CSNB334 Advanced Operating Systems5. Memory Management

Review of basic concepts

What are the requirements of memory management? Relocation

For managing the available memory in a multiprogramming environment.

Protection Must be satisfied by the hardware (processor) rather than the

OS. Permissibility of a memory reference by an instruction can only be

checked at the time of execution of the instruction. Sharing Physical Organization

Moving information between the main memory and secondary memory.

Relocation

Phenomenon by which a process may occupy different partitions during the course of its life.

Three types of addresses Logical Address

Reference to a memory location independent of the current assignment of the process to memory.

Relative Address Type of logical address – address is expressed as a location

relative to some known point. Physical Address/ Absolute Addres

Actual location in main memory.

Loading

Absolute Loading Decision of where to load a module in the memory is made at

compile time. Thus, a given module is always loaded into a specific location in main

memory. Relocatable Loading.

Decision is made at load time. Thus a module can be loaded anywhere in the main memory. But, is swapped back to the same memory.

Dynamic Run-Time loading Decision is made at run-time.

Therefore, we can swap a process image into different locations at different times.

Done by special processor hardware rather than software. Base Register, Bounds Register, Adder, Comparator.

Memory Management Techniques Fixed partitioning

Main memory is divided into a number of static partitions at system generation time. Equal-size partitions Unequal-size partitions

Pros Simple to implement; little OS overhead

Cons Inefficient : internal fragmentation Maximum number of active processes – fixed.

Placement Algorithm One process queue per partition. Single queue.

Fixed partitioning Placement Algorithm

Memory Management Techniques Dynamic partitioning

Partitions are created dynamically Each process is loaded into a partition of exactly the same size as

that process. Pros

No internal fragmentation. Cons

External fragmentation. Counteract : by compaction.

But an overhead for the processor. Placement Algorithm : (because compaction is time consuming)

Best-fit First-fit. Next-fit.

Dynamic partitioning

Dynamic partitioning

Memory Management Techniques Paging

Main memory is divided into a number of equal-sized, relatively small frames.

Each process is divided into a number of equal-sized pages – same length as a frame.

A process is loaded by loading all of its pages into available frames. Not necessarily be contiguous.

Possible thru the use of a page table for each process. Logical address (page number, offset) --- Physical Address (frame

number, offset). Pros

No external fragmentation Cons

A small amount of internal fragmentation.

Address Translation in a Paging System

Program Paging Main Memory

Logical address

Register

Page Table

PageFrame

Offset

P#

Frame #

Page Table Ptr

Page # Offset Frame # Offset

+

Memory Management Techniques Segmentation

Each process is divided into number of segments. Need not be of same size.

A process is loaded by loading all of its segments into dynamic partitions. Need not be contiguous

Use segment table. Difference with dynamic partitioning

A process may occupy more than one partition. Partitions need not be contiguous.

Pros No internal fragmentation.

Cons External fragmentation : though less severe than dynamic partitioning

because of the small size of the segments.

Hardware support

Base + d

Program Segmentation Main Memory

Virtual Address

Register

Segment Table

Seg

men

t

d

S#

Length Base

Seg Table Ptr

Seg # Offset = d

Segment Table

+

+

Memory Management Techniques Virtual memory

Similar to paging/segmentation except that it is not necessary to load all of the segments/pages of a process into main memory. Nonresident pages/segments that are needed are brought in later

automatically. May require writing a page/segment out to disk if the memory is full.

Pros: Large virtual address space.

More processes may be maintained in main memory. A process may be larger than all of main memory.

Cons Overhead of complex memory management.

Thrashing : The system spends most of its time swapping pieces rather than executing instructions.

Translation Lookaside Buffer

Every virtual memory reference causes two physical memory access: To fetch the appropriate page table entry. To fetch the desired data.

Thus, the memory access time is doubled. To overcome this: Use a special high-speed cache for page table

entries – Translation Lookaside Buffer Similar to a memory cache. Contains those page table entries – most recently used.

Use of a Translation Lookaside Buffer

Virtual Memory

Virtual memory can be based on Paging only

Virtual Address : Page Number + Offset Page table entry : P(bit)+M(bit)+ Frame Number

Segmentation Virtual Address : Segment Number + Offset Segment table entry : P(bit)+M(bit)+ Length + Segment Base

Or, a combination of the two. Virtual Address : Segment Number + PageNumber + Offset Segment table entry : Length + Segment Base Page table entry : P(bit)+M(bit)+ Frame Number

Memory Management in Linux Linux uses demand paged virtual memory for memory

management design. It's a dynamic memory allocation technique that consists of

deferring page frame allocation until the last possible moment, for example, when a process attemps to access a page that is not present in RAM.

Basic unit of memory allcation – page. Page size : 212 (4096 bytes or 4KB).

Allocation of blocks in physical memory is as page frames Protection mechanism is page by page Sharing is also based on pages Swapping controls automatic movement through the memory

hierarchy.

Getting the Page size

The standard POSIX method #include <unistd.h> long sysconf (int name); long page_size = sysconf(_SC_PAGESIZE);

Linux also provides int getpagesize (void); Returns the page size in bytes.

PAGE_SIZE macro int page_size = PAGE_SIZE Retrieves the page size at compile time.

Abstract view of memory management

Managing the Virtual Address Space in Linux Each process : its own virtual address space.

In i386 arch, the virtual address is 32-bits wide. Therefore, the total virtual memory that a virtual address can

reference = 232 = 4GB. Page Size = 212. Therefore, number of pages that a virtual

address can reference = 220. Assuming that each PTE is 4 bytes, how many pages are

needed to store the page table?

Two Level Page Table The amount of memory devoted to page tables alone is

quite high. Therefore, page tables are stored in virtual memory

rather than main memory. This is achieved thru the use of a two-level hierarchical page table.

Root Page Table (4KB)

User Page Table (4MB)

User Address Space (4 GB)

Two-Level Page-Table Scheme

Hierarchical paging

Address translation scheme:

Virtual -> physical address translation……is a three level process in Linux

Virtual address

4 parts: Page directory offset j.pgd Page middle director offset j.pmd Page table offset j.pte Offset within page j.offset

The physical address i for a virtual address j is : i = PTE(PMD(PGD(j.pgd)+j.pmd)+j.pte)+j.offset.

•The x86(32-bit addressing) only supports a two level conversion of the address.•This is dictated by the hardware’s MMU…

•This is accomplished by reducing each page middle directory to only a single entry.

Segmentation in Linux Linux uses the segmentation model in a limited way. Each virtual address space is divided into two segments: User segment (3 Gb) to contain the applications code and

data. Addressable by the user. Unmapped virtual addresses are simply not used.

Kernel segment (1 Gb) permanently mapped and associated with fixed physical memory addresses used by the kernel.

System calls execute in kernel segment(mode).

4 GB

3 GB

2 GB

1 GB

ox00000000

Kernel

Tasks

Kernel Space (Code + Data)

User Space (Code + Data)

Per-Process Virtual Memory Layout Code (also called text)

segment Static Data segments

Initialized global (and C static) variables

Uninitialized global variables (zeroed when initializing the process, also called bss)

Stack segment: function calls, local variables (also called automatic in C)

Heap segment (malloc())

Page Table Flags

Each entry in the theoretical page table contains the following information: Valid flag. This indicates if this page table entry is valid, The physical page frame number that this entry is describing, Access control information. This describes how the page may

be used. Can it be written to? Does it contain executable code?

Flags in the page table entry indicate The legal access modes into the page. The page’s status.

A page’s status can give vital information for how memory management is performed.

Page Table Flags

PAGE_NONE – No physical memory page associated with entry.

PAGE_SHARED – All types of access permitted. PAGE_READONLY – No writing. “Copy-on-Write”

can be used. PAGE_KERNEL – kernel segment only allowed

access. PAGE_KERNEL_RO – kernel read-only access.