Lecture 7: Introduction to virtualmemory

Lecture 7: Introduction to virtualmemory

Mythili VutukuruIIT Bombay

Why virtualize memory?• Because real view of

memory is messy!• Earlier, memory had only

code of one running process(and OS code)

• Now, multiple activeprocesses timeshare CPU– Memory of many processes

must be in memory– Non-contiguous too

• Need to hide this complexityfrom user

2

• Because real view ofmemory is messy!

• Earlier, memory had onlycode of one running process(and OS code)

• Now, multiple activeprocesses timeshare CPU– Memory of many processes

must be in memory– Non-contiguous too

• Need to hide this complexityfrom user

Abstraction: (Virtual) Address Space

• Virtual address space: everyprocess assumes it has accessto a large space of memoryfrom address 0 to a MAX

• Contains program code (andstatic data), heap (dynamicallocations), and stack (usedduring function calls)• Stack and heap grow

during runtime• CPU issues loads and stores

to virtual addresses3

• Virtual address space: everyprocess assumes it has accessto a large space of memoryfrom address 0 to a MAX

• Contains program code (andstatic data), heap (dynamicallocations), and stack (usedduring function calls)• Stack and heap grow

during runtime• CPU issues loads and stores

to virtual addresses

How is actual memory reached?• Address translation from

virtual addresses (VA) tophysical addresses (PA)– CPU issues loads/stores to VA

but memory hardwareaccesses PA

• OS allocates memory andtracks location of processes

• Translation done by memoryhardware called MemoryManagement Unit (MMU)– OS makes the necessary

information available

• Address translation fromvirtual addresses (VA) tophysical addresses (PA)– CPU issues loads/stores to VA

but memory hardwareaccesses PA

• OS allocates memory andtracks location of processes

• Translation done by memoryhardware called MemoryManagement Unit (MMU)– OS makes the necessary

information available4

Example: Paging

• OS divides virtual address spaceinto fixed size pages, physicalmemory into frames

• To allocate memory, a page ismapped to a free physical frame

• Page table stores mappings fromvirtual page number to physicalframe number for a process (e.g,page 0 to frame 3)

• MMU has access to page tables,and uses it to translate VA to PA

• OS divides virtual address spaceinto fixed size pages, physicalmemory into frames

• To allocate memory, a page ismapped to a free physical frame

• Page table stores mappings fromvirtual page number to physicalframe number for a process (e.g,page 0 to frame 3)

• MMU has access to page tables,and uses it to translate VA to PA

5

Goals of memory virtualization

• Transparency: user programs should not beaware of the messy details

• Efficiency: minimize overhead and wastage interms of memory space and access time

• Isolation and protection: a user processshould not be able to access anything outsideits address space

• Transparency: user programs should not beaware of the messy details

• Efficiency: minimize overhead and wastage interms of memory space and access time

• Isolation and protection: a user processshould not be able to access anything outsideits address space

6

How can a user allocate memory?

• OS allocates a set ofpages to the memoryimage of the process

• Within this image– Static/global variables

are allocated in theexecutable

– Local variables of afunction on stack

– Dynamic allocation withmalloc on the heap

• OS allocates a set ofpages to the memoryimage of the process

• Within this image– Static/global variables

are allocated in theexecutable

– Local variables of afunction on stack

– Dynamic allocation withmalloc on the heap

7

Memory allocation system calls• malloc implemented by C

library– Algorithms for efficient memory

allocation and free spacemanagement

• To grow heap, libc uses thebrk/sbrk system call

• A program can also allocate apage sized memory using themmap()system call– Gets “anonymous” page from OS

brk

• malloc implemented by Clibrary– Algorithms for efficient memory

allocation and free spacemanagement

• To grow heap, libc uses thebrk/sbrk system call

• A program can also allocate apage sized memory using themmap()system call– Gets “anonymous” page from OS

8

mmap

A subtle point: what is the addressspace of the OS?

• OS is not a separateprocess with its ownaddress space

• Instead, OS code is part ofthe address space of everyprocess

• A process sees OS as partof its code (e.g., library)

• Page tables map the OSaddresses to OS code

• OS is not a separateprocess with its ownaddress space

• Instead, OS code is part ofthe address space of everyprocess

• A process sees OS as partof its code (e.g., library)

• Page tables map the OSaddresses to OS code

9

OS

How does the OS allocate memory?

• OS needs memory for its data structures• For large allocations, OS allocates a page• For smaller allocations, OS uses various

memory allocation algorithms (more later)– Cannot use libc and malloc in kernel!

• OS needs memory for its data structures• For large allocations, OS allocates a page• For smaller allocations, OS uses various

memory allocation algorithms (more later)– Cannot use libc and malloc in kernel!

10