Stevens Institute Of Technology CS 492 _ Complete Midterm Review.

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CS492 Complete Midterm Review CS492 Complete Midterm Review Processes: ● Process - a program in execution ● A single CPU at any given time may only run one process ● CPU Utilization: 1 - p​n ● Processes don’t share memory address space Syscalls: Calling a kernel mode service from user mod program ● switches execution to kernel mode ● Programming interface to services provided by the OS ● Typically written in a high-level language ● APIs: ○ Win32 API (Windows) ○ POSIX API (POSIX-based systems: UNIX, Linux, MacOS) ● Types of syscalls: ○ Process control ○ File management ○ Device management ○ Information maintenance ○ Communications ● Return codes ○ s is -1 if error has occurred ○ fd is a file descriptor ○ n is a byte count ○ position is an offset within the file ● OS-to-Process Communication: a way for the OS to inform the process about something ○ “Asks” process to take an action based on the information Threads: ● Inter-Process Communications: Two main functions of an OS: ● Resource management ● User-friendliness for interaction with the hardware

Scheduling: ● Scheduling decisions may take place when a process/thread ○ Is created ○ In running state exits ○ Blocks on IO, or an event ● Scheduling decisions may take place when an interrupt occurs

○ Clock interrupt ■ Switch from running to ready ○ IO interrupt, or (unblocking) syscall ■ Switch from blocked to ready ● Scheduling takes place at clock interrupts ● A way for a system to keep track of time ○ Interrupt at periodic intervals ○ Implemented using a hardware clock interrupt ○ High priority ● Non-preemptive scheduling ○ Voluntary process switch(yield) ○ Process switch on blocking calls ○ The scheduler gets involved only at exit or on request ● Preemptive scheduling ○ Involuntary process switch ○ For every clock interrupt, running process may be suspended and switched with another process(if there is any) ● Different environments different scheduling algorithms are needed ● Different application areas have different goals ● Environments ○ Batch Systems ○ Interactive Systems ○ Real-time Systems ● Scheduling Algorithms Goals ○ All Systems ■ Fairness: giving each process a fair share of the CPU ■ Policy enforcement: seeing that stated policy is carried out ■ Balance: keeping all parts of the system busy ○ Batch Systems ■ Throughput: maximize jobs per hour ■ Turnaround time: minimize time between submission and termination

■ CPU utilization: keep the CPU busy all the time ○ Interactive Systems ■ Response time: respond to requests quickly ■ Proportionality: meet user’s expectations ○ Real-time Systems ■ Meeting deadlines: avoid losing data ■ Predictability: avoid quality degradation in multimedia systems ● Batch Systems ○ First come First served ○ Shortest Job first and Shortest Remaining Time Next ● Interactive Systems ○ Round-robin ○ Priority ○ Multiple Queues ○ Shortest Process next, Guarantee, Lottery, Fair-share ● Real-time Systems ○ Fixed priority ○ Dynamic Priority ● Scheduling in Batch Systems ○ Assumptions ■ How much time is needed to complete a job is maybe known ahead of time ○ Scheduling Goals ■ Throughput- maximize jobs per unit time(hour) ■ Turnaround time- minimize time users wait for jobs ■ CPU utilization- keep the CPU as busy as possible ○ Algorithms ■ First-Come First-Served (FCFS) ■ Shortest job first (SJF) ● First Come First Served ○ Processes are assigned to the CPU in the order they request it(or they arrive)

○ Non-preemptive ○ Turnaround time is just the added process times ○ Calculate average turnaround times with calculating the basic average ● Shortest Job First ○ Associate with each process the length of its CPU time ○ Use the CPU time length to schedule the process with the shortest CPU time first ○ Two variations ■ Non-preemptive ■ Preemptive- if a new process arrives(Shortest Remaining Time Next, SRTN) ● Scheduling in Interactive Systems ○ Assumptions ■ None ○ Scheduling Goals ■ Response time: respond quickly to user’s requests ■ Proportionality: meet user’s expectations ○ Algorithms ■ Round-robin scheduling ■ Priority Scheduling ■ Multiple Queues ● Round Robin Scheduling ○ Each process is able to run for a specified interval ■ Called quantum ○ After this time has elapsed ■ Process is preempted ■ Added to the end of the ready queue ■ Next process scheduled ○ If the process terminates or blocks before this time ■ Next process is scheduled ● Advantages of Round Robin ○ Solution to fairness and starvation

○ Fair allocation of CPU across jobs ○ Low average waiting time when job lengths vary ○ Good for responsiveness if small number of jobs ● Disadvantages ○ Context-switching time may add up for long jobs ● Priority Scheduling ○ Execution Plan ■ Always execute the highest-priority runnable jobs to completion ■ Each queue processed in Round-Robin fashion with a time quantum ○ Problems ■ Starvation ■ Deadlock ● Priority Inversion ● How are priorities assigned ○ Statically ■ Process type ■ User ■ How much the user paid ○ Dynamically, based on how much they run vs IO ■ Priority = 1/f ■ F = size of quantum used last ● Multiple Queue Scheduling ○ Execution Plan ■ Same as priority scheduling ■ But each queue has a different time quana ● Shortest for high prio ● Longer for low prio ■ Processes start at the highest priority ■ When a process exceeds its quanta is moved to the lower priority

■ When a process becomes interactive is moved to higher priority ○ Problem ■ If the user discovers how make his tasks interactive, he can play the system Scheduling in Real-Time Systems ● Time plays an essential role ○ One or more physical devices external to the computer generate stimuli ○ The computer must react appropriately to them within a fixed amount of time ○ If the computer reacts to late, it is not as bad as not reacting ● Categorization ○ Hard real-time ■ There are absolute deadlines that must be met ○ Soft real-time ■ Missing an occasional deadline is undesirable but tolerable ● Assumptions ○ Processes behavior is predictable and known in advance ○ Periodic, Sporadic, Aperiodic process ● How many periodic processes are schedulable? m

● ∑ CP <= 1 i=1

● M periodic events

Synchronization Problems: Dining Philosophers/Lawyers problem: ● 5 philosophers, each with a spaghetti dish and a fork ● To eat, a philosopher needs two forks. ● Philosopher alternates periods of eating and thinking ● Deadlock problem: What if all philosophers grab the one on their left at the same time and wait for the one on their right?

○ They will all indefinitely wait, because no one can eat ● Cause of deadlocks: ○ Each entity needs what another entity has. This results from sharing resources​. ● Cause of starvations: ○ One or a set of entities monopolizing resources for an indefinite time. ● A philosopher is equivalent to a process ● A fork is equivalent to a resource ● In order to use a resource you must ○ Request the resource(Get_resource) ○ Use the resource ○ Release the resource(Release_resource) ● If a resource is already in use ○ Get_resource will either block or return a “busy” error code ● Deadlock Example

● Neither process can run, because both processes are blocked ● Conditions for Resource Deadlocks: ○ Mutual Exclusion

■ Each resource is currently assigned to exactly one process or is available ○ Hold and Wait ■ Processes currently holding resources that were granted earlier can request new resources ○ No preemption ■ Resources previously granted cannot be forcibly taken away from a process. They must be explicitly released by the process holding them ○ Circular Wait ■ There must be a circular list of two or more processes, each of which is waiting for a resource held by the next member of the chain ● Starvation vs Deadlock ○ Starvation ■ Thread waits indefinitely (but is not blocked) ■ Example, low-priority thread waiting for resources constantly in use by high-priority threads ○ Deadlock ■ Circular waiting for resources ■ One thread owns a resource 1 that is waiting for resource 2 ■ Another thread owns a resource 2 that is waiting for a resource 1 ○ Deadlock implies starvation, but starvation doesn’t necessarily imply deadlock Deadlock Modeling ● Often modeled with a resource-allocation graph

● A cycle in a graph is not sufficient to imply deadlock ● STUDY THE GRAPHS AND UNDERSTAND WHICH ARE DEADLOCK AND WHICH ARENT