Chapter 10- Virtual memory

For EAT = (1-p) x memory access + p(?) ; P(?) is (4 points)

(page fault overhead + swap page out + swap page in + restart overhead )

For most computer systems the memory access time ranges from _____ to ____ nanoseconds

0 to 200

A page fault is triggered when p = _ and m = _

0,0

The modified bit is set to _ automatically by any instruction that stores data into the page and is used by the OS to minimize the movement of data to disk.

1

Immediately after a page replacement, the bits in the page table are p = _ and m = _

1,0

A page must be written back to disk when p = _ and m = _

1,1

Virtual memory can be implemented via : (2 points)

1- Demand Paging 2- Demand Segmentation

Benefits of program that is only partially in memory : (3points)

1- Program would no longer be constrained by the amount of physical memory that is available -> Users would be able to write programs for extremely large virtual address space 2- Increase in CPU utilization and throughput, and no increase in response time or turnaround time —-> because each user program could take less physical memory —> so more programs can run at same time 3 - Each user program will run faster —> because of less I/O will be needed to load or swap user programs into memory

Benefits of sharing :

1- System libraries can be shared by several processes through mapping of the shared object into virtual address space . 2- virtual memory enables processes to share memory -> Two or more processes can communicate through the use of shared memory 3- virtual memory can allow pages to be shared during process creation with the fork() system call —> thus speeding up process creation

Steps of basic page replacement :

1- find location of desired page on disk 2- find a free frame , __> if there is free frame use it, if no free frame use page replacement algorithm to select a victim frame 3- bring desired page into newly free frame then update page and frame tables 4- restart process

Steps of operating systems when it reads from table : 6 steps

1- operating systems looks at another table to decide either if it is invalid reference -> it get aborted, or just not in memory 2- Get empty frame 3- Swap page into frame 4- resets tables 5- set validation bit to v 6- restart the instruction that caused page fault

Three major components of page fault service time which they are :

1- service page-fault interrupt 2- read page 3- restart process

12 steps of demand paging :

1- trap the os 2- save user registers and process state 3- identify that this interrupt is a page fault 4- check if pager reference is legal and determine page location in disk 5- issue a read from disk to a free frame a - wait in queue until read request is serviced b- wait for device seek/latency time c- transfer page to free frame 6- allocate cpu to other user 7- I/O completed 8- save registers and process states for other users 9- determine that the interrupt from disk 10- correct page table 11- wait CPU to allocate to this process 12-restore user registers, process state, and new page table , then resume the interrupted instructions

For FIFO , the reference string is :1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 with 4 frames , # number of faults is ?

10

For optimal algorithm , with 4 frames , and string of 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 . How many page faults are there ?

6 page faults

If memory access time is 200 nanoseconds , and average page fault service time is 8 millisecond the EAT equal ___

8 milliseconds equals = 8,000,000 nanoseconds (1-p) x 200 + p(8milliseconds) = 200 - 200p +8,000,000p = 200 - 7,999,800p (so the difference is from 8,000,000 nanoseconds to 7,999,800 nanseconds with is an decrease of 200 ns therfore the EAT is 8.2 microseconds)

For FIFO , the reference string is :1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 with 3 frames , # number of faults is ?

9 faults

The fork () system call creates ____________

A child process that is duplicate of its parent

Creates a single large continuous address space per process.

A paged virtual memory

Swapper that deals with pages is called ________

A pager

Since few programs make use of large address spaces that can be found in virtual memory, much of the space taken up by a segment or page table in memory would be wasted. How can we reduce this overhead?

A table can be divided into individual pages and brought into memory as needed using demand paging and page replacement.

Invalid reference means _________

Abort

Not-in-memory means ________

Bring to memory

_____. Allows more efficient _________ as only modified pages are _____

COW , process creation , copied

What is the process that is similar to page sharing , and provides rapid process creation and minimizes the number of new pages that should be allocated to newely created process

Copy on write

_________ allows both parent and child process to initially share same pages in memory

Copy on write (COW)

Virtual memory allows other benefits during process creation :

Copy on write (COW) Memory mapped files

The principle of loading a page into memory only when the page is needed, rather than at the start of the execution.

Demand paging

EAT equation is ___

EAT = (1-P) * ma + P * page fault time , P here is the page fault rate

Computing the __________ for a demand paged memory will show the affect of performance in computer systems

Effective access time

EAT means ____

Effective access time

How to evaluate a page replacement algorthim ?

Evaluate it by running the algorithm on particular string of memory references and compute the number of page faults on that string

What is the counter implemention that is in LRU?

Every page has a counter, every time page is referenced through this entry, copy the clock into the counter.

True or false? VM cannot be implemented without the modified bit.

False

The hole between the heap and stack is not part of the virtual address ?

False , its part of the virtual address

Virtual memory allows ________ and _______ to be shared by two or more _______ through ________________

Files , memory , processes, page sharing

What is page replacement?

Find some page in memory, but not really in use, swap it out.

FIFO stand for :

First in first out

Process creation happens using _____ () system call

Fork

We allow for the stack to _________ in memory

Grow downward

We allow for the heap to ____________ in memory as it used for _____________

Grow upward, dynamic memory allocation

Who is the big boss among you?

I know that you say Mohammed Alameri, but no hw is not. He is the Boss of Bosses

How copy on write works?

It allows parent and child to share the same page, these pages are marked as copy on write pages, if process writes to shared page, a copy of shared page is created

This is the case if a page replacement occurs for a page that has not been modified during execution.

It does not have to be copied back to disk because an exact copy still exists on the disk.

Logical address space can therefore be much ______ than physical address spzce

Larger

______ never swaps a page into memory unless page will be _______

Lazy swapper, needed

What LRU stands for ?

Least recently used

The virtual address space of a process refers to a __________ or _______ view of how process is stored in memory

Logical , or virtual

Page replacements completes the separation between _________ and ____________

Logical memory and physical memory

Use _________ to reduce the overhead of page transfers , where only ___________________

Modify(dirty) bit, modified pages are written to disk

Prevent over-allocation of memory by ______

Modifying page-fault service routine to include page replacement

What is belady's anomaly ?

More frames = more page faults

If page fault is 0 this means _________

No page faults

Page fault rate represented by the letter ___ and ranges from ____ to ____

P , 0 to 1.0

IF there is a reference to a page, first reference to that page will trap to operating system is called _______

Page Fault

During address translation if a vlid-invalid bit in page is set to i then that will cause _______

Page fault

The act of overwriting a page in memory with a different page loaded from the disk when needed.

Page replacement

During __________ , same page could be brought into memory several times

Page replacment

Free _____ are allocated from a pool of ________________

Pages, zeroed-out pages

What is the optimal algorithm ?

Replace the page that will not be used for longest period of time

To compute the effective access time , we must know how much time is neeeded to _______

Service a page fault

If we want performance degradation to be less than 10 percent we need, write the calculations :

Since 10% less so from 200 it will be 220 (because 200 x 10% is 220) 220 > 200 - 7,999,800p We send 200 to the right side 20 > -7,999,800p We divide 20 by - 7,999,800 to make the p alone P > - 0.0000025 To take away the sign we will flip the logical operator from > to < P < 0.0000025

Virtual address spaces that include holes are known as __________

Sparse address spaces

When a page needs to be changed , look at _______

The counter to determine which are to change

What must happen when all frames are occupied and a page fault occurs?

The resident pages must be removed from memory to create a free frame.

Exaplain the logical virtual view:

The view where a process begins at a certain logical address and exists in contiguous memory

As long as we have no page faults the effective access time will equal to memory access time (T/F)

True

Demand paging can affect the perfomance of computer system (T/F)

True

If either process modifies a shared page , only then is the page copied (T/F)

True

T/F : Only part of program needs to be in memory

True

True or false? VM (virtual memory) cannot be implemented without demand paging.

True

True or false? VM cannot be implemented without page replacement.

True

True or false? When a page fault occurs and no free frame is available, the algorithm chooses one of the resident pages for replacement, regardless of which process the page belong to.

True

____ seperartion of user logical memory from physical memory

Virtual memory

Large _________ can be provided on smaller __________

Virtual memory , physical memory

________ allows address spaces to be shared by several ________

Virtual memory , processes

________allows for more efficient process ______

Virtual memory, Creation

Copy on write technique is used by several os like ____

Windows xp, linux and solaris

OS allocate free pages using ______, where ____________ have been zeroed-out before being allocated thus erasing the previous contents

Zero fill on demand , zero-fill-on-demand pages

Bring a page into memory only when it is needed is called _________

demand paging

Makes the implementation of virtual memory possible by moving pages to main memory only as needed.

demand paging

If page fault is 1 this means______

every reference is a fault

Memory consists of a fixed number of _______

frames

Initially valid-invalid bit is set to ___ on all entries

i

Virtual memory is typically much ______ than the available physical memory.

larger

A binary flag in each page table entry that indicates whether the corresponding page has been modified during execution.

modified-bit (m-bit)

When the present bit is 0, the corresponding page resides _______

on disk only (not in physical memory)

An interrupt that occurs when a program attempts to reference a non-resident page. The interrupt triggers the OS to find the page on disk, copy the page into a frame, and set the present bit to 1.

page fault

If a _____ occurs ,we must first read the relevant page from disk and then access the desired word

page fault

Both a segment table and page table can be divided into pages and kept track of by a ___________

page table

Both the modified bit and the present bit can be found in the __________.

page table

Creates multiple large address spaces per process, each of which is paged.

paged virtual memory with segmentation

When determined that a page need to be duplicated , the location of the free page to be allocated need to be determined where some systems provide_____ to such requests.

pool

A binary flag in each page table entry that indicates whether the corresponding page is currently resident in memory. If a page is resident, then the entry points to the frame that holds the page.

present bit

Page is needed means ______

reference to it

Moving pages between memory and the disk is very ________

time consuming

With each page table entry _______ is associated , ___ means in memory, and ___ means not-in-memory

valid-invalid bit , v, i

A logical address in virtual memory.

virtual address

Collection of one or more logical address spaces, each of which may exceed the size of physical memory.

virtual memory

Benfits of demand paging (4 points)

١- Less I/O needed ٢- Less memory needed ٣- Faster response ٤- More users