Memory Allocation. Copyright : University of Illinois CS 241 Staff 1

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1 Memory Allocation Copyright : University of Illinois CS 241 Staff 1

2 Recap: Virtual Addresses A virtual address is a memory address that a process uses to access its own memory Virtual address actual physical RAM address When a process accesses a virtual address, the MMU hardware translates the virtual address into a physical address The OS determines the mapping from virtual address to physical address Copyright : University of Illinois CS 241 Staff 2

3 Recap: Virtual Addresses Benefit: Isolation Virtual addresses in one process refer to different physical memory than virtual addresses in another Exception: shared memory regions between processes (discussed later) Benefit: Illusion of larger memory space Can store unused parts of virtual memory on disk temporarily Benefit: Relocation A program does not need to know which physical addresses it will use when it s run Can even change physical location while program is running Copyright : University of Illinois CS 241 Staff 3

4 Mapping virtual to physical addresses Stack How does this thing work?? MMU Heap Data segment Code segment Physical RAM Copyright : University of Illinois CS 241 Staff 4

5 MMU and TLB Memory Management Unit (MMU) Hardware that translates a virtual address to a physical address Each memory reference is passed through the MMU Translate a virtual address to a physical address Lots of ways of doing this! Virtual address CPU MMU Translation mapping Physical address Memory TLB Cache of translations Copyright : University of Illinois CS 241 Staff 5

6 MMU and TLB Translation Lookaside Buffer (TLB) Cache for MMU virtual-to-physical address translations Just an optimization but an important one! Virtual address CPU MMU Translation mapping Physical address Memory TLB Cache of translations Copyright : University of Illinois CS 241 Staff 6

7 Translating virtual to physical Can do it almost any way we like But, some ways are better than others Strawman solution from last time Base and bound Copyright : University of Illinois CS 241 Staff 7

8 Base and bounds if (virt addr > bound) trap to kernel } else { phys addr = virt addr + base } physical memory size physical memory Process has the illusion of running on its own dedicated machine with memory [0,bound) Provides protection from other processes also currently in memory bound virtual memory base + bound base 0 0 Copyright : University of Illinois CS 241 Staff 8

9 Base and Bounds Registers CPU Address Bounds Register Logical Address LA Memory Address MA Base Register < + Fault Base Address BA Physical Address PA Base: start of the process s memory partition Limit: max address in the process s memory partition Base Address MA+BA Memory Limit Address Copyright : University of Illinois CS 241 Staff 9

10 Base and bounds Problem: Process needs more memory over time Stack grows as functions are called Heap grows upon request (malloc) Processes start and end How does the kernel handle the address space growing? You are the OS designer Design algorithm for allowing processes to grow bound Process 1 virtual memory base + bound base 0 0 Process 2 physical memory Copyright : University of Illinois CS 241 Staff 10

11 But wait, didn t we solve this? grows dynamically grows dynamically fixed size Stack Heap Data segment Problem: wasted space And must have virtual mem phys mem base + bound physical memory fixed size Code segment base Copyright : University of Illinois CS 241 Staff 12

12 Another attempt: Segmentation Segment Region of contiguous memory Segmentation Generalized base and bounds with support for multiple segments at once Copyright : University of Illinois CS 241 Staff 13

13 Segmentation fff virtual memory segment 3 physical memory Seg # Base Bound Description 0 stack code 46ff Code segment Data segment 2 Unused Stack segment 04ff 0 Virtual memory segment 1 data stack 2fff ff 0 Virtual memory segment 0 code data 4ff Copyright : University of Illinois CS 241 Staff 14

14 Segmentation Segments are specified many different ways Advantages over base and bounds? Protection Different segments can have different protections Flexibility Can separately grow both a stack and heap Enables sharing of code and other segments if needed fff 0 04ff 0 6ff 0 virtual memory segment 3 stack Virtual memory segment 1 data Virtual memory segment 0 code physical memory code stack data 46ff fff ff Copyright : University of Illinois CS 241 Staff 15

15 Segmentation Segments are specified many different ways Advantages over base and bounds? What must be changed on context switch? Contents of your segmentation table A pointer to the table, expose caching semantics to the software (what x86 does) fff 0 04ff 0 6ff 0 virtual memory segment 3 stack Virtual memory segment 1 data Virtual memory segment 0 code physical memory code stack data 46ff fff ff Copyright : University of Illinois CS 241 Staff 16

16 Recap: Mapping Virtual Memory Base & bounds Problem: growth is inflexible Problem: external fragmentation As jobs run and complete, holes left in physical memory Segments Resize pieces based on process needs Problem: external fragmentation Note: x86 used to support segmentation, now effectively deprecated with x86-64 Modern approach: Paging Copyright : University of Illinois CS 241 Staff 17

17 Paging Solve the external fragmentation problem by using fixed-size chunks of virtual and physical memory Virtual memory unit called a page Physical memory unit called a frame (or sometimes page frame) virtual memory ( process (for one page 0 page 1 page 2 page 3 page X physical memory frame 0 frame 1 frame 2 frame Y Copyright : University of Illinois CS 241 Staff 18

18 Application Perspective Application believes it has a single, contiguous address space ranging from 0 to 2P 1 bytes Where P is the number of bits in a pointer (e.g., 32 bits) In reality, virtual pages are scattered across physical memory This mapping is invisible to the program, and not even under it's control! Copyright : University of Illinois CS 241 Staff 19

19 Application Perspective (Reserved for OS) Stack Lots of separate processes Heap Uninitialized vars (BSS segment) Initialized vars (data segment) Code (text segment) (Reserved for OS) Stack Heap Uninitialized vars (BSS segment) Initialized vars (data segment) Code (text segment) (Reserved for OS) Stack Heap Uninitialized vars (BSS segment) Initialized vars (data segment) Code (text segment) MMU Physical RAM Copyright : University of Illinois CS 241 Staff 20

20 Translation process Virtual-to-physical address translation performed by MMU Virtual address is broken into a virtual page number and an offset Mapping from virtual page to physical frame provided by a page table (which is stored in memory) 0xdeadbeef = 0xdeadb 0xeef Virtual page number Offset Copyright : University of Illinois CS 241 Staff 21

21 ... 0xdeadb Translation process virtual address virtual page # offset page table page frame # 0xeef physical address page frame # offset Page table entry page frame 0 page frame 1 page frame 2 page frame 3 page frame Y Copyright : University of Illinois CS 241 Staff 22

22 Translation process if (virtual page is invalid or non-resident or protected) trap to OS fault handler else physical frame # = pagetable[virtpage#].physpagenum Each virtual page can be in physical memory or swapped out to disk (called paged out or just paged ) What must change on a context switch? Could copy entire contents of table, but this will be slow Instead use an extra layer of indirection: Keep pointer to current page table and just change pointer Copyright : University of Illinois CS 241 Staff 23

23 Where is the page table? Page Tables store the virtual-to-physical address mappings. Where are they located? In memory! OK, then. How does the MMU access them? The MMU has a special register called the page table base pointer This points to the physical memory address of the top of the page table for the currently-running process Copyright : University of Illinois CS 241 Staff 24

24 Where is the page table? MMU pgtbl base ptr Process A page tbl Process B page tbl Physical RAM Copyright : University of Illinois CS 241 Staff 25

25 Paging Can add read, write, execute protection bits to page table to protect memory Check is done by hardware during access Can give shared memory location different protections from different processes by having different page table protection access bits How does the processor know that a virtual page is not in memory? Resident bit tells the hardware that the virtual address is resident or non-resident Copyright : University of Illinois CS 241 Staff 26

26 Valid vs. Resident Resident Virtual page is in memory NOT an error for a program to access non-resident page Valid Virtual page is legal for the program to access e.g., part of the address space Copyright : University of Illinois CS 241 Staff 27

27 Valid vs. Resident Who makes a page resident/non-resident? OS memory manager Who makes a virtual page valid/invalid? User actions Why would a process want one if its virtual pages to be invalidated? Avoid accidental memory references to bad locations Copyright : University of Illinois CS 241 Staff 28

28 Page Table Entry Typical PTE format (depends on CPU architecture!) M R V prot page frame number Various bits accessed by MMU on each page access: Modify bit: Indicates whether a page is dirty (modified) Reference bit: Indicates whether a page has been accessed (read or written) Valid bit: Whether the PTE represents a real memory mapping Protection bits: Specify if page is readable, writable, or executable Page frame number: Physical location of page in RAM Why is this 20 bits wide in the above example? Copyright : University of Illinois CS 241 Staff 29

29 Page Faults What happens when a program accesses a virtual page that is not mapped into any physical page? Hardware triggers a page fault Page fault handler Find any available free physical page If none, evict some resident page to disk Allocate a free physical page Load the faulted virtual page to the prepared physical page Modify the page table Copyright : University of Illinois CS 241 Staff 30

30 Advantages of Paging Simplifies physical memory management OS maintains a free list of physical page frames To allocate a physical page, just remove an entry from this list No external fragmentation! Virtual pages from different processes can be interspersed in physical memory No need to allocate pages in a contiguous fashion Copyright : University of Illinois CS 241 Staff 31

31 Advantages of Paging Allocation of memory can be performed at a (relatively) fine granularity Only allocate physical memory to those parts of the address space that require it Can swap unused pages out to disk when physical memory is running low Idle programs won't use up a lot of memory (even if their address space is huge!) Copyright : University of Illinois CS 241 Staff 32

32 Paging Example Request Address within Virtual Memory Page 3 Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 33

33 Paging Example Request Address within Virtual Memory Page 1 Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 34

34 Paging Example Request Address within Virtual Memory Page 6 Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 35

35 Paging Example Request Address within Virtual Memory Page 2 Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 36

36 Paging Example Request Address within Real Memory Virtual Memory Page 8 Page Table 1 Cache VM Frame What happens when there is no more space 2 4 in the cache? Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 37

37 Paging Example Store Virtual Memory Page 1 to disk Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 38

38 Paging Example Process request for Address within Virtual Memory Page 8 Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 39

39 Paging Example Load Virtual Memory Page 8 to cache Cache Page Table VM Frame Real Memory Virtual Memory Stored on Disk Disk Copyright : University of Illinois CS 241 Staff 40

40 Is paging enough? (Reserved for OS) How do we allocate memory in here? Stack Heap MMU Uninitialized vars (BSS segment) Initialized vars (data segment) Code (text segment) Physical RAM Copyright : University of Illinois CS 241 Staff 41

41 Memory allocation within a process What happens when you declare a variable? Allocating a page for every variable wouldn t be efficient Allocations within a process are much smaller Need to allocate on a finer granularity Copyright : University of Illinois CS 241 Staff 42

42 Memory allocation within a process Solution (stack): stack data structure Function calls follow LIFO semantics So we can use a stack data structure to represent the process s stack no fragmentation! Solution (heap): malloc This is a much harder problem Need to deal with fragmentation Copyright : University of Illinois CS 241 Staff 43

43 Problems What was the key abstraction not supported well by segmentation and by B&B? Supporting an address space larger than the size of physical memory How could you support this using B&B and segmentation? Use lots of segments and have the user switch between them (this is kind of how x86 segmentation works) Note: x86 used to support segmentation, now effectively deprecated with x86-64 Copyright : University of Illinois CS 241 Staff 44

44 Paging On heavily-loaded systems, memory can fill up Need to make room for newly-accessed pages Heuristic: try to move inactive pages out to disk Paging What constitutes an inactive page? Refers to moving individual pages out to disk (and back) We often use the terms paging and swapping interchangeably Different from context switching Background processes often have their pages remain resident in memory Copyright : University of Illinois CS 241 Staff 45

45 Demand Paging Never bring a page into primary memory until its needed Fetch Strategies When should a page be brought into primary (main) memory from secondary (disk) storage. Placement Strategies When a page is brought into primary storage, where should it be put? Replacement Strategies Which page now in primary storage should be removed from primary storage when some other page or segment needs to be brought in and there is not enough room Copyright : University of Illinois CS 241 Staff 46

46 Page Eviction When do we decide to evict a page from memory? Usually, at the same time that we are trying to allocate a new physical page However, the OS keeps a pool of free pages around, even when memory is tight, so that allocating a new page can be done quickly The process of evicting pages to disk is then performed in the background Copyright : University of Illinois CS 241 Staff 47

47 Page Eviction: Which page? Hopefully, kick out a less-useful page Dirty pages require writing, clean pages don t Where do you write? To swap space Goal: kick out the page that s least useful Problem: how do you determine utility? Heuristic: temporal locality exists Kick out pages that aren t likely to be used again Copyright : University of Illinois CS 241 Staff 48

48 Basic Page Replacement How do we replace pages? Find the location of the desired page on disk Find a free frame If there is a free frame, use it If there is no free frame, use a page replacement algorithm to select a victim frame Read the desired page into the (newly) free frame. Update the page and frame tables. Restart the process Copyright : University of Illinois CS 241 Staff 49

49 Page Replacement Strategies Random page replacement Choose a page randomly FIFO - First in First Out Replace the page that has been in primary memory the longest LRU - Least Recently Used Replace the page that has not been used for the longest time LFU - Least Frequently Used Replace the page that is used least often NRU - Not Recently Used An approximation to LRU. Working Set Keep in memory those pages that the process is actively using. Copyright : University of Illinois CS 241 Staff 51

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