Compare Memory Management Of Unix And Linux Computer Science Essay

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Primary memory is a very precious resource which frequently cannot contain every active processes in the system

The memory management system has the ability to decide which processes should reside (at least partially) in main memory

It also monitors the amount of available primary memory and may also periodically write processes to a secondary device which is called the swap device to provide more space in primary memory

At a later time, the kernel also reads the data from swap device back to the main memory

UNIX Memory Management Policies

Has two main areas:

Swapping

It's Easy to implement

It utilizes Less system overhead

Demand Paging

Has Greater flexibility

Swapping

The swap device is like a block device in a configurable section of a disk

Kernel allocates contiguous space on the swap device without any fragmentation

It also maintains free space of the swap device in an in-core table, called map

The kernel treats each unit of the swap map as a group of disk blocks

As kernel allocates and frees resources, it also updates the map accordingly

Swapping Process Out

Memory à Swap device

Kernel swap out whenever it needs memory

When fork() called for allocate child process

When called for increase the size of process

When the process become larger by growth of its stack

Previously swapped out processes want to swap in but there is not enough memory

The kernel should gather the page addresses of data at primary memory to be swapped out

Kernel copies the physical memory assigned to a process to the allocated space which is on the swap device

The mapping between physical memory and swap device is kept in page table entry

Fork Swap

There may not be enough free memory when fork() called

Child process swap out and "ready-to-run"

Swap in when kernel schedules it

Expansion Swap

It reserves enough space on the swap device to contain the memory space of the process, including newly requested space

Then it adjusts the address translation mapping of the process

Finally, it swaps the process out on newly allocated space in the swapping device

When the process swaps the process into memory, it will allocate physical memory according to the new address translation map

Demand Paging

Not all pages of process resides in memory

Locality

When a process accesses a page which is not part of its working set, it incurs a page fault.

The kernel suspends the execution of a process until it reads the page into memory and makes it accessible to the process

Data Structure for Demand Paging

Page table entry

Disk block descriptors

Page frame data table

Swap use table

Page Table Entry

It contains the physical address of page and the following bits:

Valid: whether the page content is legal

Reference: whether the page is referenced recently or not

Modify: If the page content is modified

copy on write: kernel must create a new copy when a process modifies its content

Age: Age of page

Protection: Read/ write permission

Disk Block Descriptor

Swap Device number as there can be several swap devices

Block number that contains the page

Memory Management in Linux

It contains two Parts

Architecture Independent Memory

It should be flexible and portable enough across platforms

Implementations for a specific architecture

Architecture Independent Memory Model

Process virtual address space which is divided into pages

Page size given in PAGE_SIZE macro in asm/page.h

(4K for x86 and 8K for Alpha)

The pages are mostly divided between 4 segments

User Code, User Data, Kernel Code, Kernel Data

In User mode, access is only User Code and User Data

But in Kernel mode, access is also needed for User Data

put_user(), get_user(), memcpy_tofs(), memcpy_fromfs() allow kernel to access user data

Registers cs and ds point to the code and the data segments of the current mode

fs points to data segment of the calling process in kernel mode.

Get_ds(), get_fs(), and set_fs() are also defined in asm/segment.h

Segment + Offset = 4 GB Linear address (32 bits)

Of this, user space = 3 GB and kernel space = 1GB

Linear Address are converted to physical address using 3 levels

Page Table Entry (pte_t)

Various Attributes

Presence (is the page present in VAS?)

Read, Write and Execute

Accessed ?

Dirty

Macros of Pgprot_type

PAGE_NONE (invalid)

PAGE_SHARED (read-write)

PAGE_COPY/READ_ONLY (read only, used by copy-on-write)

PAGE_KERNEL (accessibe only by kernel)

Page Table Functions

mk_pte(), Pte_clear(), set_pte()

pte_mkclean(), pte_mkdirty(), pt_mkread(), ….

pte_none() (checks whether entry is set)

pte_page() (returns the address of page)

pte_dirty(), pte_present(), pte_young(), pte_read(), pte_write()

Traditional mmap()

int do_mmap(struct file *, unsigned long addr, unsigned long len, unsigned long prot, unsigned long flags, unsigned long off);

It creates a new memory region

Also creates the required PTEs

Sets the PTEs to fault later

The handler (nopage) will either copy-on-write if anonymous mapping, or will bring in the required page of the file.

How is brk() implemented?

Checks whether to allocate (deny if not enough physical memory, exceeds its VA limits, or crosses stack).

Then call do_mmap() for anonymous mapping between the old and the new values of brk (in process table).

Returns the new brk value.

Kernel Segment

On a sys call, CS points to the kernel segment. DS and ES are set to the kernel segment as well.

Next, FS is set to the user data segment.

Put_user() and get_user() can then access the user space if needed.

The address parameters to these functions cannot exceed 0xc0000000.

Violation of this results in a trap, together with any writes to a read-only page (creates a problem on 386, while the problem does not exist in 486/Pentium)

Hence, verify_area() is typically called before performing any such operations.

Physical and Virtual addresses are the same except for those allocated using vmalloc().

Kernel segment is shared across processes (not switched !)

Memory Allocation for Kernel Segment

Static

Memory_start = console_init(memory_start, memory_end);

Typically done for drivers to reserve areas, and for some of the other kernel components.

Dynamic

Void *kmalloc(size, priority), Void kfree (void *) // in mm/kmalloc.c

Void *vmalloc(size), void *vmfree(void *) // in mm/vmalloc.c

Kmalloc is mainly used for physically contiguous pages while vmalloc does not allocate physically contiguous pages

Memory allocated is not initialized (and is not paged out).

vmalloc()

Allocates virtually contiguous pages, but they do not need to be physically contiguous.

Uses get_free_page() to allocate the physical frames.

Once all the required physical frames are found, the virtual addresses are then created at an unused part.

The virtual address searches for unused parts on x86 begins at the next address after physical memory on an 8 MB boundary.

One (virtual) page is also left free after each allocation for cushioning.

vmalloc vs kmalloc

Contiguous vs non-contiguous physical memory

kmalloc is faster and less flexible

vmalloc involves get_free_page() and sometimes needs to block to find a free physical page

DMA requires contiguous physical memory

Paging

All kernel segment pages are locked in the memory (no swapping)

The user pages can be paged out:

Complete block device

By fixed length files in a file system

First 4096 bytes are like a bitmap indicating that space for that page is available for its paging.

At byte 4086, string "SWAP_SPACE" is stored.

Hence, max swap of 4086*8-1 = 32687 pages = 130784KB per device or file

MAX_SWAPFILES specifies the number of swap files or devices

Swap devices are more efficient than swap file.

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