CS2506 Quick Revision
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1 CS2506 Quick Revision OS Structure / Layer Kernel Structure Enter Kernel / Trap Instruction Classification of OS Process Definition Process Context Operations Process Management Child Process Thread Process Scheduling Numeric example of Shortest Process First Priority Scheduling Dynamic Priorities Communicates with OS -> pre-written functions Kernel -> collection of functions Repertoire of commands -> Virtual Machine Software -> Non-portable, low level details Example -> Control and State Registers / DMA Hardware -> CPU/Memory/Buses/Keyboard Device Driver -> Hardware Unit Managers Process Management -> Resources / Scheduling Memory Management -> Physical & Virtual File Sys Man -> Data = Files and Directories Network Services -> host-to-host / p-to-p Enters = Trap Instruction Switches = Kernel Mode Parameters = Trap Instruction what to do! Finished = User Mode Powerful / Multitasking / Functionality Instance of a running program Control of CPU to execute instructions Context -> Distinguish Info on current process running or next to run. Example PID PID Parent PID Real User ID Priority Current Directory Create Terminate Change Program Set Process Parameters Get Process Parameters Awaken Process Copy Different value computed Internal Rep / Resource Release all resources exec() Priority CPU Time so far Run again Int I; If( fork()) { for(i=0; i<1000; i++) print (Parent, i); Lightweight Process Process = one or more threads Provides concurrency in a program Exploited in multicore computers Exit() Time = Starve other threads Order Complete with highest level of efficiency Queue = next one to take control of CPU 3 Processes a(35), b(40), c(15) t a = 35, t b = 75, t c = 90 t aa = 200/3 t c = 15, t a = 50, t b = 90 t aa = 155/3 Multilevel feedback queues Dynamic properties implementation Initial priority Time slice lowered acceptable level ROUND ROBIN Blocked Higher Priority PRIORITY BOOST Allow to avoid process starvation 1 P a g e
2 Two-level scheduling Earliest First Scheduling Challenge of scheduler Multicore System Idea of Group Scheduling Scheduling Domain (including example of Policy) Policy Example Active Balancing UNIX system calls UNIX uses the Process Table UNIX scheduling Elements WIN NT Scheduling Strategies Tiny OS Example medium-level priority process = computation strong / never blocks. Raise lower level who are starving. Active processes main memory Moving difficult Higher Level Slower Scheduler subset to reside in memory for longer. EDF Attribute value time completed Always select earliest deadline Finished time-slice ready list Identify and predict resource needs Schedule them minimise shared resource contention Maximise shared resource utilisation Exploit advantage of SR between cores Group minimise shared resource contention / duplication Co-scheduled on processers sharing the same cache Sets of cores which share properties and policies Processes from one core can be moved to spread the load Contains one or more groups subsets of cores treated as one entity. If processor = IDLE domain has SD_BALANCED_NEWIDLE flag, scheduler looks for processes on a busy processor to move over. High priority process on one processor can IDLE another. CPU-hungry processes competing for access to a HYPER THREADED PROCESSOR. Scheduler does not normally move a process running. BALANCING CODE = CPU clog out of the processor for it to be moved. FORK() copy EXEC() replace EXIT() terminate KILL() terminate (appropriate privileges) Process Table divided into TWO; 1 Array of Structures admin info / state info / sched info 2 Not needed data swapped out stored on user structure. Process Priorities Scheduling Code = SWTCH() searches for highest priority Migrate = SCHED() SWTCH() and SCHED() represent two-level scheduler. Multi-level feedback queue 0 Kernel 1 15 Dynamic levels APPLICATION THREADS Real-Time Levels HIGHER PRIORITY Tiny Sensors ONLY necessary parts Set of software components Complete system = SCHEDULER & SET OF COMPONENTS COMPONENT PARTS Set of Command Handlers Set of Event Handlers Fixed-sized Frame Bundle of Tasks 2 P a g e
3 Compare Two different OS UNIX All processes created by other ones except the first one after system boot. Fork() EXEC() EXIT() WinNT Sets of threads that share common memory space Threads = UNITS OF EXECUTION CreateProcess() CreateThred() ExitProcess() 3 P a g e
4 Process Id Linux alloc_pid last_pid +1 Bitmap Linear Search EAGAIN Free allocate Linux Scheduling Two multilevel feedback queues Bitmap used determine highest priority non-empty one. INTERACTIVE Processes QUANTUM moved to ready ROUND ROBIN COMPUTE-BOUND Processes QUANTUM Expired Queue Linux compute priorities $ nice (high or Low) Virtual Address and Physical Addresses Memory = real Id more locations Create physical address space Process Layout = Virtual Set represents the virtual add space Methods used to translate virtual to physical Base Registers Virtual Add + content of base register = physical address Segmentation Different memory segments store different parts of the program. Paging Memory separated 2 k Bytes = Page Frames Virtual Memory = n Bytes in length = 2 n-k pages. First n-k = Virtual Address point to an entry in the base table. Page Table -> Page Frame Number Page Frame Number + last n = Physical. 4 P a g e
5 Mechanism of Memory Pages 2 k Bytes Sized Areas Virtual = n bits = 2 n-k Upper n-k = Page Table Entry Page Frame Number + last n = Physical Address Hierarchical Paging = upper bits split in two <> Upper = indexes a page directory <> Lower = indexes a page table to get physical address Page Table Information Page Frame Number Protection Bits Present Bit Modified Bit Accessed Bit OS Free Memory Space Managed Needs to locate free IF fixed-sized then one bit/[page can show its state free or not This is called FREE BITMAP Linked List Solution Take off the list Append it to the list Search is required if VARIABLE and not FIXED Fragmentation If FIXED then there is WASTE MORE = INTERNAL LEFT UNALLOCATED = EXTERNAL First Fit Equal or Greater size than is required Clustered to lower addresses Next Fit Start search at next block Evenly distributed blocks Best Fit Equal to required size Leads to EXTERNAL fragmentation Worst Fit Large blocks used for all requests Buddy Memory System Split free block into two equally sized blocks One for allocation One free Can be combined later Swapping Technique Transferring one blocked process memory space to disk. Restore when active. Demand Paging Loading pages when they are needed. First In and First Out Replace the page which has been in memory the longest Done by a queue new pages to tail of the queue. Second Chance FIFO Extension A = O Swap A!= 0 Clear and add to tail of queue Clock Algorithm Similar to Second Chance Two clock hands moving synchronously over a page Distance between them = time to use <> else swap Not Recently Used Similar to Second Chance M bit of memory block checked as well as the accessed A bit Lowest AM Values are swapped Least Recently Used Based on TIMING when block last used MOST TIME = Swapped Working Set for Memory Replacement Working Set of Pages = Number of Pages a specific process is using at a time. Process MORE pages than working set then SWAP Process LOWER then SWAP ALL PAGES WinNT uses this strategy Admin determined MIN and MAX Working set size. Linux Slab Collections of FREE BLOCKS EMPTY SLAB one or more pages divided into blocks Block released ADDED to SLAB Device Driver INTERFACE with DEVICE CONTROLLERS on behalf of OS Two Threads of Control User Processes request for I/O 5 P a g e
6 Interrupts ( If a user process requests a read operation, the driver loads the called function s arguments into the controller registers and starts the read. Until this read is finished, any other requests are put in the request queue. However, the user control can be interrupted. ) Device Driver Structure UPPER = User Requests LOWER = Controller Both User MUTUAL EXCLUSION to prevent corruption Watermarks Used with BUFFERS Mark close to the end BUFFER about to become FULL Mark close to the EMPTY signals BUFFER is about to become this. I/O Representation in Linux Device Name Referenced then ; <> File System Translates it into three items <><> MAJOR DEVICE NUMBER <><> MINOR DEVICE NUMBER <><> FLAG (indicating character or block one) I/O Schedulers NOOP SCHEDULER <>merges adjacent requests COMPLETE FAIR QUEUEING SCHEDULER <>Separate queue for each process <> Requests merged and inserted in order of sector number <> Round Robin OS Exclusive access ALGORITHM <> Q (queue of processes) is nonempty Then add P to the tail of Q and return. <> If no process then P access and return <> P requests read-only and Q is empty P access and return <> Add P to the tail of Q and return. File Metadata Managed by OS and some applications also Data = NAME / SIZE / LAST MODIFICATION File System Metadata Total Size of the System The Amount of Free Space Date of Last Mount OS Manage Free Storage Space Fixed Sized Blocks Free Bitmap Free Lists embedded in the Free Blocks Simple List Stored in a Free Block Purpose of Linux Virtual File System Implement a number of COMMON SERVICES & GENERIC FILE OPERAPTIONS Superblock When a file is MOUNTED, a file system function call to load an internal representation of the file system metadata. Member of SB points to a structure of type STRUCT SUPER_OPERATIONS. Fetch and Manipulate structures called I_NODES RAID 0 Data Striping no redundancy 1 Data Mirroring Duplicated to two Drives 2 Error-Correcting Code Disk One 1 st bit Disk Two 2 nd bit etc 3 Bit Interleaved Parity Only one disk for parity 4 Block Interleaved Parity Parity Block on separate disk 5 Block Interleaved DISTRIBUTION Data & Parity spread on Resource Protection Case of ACCESS MATRIX STRATEGY Access either GRANTED or DENIED all disks 6 P a g e
7 Attributes of Access Rights Revocation Java Protection OS User Authentication MIDP Security Model Public Key Infrastructure & Certification Policies Load Balancing Admin can delete file but user cant Immediate vs Delayed Selective vs General Partial vs Total Temporary / Permanent Class Loaded JVM assigns to it a protection domain that gives permissions Depends on the URL Usernames and Passwords = Database Fingerprint Trusted MIDLet Suites : Origin and Integrity Protected API s Access Level Public key -> Trusted Certificate Authority (CA) ROOT Cert (ROOT KEYS) contain details of the CA and their PUBLIC KEYS PKI Hierarchy of CA s up to the trust anchor Distribute Workload Achieve optimal resource utilisation Max throughput Min response time Avoid Overload GOAL : Increase reliability through redundancy 7 P a g e
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