Allocating new pages, Operating System

Assignment Help:

Processes have valid and invalid entries on their page tables. The valid entries all point to some where "real" (e.g. a physical page, or some portion of disk in case of non-resident pages, etc). The entries that don't point anywhere are the entries that we will use when allocating a new page.

The allocation of new pages can be done in two ways: either via sbrk(), or via mmap(). If you want to increase the size of the heap (i.e. the number of valid pages), you can use sbrk().

Using mmap(), on the other hand, maps a ?le into a process' virtual address space. In the allocator you implemented, for example, you used mmap() to map memory addresses to the ?le /dev/zero. This makes it seem like you were allocating space from /dev/zero each time you called mmap(). (Remember that whenever you read something from /dev/zero, you get only zeroes for as long as you want to keep reading.) But, since /dev/zero is a read-only ?le and we usually call mmap() using the MAP PRIVATE ?ag, we follow the COW rules. When you actually try to write to the memory mmap()'d from /dev/zero, the OS intervenes and clones the corresponding page. So, instead of actually writing to /dev/zero, you end up writing to a new memory page.

Now suppose you mmap 3 pages to /dev/zero. Right after you do this, the process' page table contains three mappings to /dev/zero. These are all COW mappings to a same single page in memory, which itself maps to /dev/zero3. However, the ?rst time some of these pages is modi?ed, a new page is created, and the corresponding mapping in one of the page tables is modi?ed. Notice that we could have used mmap with any other ?le instead of /dev/zero; say, an MP3 ?le.

In this case, whenever we mmap'd, we would be actually mapping memory addresses to portions of the MP3 ?le. If we then tried to write to those areas of memory, we would be indirectly over writing the ?le! Notice, however, that we could be careful enough and used the mmap parameter MAP PRIVATE; then, we would still be able to read from the MP3 ?le, but all writings to it would be done using Copy On Write.


Related Discussions:- Allocating new pages

Define memory management, Define memory management In multiprogramming ...

Define memory management In multiprogramming system the user part of memory should be further subdivided to accommodate multiple processes. The task of subdivision is carried o

Explain about threading issues, Explain about threading issues? The for...

Explain about threading issues? The fork and exec system calls In a multithreaded program of few UNIX systems have chosen to have two versions of fork, one that duplicates e

Transfer time, A hard-disk drive reads “120 GB HDD 7200 rpm 3 GB/sec transf...

A hard-disk drive reads “120 GB HDD 7200 rpm 3 GB/sec transfer rate”. If the drive has a sector size of 512 bytes, what is the average rotational latency and transfer time to read

Nachos oprating system assignment, i am just wondering if you have expert i...

i am just wondering if you have expert in nachos operating system thank you

Explain optimal page replacement, Optimal page replacement An optimal ...

Optimal page replacement An optimal page replacement algorithm has the least page fault rate of all algorithms. The algorithm states that a 'replace the page that will not be

Sector sparing, What is sector sparing is proper definition

What is sector sparing is proper definition

If a process is blocked, There is a process bloacked by a semaphore and no ...

There is a process bloacked by a semaphore and no other process in the ready queue, can the blocked process get back to the ready queue without the semsignal() function?

Memory Management Problems, Sir i want formulas in Memory management starti...

Sir i want formulas in Memory management starting from paging till the demang paging in a easy manner for solving problems in competitive exams

Define the turn-around time of a user job, Define the “turn-around” time of...

Define the “turn-around” time of a user job Turn- around time is total time taken to execute the job.

What are race conditions?, What are race conditions? Race conditions ar...

What are race conditions? Race conditions are problems that take place due to the sharing of the same file by several processes. In such a case none of the processes is able to

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd