find a free frame: If there is a free frame, use it If there is no free frame, use a page replacement algorithm to select a victim frame; if the victim page is modified, write it back to disk.

3. Bring the desired page into the (new) free frame; update the page and frame tables

4. Restart the process

Page-replacement algorithm

Want lowest page-fault rate on both first access and re-access

There is particular string of memory references. String is just page numbers, not full addresses Repeated access to the same page does not

cause a page fault Results depend on number of frames available

In all our examples, the reference string of referenced page numbers is 7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1

First In First Out (FIFO) Algorithm

Oldest page in main memory is the one which will be selected for replacement

Easy to implement, keep a list, replace pages from the tail and add new pages at the head.

First-In-First-Out (FIFO) Algorithm

Adding more frames can cause more page faults!

Belady’s Anomaly

15 page faults

FIFO Illustrating Belady’s Anomaly

Optimal Algorithm Replace page that will not be used for longest period of time How do you know this?

Used for measuring how well your algorithm performs

Least Recently Used (LRU) Algorithm Use past knowledge rather than future Replace page that has not been used in the most amount of time

12 faults – better than FIFO but worse than OPT Generally good algorithm and frequently used

LRU Approximation Algorithm

Reference bitWith each page associate a bit, initially= 0When page is referenced bit set to 1

Additional Reference Bits We can gain additional ordering information

by recording the reference bits at regular interval

We can keep 8-bit byte for each page in a table in memory

The OS shifts the reference bit for each page into the high-order bit of its 8-bit byte, shifting the other bit right by 1 bit and discard the low-order bit

Second Chance Algorithm

This Algorithm is a FIFO replacement algorithm wit consideration given to the reference bit .when a page has been selected for replacement , its reference bit is inspected .

When a page is given a second chance its reference bit is cleared and its arrival time reset to the current time

Enhanced Second-Chance Algorithm

Improve algorithm by using reference bit and modify bit (if available) in concert

1. (0, 0) neither recently used not modified – best page to replace

2. (0, 1) not recently used but modified – not quite as good, must write out before replacement

3. (1, 0) recently used but clean – probably will be used again soon

4. (1, 1) recently used and modified – probably will be used again soon and need to write out before replacement

Allocation of Frames

Each process needs minimum number of pages Various allocation approaches

› fixed allocation › global allocation› local allocation

Fixed Allocation Equal allocation – For example, if there are 100

frames and 5 processes, give each process 20 frames.

Proportional allocation – Allocate according to the size of process

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Global versus Local Allocation

When a page fault occurs for a process and we need page replacement, there are two general approaches: › Global replacement – select a victim frame

from the set of all frames; one process can take a frame from another

› Local replacement – select a victim frame only from the frames allocated to the process. A process uses always its allocated frames

Thrashing

At some time , the processor may be spend most of its time swapping pages and doing little productive work, this is known as Thrashing.

Figure:

Thrashing Cause

It is caused by under allocation of minimum no. of pages required by a process , forcing it to continuously page fault.

Working setIt is a collection of pages, which is actively referenced by a process . It should be kept in main memory to run efficiently.

Working sets change during process execution . pages are added or deleted and critical changes may occur.

Page Size:It is a hardware design issue . Small pages reduce the amount of internal fragmentation. Large pages reduce the size of the page table.

In general small pages have better locality than large pages.