Mari Sama-sama Present OS

8/8/2019 Mari Sama-sama Present OS

1/21

8.4 LINUX MEMORYMANAGEMENT

Present by:

Mohamad Taufiq b. Mat Zakri CC08133

Muhammad Izzat b. Ibrahim CC08110

Muhammad Iman b. Abd. Razak CC08128

Muhammad Isamudin b. Abd. Ghani CC08056


2/21

` Since the early days of computing, there has been a

need for more memory than exists physically in a

system. Strategies have been developed to overcome

this limitation and the most successful of these is virtual

memory. Virtual memory makes the system appear to

have more memory than it actually has by sharing it

between competing processes as they need it.

Memory Management


3/21

` Large Address Spaces The operating system makes

the system appear as if it has a larger amount of

memory than it actually has. The virtual memory can be

many times larger than the physical memory in the

system,

` Protection Each process in the system has its own

virtual address space. These virtual address spaces are

completely separate from each other and so a processrunning one application cannot affect another. Also, the

hardware virtual memory mechanisms allow areas of

memory to be protected against writing. This protects

code and data from being overwritten by roguea lications.

Memory Management Provides:


4/21

` Fair Physical Memory Allocation The memory

management subsystem allows each running process in

the system a fair share of the physical memory of the

system,

` Shared Virtual MemoryAlthough virtual memory

allows processes to have separate (virtual) address

spaces, there are times when you need processes to

share memory. Shared memory can also be used as anInter Process Communication (IPC) mechanism, with

two or more processes exchanging information via

memory common to all of them. Linux supports the Unix

TMS

ystem V shared memory IPC.


5/21

Kernel Memory AllocationProcess Virtual Memory


6/21

LINUX VIRTUAL MEMORY

` -Virtual Memory Addressing Page Directory An active process has a single page directory

that is the size of one page. Each entry in the page directory

points to one page of the page middle directory. The page

directory must be in main memory for an active process. Page Middle Directory The page middle directorymay span

multiple pages. Each entry in the page middle directory points

to one page in the middle table.

Page Table The page table may also span multiple page.

Each page table entry refers to one virtual process.


7/21


8/21


9/21

` Linux page table structure is platform independent and

was designed to accommodate the 64-bit Alpha

processor

` With 64-bit addresses, the use of only two levels of

pages on the Alpha would result large page table and

directories

` Extra level of indirection are optimized away at compile

time, not at runtime

` Therefore, there is no performance overheadfor usinggeneric three levels design on platforms which supports

only two levels in hardware


10/21

Linux Memory Management

Page Allocation


11/21

Page Allocation Linux uses the Buddy algorithm to

effectively allocate and deallocate blocks of pages.

The page allocation code attempts to allocate ablock of one or more physical pages.

Pages are allocated in blocks which are powers of2 in size.(ex: 1, 2, 4, 8, 12, )


12/21

Page Allocation (cont.) as there are enough free pages in the system to

grant this request (nr_free_pages > min_free_pages)the allocation code will search the free_area for a

block of pages of the size requested.

Each element of the free_area has a map of theallocated and free blocks of pages for that sizedblock.

Example : element 2 of the array has amemory map that describes free andallocated blocks each of 4 pages long.


13/21

Page Allocation (cont.) allocation algorithm first searches for blocks of

pages of the size requested.

follows the chain of free pages that is queued onthe list element of the free_area data structure.

If no blocks of pages of the requested size arefree, blocks of the next size (which is twice that ofthe size requested) are looked for.

This process continues until all of the free_areahas been searched or until a block of pages hasbeen found.


14/21

Page Allocation (cont.) Indicate the financial advantages for the customer

If the block of pages found is larger than that

requested it must be broken down until there is ablock of the right size.

Because the blocks are each a power of 2 pagesbig then this breaking down process is easy as

you simply break the blocks in half. The free blocks are queued on the appropriate

queue and the allocated block of pages isreturned to the caller.


15/21

Page Allocation (cont.)


16/21

Page replacement algorithms

Linux page replacement algorithm is based on the clock algorithm

Clock is a more efficient version of FIFO than Second-chance because pages don't

have to be constantly pushed to the back of the list

but it performs the same general function as Second-Chance

The clock algorithm keeps a circular list of pages in memory, with the "hand"

(iterator) pointing to the oldest page in the list. When a page fault occurs and no empty frames exist, then the R (referenced) bit is

inspected at the hand's location.

If R is 0, the new page is put in place of the page the "hand" points to, otherwise the

R bit is cleared.

Then, the clock hand is incremented and the process is repeated until a page is

replaced.


17/21


18/21

Buddy allocator

Allocate and deallocate in units of one or more pages.

Allocate memory using power of two allocator

If a block of the correct size is not currently available, thenone is formed by splitting the next larger block in two,

forming two matched buddies. ( And if that larger size is

not available, then the next largest available size is split,

and so on. )


19/21


20/21

Slab allocator

Allocate smaller block of memory

Consisting of one or more contiguous pages

The kernel then creates separate caches for each type ofdata structure it might need from one or more slabs.

Initially the caches are marked empty, and are marked full

as they are used


21/21

Mari Sama-sama Present OS

Documents

Transcript of Mari Sama-sama Present OS