Structure of an Operating System

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1 Fö 11: OS Minneshantering och virtuellt minne Virtuella I/U-enheter och Filsystemet Multisystem Zebo Peng, IDA, LiTH 1 Structure of an Operating System The Onion Model [Lökmodellen]: Användarprocesser Shell [Skal] Jobbhantering Filåtkomst I/O Minneshantering Kärna Hårdvara Zebo Peng, IDA, LiTH 2 1

2 Static Partitioning The primary memory is divided into fixed-sized partitions: 0 The partitions can be of different sizes, to make it more efficient. 0 A process is fitted into the smallest partition that will take it (best fit). 0 Some wasted memory, which is unavoidable. Primärminne Program D Program C Program B Region 4 Region 3 Region 2 Program A Region 1 Zebo Peng, IDA, LiTH 3 Static Partitioning The primary memory is divided into fixed-sized partitions: 0 The partitions can be of different sizes. 0 A process is fitted into the smallest partition that will take it (best fit). 0 Some wasted memory. 0 It is good to have variable sized partitions. Primärminne Program D Program C Program B Region 4 Region 3 Region 2 Program A Region 1 Zebo Peng, IDA, LiTH 4 2

3 Dynamic Partitioning (1) Allocate exactly the required memory to a process. Program E Program E Primärminne Program D Program C Program F Waiting for memory space Program B Program A Zebo Peng, IDA, LiTH 5 Dynamic Partitioning (2) Allocate exactly the required memory to a process. Program V 0 Eventually the memory has lots of holes (fragmentation) 0 Solutions: Memory Compaction, i.e., from time to time go through the memory and move all hole into one free block, which is called garbagecollection. Primärminne Program W Program X Program Y Program Z Waiting for memory space Zebo Peng, IDA, LiTH 6 3

4 Garbage-Collection Expensive Program V Primärminne Program W Program X Program Z Waiting for memory space Program Y Zebo Peng, IDA, LiTH 7 Garbage-Collection Expensive Primärminne Program V Program W Program X Program Z Waiting for memory space Program Y Zebo Peng, IDA, LiTH 8 4

5 Memory Relocation No guarantee that a process will load into the same place in memory. Machine instructions contain addresses: 0 Locations of data 0 Addresses for instructions (branching) Logical addresses relative to beginning of program should be used. Physical addresses actual location in memory (at this moment) should not be wired in the programs. Automatic conversion using base address is managed by the operating system. Zebo Peng, IDA, LiTH 9 Paging Divide programs (processes) into equal sized, small blocks, called pages. Divide the primary memory into equal sized, small blocks called page frames. Allocate the required number of page frames to a program. A program does not require continuous page frames! OS maintains a list of free frames. Use page table to keep track of the mapping between pages and page frames. Zebo Peng, IDA, LiTH 10 5

6 Logical and Physical Addresses Implementation of the page-tables: Main memory slow since an extra memory access is needed. Separate registers fast but expensive. Cache. Zebo Peng, IDA, LiTH 11 Demand paging Virtual Memory 0 Do not require all pages of a process in memory. 0 Bring in pages as required. Page fault 0 Required page is not in memory. 0 Operating system must swap in the required page. 0 May need to swap out a page to make space. 0 Select page to throw out based on recent history. Zebo Peng, IDA, LiTH 12 6

7 Objective of Virtual Memory To provide the user/programmer with a much bigger memory than the main memory with the help of the operative system. 0 Virtual memory >> real memory Programadresser Primärminnesadresser Sekundärminne Zebo Peng, IDA, LiTH 13 Page Fault When accessing a VM page which is not in the main memory, a page fault [sidmiss] occurs. The page must then be loaded from the secondary memory into the main memory by the OS. Page Number Virtual Address Offset Page Map Page Fault (Interrupt to OS) Pages in MM Zebo Peng, IDA, LiTH 14 7

8 Page Replacement [ [Sidbyte] When a page fault occurs and all page frames are occupied, one of them must be replaced. If the replaced page has been modified during the time it resides in the main memory, the updated version should be written back to the secondary memory. Our wish is to replace the page which will not be accessed in the future for the longest amount of time. Problem We don t know exactly what will happen in the future. Solution We predict the future by studying the access patterns up till now ( learn from history ). Zebo Peng, IDA, LiTH 15 Replacement Algorithms FIFO (First In First Out) To replace the one in MM the longest of time. LRU (Least Recently Used) To replace the one that has not be accessed the longest time. LFU (Least Frequently Used) To replace the one that has the smallest number of access during the latest time period. Zebo Peng, IDA, LiTH 16 8

9 Replacement for Cache vs. VM Both may make use of the three algorithms: FIFO (First In First Out) LRU (Least Recently Used) LFU (Least Frequently Used) But the replacement by random in not used for VM! Why? Zebo Peng, IDA, LiTH 17 Fö 11: OS Minneshantering och virtuellt minne Virtuella I/U-enheter och Filsystemet Multisystem Zebo Peng, IDA, LiTH 18 9

10 Objectives of I/O Management To make program codes independent of: 0 The type of devices (as far as it goes): e.g., write onto the screen should be the same as write to some papers. 0 The particular device of a given type: e.g., there should be no difference printing on a laser printer next door or the one downstairs. 0 This is related to the convenience issue. To make sure the I/O operations will not have a strong negative impact of the CPU performance the efficiency issue. Zebo Peng, IDA, LiTH 19 Virtual I/O Devices A program will only work with virtual I/O devices, not directly with the physical devices. The operating system will be responsible for the coupling between the virtual devices and the physical ones. The physical printers Process Virtual printer OS Laser printer 1 Laser printer 2 Laser printer 3 Zebo Peng, IDA, LiTH 20 10

11 File System [ [Filesystem] An operating system should provide a file system with the following functions: 0 To hide the physical organization of file storages from the end-users: e.g., placement of the files in the hard disc. 0 To translate file-names to physical storage location. 0 To support file management, such as file creation and removal as well as reading and writing of files. 0 To facilitate the sharing of files among processes. 0 To prevent undesirable access of files: e.g., students shouldn t have access to examination questions. 0 To make it possible to let a program work with different files in different occasions. Zebo Peng, IDA, LiTH 21 Fö 11: OS Minneshantering och virtuellt minne Virtuella I/U-enheter och Filsystemet Multisystem Zebo Peng, IDA, LiTH 22 11

12 Multi-Processor System A multi- system has two or several s which is connected to the same main memory: Floatingpoint Array Graphics Main Main memory I/O I/O Database External devices External devices Zebo Peng, IDA, LiTH 23 OS for Multi-Processor Systems Master-Slave: The operative system kernel runs on a given, usually the main, which unfortunately becomes the bottleneck. Separate OS: An OS kernel runs in each of the s this is the most expensive solution. Floating OS: Only one OS kernel runs. It runs on a selected based on an algorithm and will migrate to another whenever needed. Zebo Peng, IDA, LiTH 24 12

13 Beyond Multi-Processor Systems Distributed systems: 0 A set of s with their own main memories connected together by a communication-mechanism, which has relatively short geographical distance. 0 The s usually perform certain joint activities to carry out some common task of data processing. Computer network: 0 Many computer systems, geographically wide-spread, connected by a network. 0 The computers don t usually perform joint computation tasks. Zebo Peng, IDA, LiTH 25 Summary of OS An operative system is used to coordinate the cooperation between: 0 users 0 system components 0 programs 0 software and hardware 0 application program and the OS itself It manages the sharing of the expensive resources to make them more efficient: 0 CPU time 0 Main memory space The computer hardware together with an OS defines a virtual computer to make it easy to be used. Zebo Peng, IDA, LiTH 26 13

Lecture 2: Memory Systems

Lecture 2: Memory Systems Basic components Memory hierarchy Cache memory Virtual Memory Zebo Peng, IDA, LiTH Many Different Technologies Zebo Peng, IDA, LiTH 2 Internal and External Memories CPU Date transfer