Processes

Multithreading models

• Many to One Model
• Maps many user threads to one kernel thread
• Allow the application to create any number of threads that can execute concurrently.
• All thread activity is restricted to the user space
• However only one thread can access the kernel at a time, so multiple threads are unable to run in parallel on multiprocessors

Multithreading models

• Many to Many Model
• Maps many user-level threads to many kernel-level threads
• Aka the two-level model, minimizes programming effort while reducing the cost and weight of each thread
• A program can have as many threads as are appropriate without making the process too heavy.
• This implementation provides a standard interface, a simpler programming model, and optimal performance for each process

Process Scheduling

• refers to a set of policies and mechanisms supported by operating system that controls the order in which jobs are completed.
• A scheduler is an operating system program (module) that select the next job to be admitted for execution.
• In this section we will describe:
• Scheduling objectives
• Types of schedulers
• Various scheduling algorithms

Scheduling Objectives

• Maximise throughput – service the largest possible number of processes per unit time
• Minimise overhead – scheduling should minimise the waster resources
• Balance resource use – should keep the resources of the system busy
• Maximise interactive users – maximise the number of interactive users receiving acceptable response times
• Enforce priorities – the scheduling mechanism should favour the higher-priority processes
• Avoid indefinite postponement – all processes should be treated the same and no process can suffer indefinite postponement

Types of Schedulers

• Long – term Scheduler
• Also called job scheduler
• Selects processes from the queue and loads them into memory for execution
• It is the change from of state from new to ready
• Its main objectives is to provide a balanced mix of jobs, such as I/O bound and processor bound
• Short-term Scheduler
• Also called the CPU scheduler/dispatcher
• It is the change of state from ready to running for the process
• Selects from among the processes that are ready to execute and allocates the CPU to one of them
• Faster than long term scheduler