Memory Management 9th Week

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1 Department of Electrical Engineering and Information Technology Faculty of Engineering Universitas Gadjah Mada, Indonesia Operating System - TIF 206 Memory Management 9th Week Sunu Wibirama Copyright 2011

2 Before we start... Do you really believe that CPU s clock speed defines how fast the CPU is?

3 Steve has his own words...

4 THE MEGAHERTZ MYTH

5 Outlines Memory management: Introduction Logical and Physical Addresses Swapping Paging Segmentation Memory Sharing

6 References Abraham Silberchatz, Peter Baer Galvin, Greg Gagne, Operating System Concepts with Java 6th Edition, Wiley International Publisher : Chapter 9 Abraham Silberchatz, Peter Baer Galvin, Greg Gagne, Operating System Principles 7th Edition, Wiley International Publisher : Chapter 8 Google and Youtube (very useful!)

7 Basic Memory Hierarchy

8 Type of Memory Volatile : requires power to maintain the stored information, also known as temporary memory (Random Access Memory / RAM) Non-volatile : retains the stored information even when not powered (Read-Only Memory, Flash Memory, Magnetic Storage (HDD, etc)

9 memory management OS must fit multiple process in memory Memory needs to be allocated to ensure a reasonable supply of ready processes so that CPU is never idle Memory needs to be subdivided to accommodate multiple process Memory management is an optimization task under constraints

10 Important terms

11 Important terms Relocation of address references: translating memory references to physical address

12 Important terms Relocation of address references: translating memory references to physical address Protection of memory spaces: forbid cross-process references

13 Important terms Relocation of address references: translating memory references to physical address Protection of memory spaces: forbid cross-process references Sharing of memory spaces: allow several process to access a common memory area

14 Important terms Relocation of address references: translating memory references to physical address Protection of memory spaces: forbid cross-process references Sharing of memory spaces: allow several process to access a common memory area Logical organization (of programs): programs are broken up into independent modules

15 Important terms Relocation of address references: translating memory references to physical address Protection of memory spaces: forbid cross-process references Sharing of memory spaces: allow several process to access a common memory area Logical organization (of programs): programs are broken up into independent modules Physical organization (of memory): fit multiple programs and modules in memory area

16 Memory Address Space Each process has a separate memory space Two registers provide address protection between processes: Base register: smallest legal address space (also called relocation register) Limit register: size of the legal range

17 HW Address protection CPU hardware compares every address generated in user mode with the registers Any attempt to access other process memory will be trapped and causes a fatal error

18 Various Error messages Blue Screen of Death (Windows) Kernel Panic (FreeBSD) Kernel Panic (Mac)

19 Address binding Addresses in a source program are generally symbolic - ex.: int count; A compiler binds these symbolic addresses to relocatable addresses - ex.: 100 bytes from the beginning of this module The linkage editor or loader will in turn bind the relocatable addresses to absolute addresses - ex: Each binding is mapping from one address space to another

20 Binding of instructions & data to memory Address binding of instructions and data to memory addresses can happen at three different stages: 1. Compile time: if memory location known, we can generate absolute code, but we must recompile the code if starting location change. 2. Load Time: we can generate relocatable code if memory location is not known at compile time; need only reload if starting address changes. 3. Execution Time: binding delayed until run time if the process can be moved during its execution from one memory segment to another. Need hardware support for address mapping

21 Dynamic loading v.s. Dynamic linking Dynamic Loading: Routine in a program is not loaded until it is called. Can handle large amount of code User is fully responsible to maintain the code in order to be loaded dynamically Dynamic Linking: Linking is done in execution time Using stub, to locate appropriate memory-resident library (sharedlibrary concept) Requires help from operating system to manage protection of library

22 Logical v.s. Physical Address Space Address generated by CPU is referred as logical address Address seen by the memory is seen as physical address Compile time and load time methods generate identical logical and physical addresses Execution time method generates different logical and physical addresses - Logical address referred as virtual address

Memory Management Unit (MMU) Hardware device that maps virtual to physical address In MMU scheme, the value in the relocation register (formerly known as base register)

23 Memory Management Unit (MMU) Hardware device that maps virtual to physical address In MMU scheme, the value in the relocation register (formerly known as base register) is added to every address generated by a user process at the time it is sent to memory The user program deals with logical addresses, it never sees the real physical address

24 swapping A process must be in memory to be executed However, it can be swapped temporarily out of memory to a backing store, and brought back for continued execution If address binding is done at compiling or load time, the process cannot be easily moved to a different memory space If execution time binding is used the process can be swapped into different memory space. Why?

25 swapping A process must be in memory to be executed However, it can be swapped temporarily out of memory to a backing store, and brought back for continued execution If address binding is done at compiling or load time, the process cannot be easily moved to a different memory space If execution time binding is used the process can be swapped into different memory space. Why? Because the physical addresses are computed during execution time

26 swapping

space is important, ex: Linux suggests swap space 2x RAM Location: inside normal

27 swap-space management If system runs out of swap space, it may be forced to abort processes or may crash entirely Swap file in Windows OS Overestimation of swap space is important, ex: Linux suggests swap space 2x RAM Location: inside normal file system (Windows) or in separate disk partition (Unix-based OS). Swap file in Linux OS

28 swap-space management Inside file system: a large file, but inefficient since we need extra time to navigate the directory structure and disk allocation data structure. Moreover, disk is fragmented. Separate disk partition: now file system or directory structure inside it. Here, swap space is accessed more frequently. The system optimizes the speed for data processing. When handle large data transfer with swapping mode turned ON, we prefer Unix-based operating system instead of Windows

29 Your friend did it!

30 Your friend did it!

31 Google uses Linux clusters for its data center. The widely-used term is parallel computing

33 paging Memory management scheme that permits the physical address space of a process to be noncontiguous. User submit one address, the rest will be handled by hardware. Recently, OS can take care of it Logical address should be ordered in specific way Physical memory is broken into fixed-sized blocks called frames. Logical memory is broken into blocks of the same size called pages.

34 Advantages of paging Allow noncontiguous memory allocation for a process I/O processes are more efficient if the transferred data are larger User s view of memory: one single space Actual physical memory: scattered Easier for programmer!

35 hardware of paging page number! page offset! p! d!

36 Example of Paging

37 When you code... Have you ever think the order of where your part of program is stored in memory?

38 Obviously, I don t care about the order of where the memory stores the part of my program

39 segmentation In common usage, user doesn t see memory as a linear array of bytes. User sees memory as a collection of variable-sized segments, without ordering the the logical address. Segmentation is a memory-management scheme that supports this user view of memory. User can define segment number and its offset: <segment-number, offset> In paging, user only can submit one address and HW / OS will handle the rest

40 see the difference? PAGING SEGMENTATION

41 Hardware of Segmentation d = offset to the segment

42 Memory Allocation in Linux free -t -m htop

43 Try it by yourself!

44 THANK YOU

Chapter 8: Memory Management. Operating System Concepts with Java 8 th Edition

Chapter 8: Memory Management. Operating System Concepts with Java 8 th Edition Chapter 8: Memory Management 8.1 Silberschatz, Galvin and Gagne 2009 Background Program must be brought (from disk) into memory and placed within a process for it to be run Main memory and registers are