Operating Systems {week 05}

Rensselaer Polytechnic Institute CSC 432 – Operating Systems David Goldschmidt, Ph.D. Operating Systems{week 05}

Operating system structure (i) • Using a layered approach, the operating system is divided into Nlevels or layers • Layer 0 is the hardware • Layer 1 is often the kernel • Layer N is the top-leveluser interface (GUI) • Each layer uses functionsand services of the layer(or layers) beneath it

Operating system structure (ii) • Also viewas a stackof services

Operating system kernel • The core program running inan operating system is calledthe kernel • When a computer is switched on,a bootstrap program executes from ROM • The bootstrap program initializes the system, then loads the operating system kernel and starts its execution

User and kernel modes (i) • Program instructions run eitherin user mode or in kernel mode • Kernel mode allows the operating systemto protect itself and its system components switch modesvia system calls

User and kernel modes (ii) • Kernel gives control to a user process, but may set a timer to ensure a process does not run beyond its allotted time • To avoid infinite loops, memoryleaks, memory hogs, etc. • Not always effective in practice... • Can you stop a runaway processbefore your computer crashes? Aaaaaaaugghhhh!I’m going to takethis computer and...

System calls via APIs (i) • OS services are available via system calls • System calls are made via an interface calledan Application Program Interface (API) • Common operating system APIs: • Win32 API for Windows • POSIX API for POSIX-based systems,including UNIX, Linux, Mac OS X • Java API for Java Virtual Machine • C/C++ Standard Library

System calls via APIs (ii) • Types of system calls include: • Process control (e.g. start/suspend/stop a process) • Debugging information, too • File management • Device management • Information retrieval and maintenance • Current date/time, number of current users, OS version, amount of free memory, process information, etc. • Communications (e.g. IPC, network)

System calls via APIs (iii) • An API hides the implementation details of the underlying operating system • Programmers just need to abide bythe API specifications • How do we change the API orthe operating system servicesthat it offers? the dude abides...

System calls via APIs (iv)

System calls via APIs (v) • Example using the printf()function from C • One API may callanother, which mayin turn call another,and so on...

System calls via APIs (vi) • Use registers to pass parameters: • Store memory location X that references a block of data somewhere in memory

Operating system services (i) • An operating system provides services: • Program execution • Load programs into memory, run/suspend/halt programs, handle/display errors • I/O operations • Seamlessly interact with I/O devices, includingdisks, networks connection, etc. • Filesystem manipulation • Read/write/traverse filesystem directories,read/write files, enforce permissions, search for files

Operating system services (ii) • Other operating system services: • Inter-Process Communications (IPC) • Processes exchange information via shared memory,message passing, sockets, pipes, files, etc. • Often spans multiple computers and networks • Error detection and recovery • Detect errors in CPU, memory, I/O devices,processes, network connections, etc. • Recover from errors gracefully,ensuring correct and consistent operations

Multiprogramming and timesharing • Multiprogramming goals: • Maximize CPU utilization • Maximize number of processes in memory • Timesharing goals: • Switch CPU among processes such that users interact with each program simultaneously • Maximize fairness almost!

Process scheduling and queues • Processes are created bythe operating system • Processes initially added toa job queue, which containsall processes waiting to enter the system • From the job queue, processes thatare ready for execution are addedto the ready queue

Schedulers • A long-term scheduler (i.e. job scheduler) selects processes from the job queue, adding those processes to the ready queue • A short-term scheduler (i.e. CPU scheduler) selects processes from the ready queueand allocates time with the CPU • More on this later....

Long-term scheduling (i) • The long-term scheduler isinvoked infrequently

Long-term scheduling (ii) • The degree of multiprogramming ofan operating system is defined asthe number of processes in memory • In a stable operating system,the average process arrival rate equalsthe average process departure rate

Long-term scheduling (iii) • Processes are either I/O bound or CPU bound • A CPU-bound process does little I/O and instead makes heavy use of the CPU • An I/O-bound process spends a majority of itstime performing (i.e. waiting for) I/O • The long-term scheduler should select a good process mix of CPU-bound and I/O-bound processes

Long-term scheduling (iv) • Most modern operating systems have no long-term scheduler (e.g. Windows, UNIX) • All processes are admitted tothe ready queue, regardlessof whether the operatingsystem can handle the load • Often results in userschanging their usage habits....

Medium-term scheduling • Modern operating systems implement a medium-term scheduler to swap processes to/from memory and disk

Processes (i) • A process is an active program in execution • Requires CPU time, memory, file access,network access, other I/O access • Operating system is responsible for: • Creating/deleting processes • Scheduling processes • Allocating resources to processes • Synchronizing communication between processes

Processes (ii) • For each process, the operatingsystem manages and executesprocesses by recording: • Program counter (PC) • Registers • Data section (global data) • Stack (temporary data) • Heap (dynamically allocated memory) heap stack data text/code

Process states • As a process executes, it changes its state

Process control block • Operating system represents each process via a process control block (PCB) • Process state • Process ID or number • Program counter (PC) • CPU registers • CPU-scheduling and memorymanagement information • List of open file/resource handles

Process context switch context switch takes a few milliseconds

Process scheduling

Short-term scheduling • The short-term scheduler frequently decides which process the CPU executes next • Typical time slice (t) a process has withthe CPU is 100 milliseconds • How much CPU time is wastedif t is 100ms and it takes 10msto schedule the next processand perform the context switch?

Process creation (i) • Processes are created from other processes • A parent process creates a child process, whichin turn creates child processes of its own, etc. • A tree of processes is the result:

Process creation (ii) • Operating system resources are sometimes shared amongst processes • Possibilities: • Parent and child processes share all resources • Child shares a subset of its parent’s resources • Parent and child processes share no resources

Process creation (iii) • Each process has itsown unique processidentifier (pid)

Process creation (iv) • When a new process is created,the parent has two options: • Parent process continues to execute concurrently with its children • Parent process waits for its children toterminate their execution • The child process decides what it will do: • Duplicate the parent or load a new program

Process creation (v) • In Unix, a new child process is forked viathe fork() system call • Child optionally calls the exec() system callto load a new program

Operating Systems {week 05}