Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical

Background Virtual memory – separation of user

logical memory from physical memory. Only part of the program needs to be in memory

for execution. Logical address space can therefore be much

larger than physical address space. Allows address spaces to be shared by several

processes. Allows for more efficient process creation. Virtual memory can be implemented via:

Demand paging Demand segmentation

Virtual Memory That is Larger Than Physical Memory

Demand Paging Bring a page into memory only when it

is needed. Less I/O needed Less memory needed Faster response More users

Page is needed reference to it invalid reference abort not-in-memory bring to memory

Transfer of a Paged Memory to Contiguous Disk Space

Page Table When Some Pages Are Not in Main Memory

Page Fault

If there is ever a reference to a page, first reference will trap to

OS page fault OS decides:

Invalid reference abort. Just not in memory.

Get empty frame.

Page Fault Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction

Steps in Handling a Page Fault

What happens if there is no free frame? Page replacement – find some page in

memory, but not really in use, swap it out. algorithm performance – want an algorithm which will

result in minimum number of page faults.

Same page may be brought into memory several times.

Page Replacement

Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.

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

Basic Page Replacement1. 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.

3. Read the desired page into the (newly) free frame. Update the page and frame tables.

4. Restart the process.

Page Replacement Algorithms

Want lowest page-fault rate. Evaluate algorithm by running it on a

particular string of memory references (reference string) and computing the number of page faults on that string.

In examples, the reference string is: 7,0,1,2,0, 3,0,4,2,3, 0,3,2,1,2,0,1,7,0,1

Optimal Algorithm Replace page that will not be used

for longest period of time. Used for measuring how well your

algorithm performs.

Optimal Page Replacement





First-In-First-Out

Throw out the page that has been in memory the longest.

Good when talking about a set of pages for initialization.

Bad when talking about heavily used variable.

FIFO Page Replacement

Problem with FIFO

Considers only time when loaded into system. Not whether it has been used.

Set of “second chance” algorithms that use the reference bit in page table entry to determine if it has been recently used.

Example: Clock Page Replacement Algorithm.

Second-Chance (clock) Page-Replacement Algorithm

Least Recently Used (LRU) Algorithm

Based on principal of “Locality of Reference”. A page that has been used in the near past is likely

to be used in the near future. LRU: Determine the least recently used page

in memory and evict it. Can be done but very expensive.

LRU Page Replacement

Software Approximations to LRU

Not Frequently Used (NFU). Associate a software counter with each page. On timer interrupt, OS scans all pages in memory. For each page, the R bit (Referenced bit)is added to the

counter. Page with lowest count is evicted.

Problem with NFU

It never forgets. A page referenced often in earlier phases of the

program may not be evicted long after it has been used.

Would like to have an algorithm that “ages” the count. That is, the latest references should be the most important.

Aging Algorithm for Simulating LRU

On each timer interrupt scan the pages to get the R bit.

Shift right one bit of the counter. Place the R bit in the leftmost bit of the

counter. Choose the page to evict that has the lowest

count.

Example

Assume all counters are currently 0.

Consider the case when pages 0,2,4, and 5 are referenced between last interrupt.

Simulating LRU in Software

The aging algorithm simulates LRU in software



Assume references 0,1, and 4 next window.





Assume 0,1,3,5



Allocation of Frames

Each process needs minimum number of pages. Two major allocation schemes.

fixed allocation Variable allocation.

Replacement Scope can be: Local. Global.

Fixed Allocation, Local Scope

Number of pages per process is fixed based on some criteria.

Can use equal allocation or proportional allocation. Equal allocation – e.g., if 100 frames and 5

processes, give each 20 pages. What are the drawbacks of equal allocation?


Proportional Allocation Allocate page size based on the size of the process.

Problem?


Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.

What are the drawbacks of this approach? Allocation may be too small causing significant

paging. Allocation may too large reducing number of

processes in memory and wasting memory that could be used by other processes.


Proportional Allocation Allocate page size based on the size of the process.

Problem? Process needs will vary over its execution leading to

the same problems as equal-size pages.

Problem with Fixed Allocation Schemes

All processes treated the same. No priorities. Can use process priority rather than size to allocate

frames.

Variable Allocation, Global Replacement

When a page fault occurs, new page frame allocated to the process.

Page replacement based on previous approaches: e.g., LRU, FIFO, etc.

No consideration of which process should (or can best afford) to lose a page.

Can lead to high page-fault rates.

Thrashing

If a process does not have “enough” pages, the page-fault rate is very high. This leads to:

low CPU utilization. operating system thinks that it needs to increase the

degree of multiprogramming. another process added to the system.

Thrashing a process is busy swapping pages in and out.

Thrashing

Locality In A Memory-Reference Pattern

Working-Set Model: Local Scope, Variable Allocation

working-set window a fixed number of page references Example: 10,000 instruction

WSSi (working set of Process Pi) =total number of pages referenced in the most recent (varies in time)

if too small will not encompass entire locality. if too large will encompass several localities. if = will encompass entire program.

D = WSSi total demand frames if D > m Thrashing Policy if D > m, then suspend one of the processes.

Working-set model

Page-Fault Frequency Scheme

Establish “acceptable” page-fault rate. If actual rate too low, process loses frame. If actual rate too high, process gains frame.

Other Considerations

Prepaging: Predicting future page requests.

Page size selection fragmentation table size I/O overhead


TLB Reach - The amount of memory accessible from the TLB.

TLB Reach = (TLB Size) X (Page Size)

Ideally, the working set of each process is stored in the TLB.

Increasing the Size of the TLB

Increase the Page Size. This may lead to an increase in fragmentation as not

all applications require a large page size.

Provide Multiple Page Sizes. This allows applications that require larger page

sizes the opportunity to use them without an increase in fragmentation.


I/O Interlock – Pages must sometimes be locked into memory.

Consider I/O. Pages that are used for copying a file from a device must be locked from being selected for eviction by a page replacement algorithm.

Windows NT Uses demand paging with clustering.

Clustering brings in pages surrounding the faulting page.

Processes are assigned working set minimum and working set maximum.

Working set minimum is the minimum number of pages the process is guaranteed to have in memory.

Windows NT A process may be assigned as many pages up

to its working set maximum. When the amount of free memory in the

system falls below a threshold, automatic working set trimming is performed to restore the amount of free memory.

Working set trimming removes pages from processes that have pages in excess of their working set minimum.

Documents

Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical