1.5 Synchronization, cpu scheduling  (Page 9/10)

The barbershop problem

The original barbershop problem was proposed by Dijkstra. A variation of it appears in Silberschatz and Galvin’s OperatingSystems Concepts. A barbershop consists of a waiting room with n chairs, and the barber room containing the barber chair. If there are no customers to be served,the barber goes to sleep. If a customer enters the barbershop and all chairs are occupied, then the customer leaves the shop. If the barber is busy, but chairsare available, then the customer sits in one of the free chairs. If the barber is asleep, the customer wakes up the barber. Write a program to coordinate thebarber and the customers.

Scheduling

Until now we have talked about processes, from now on we will talk about resources, the things operated upon by processes. Resourcesrange from cpu time to disk space to channel I/O time.

Resources fall into two classes:

• Preemptible: processor or I/O channel. Can take resource away, use it for something else, then give it back later.
• Non-preemptible: once given, it cannot be reused until process gives it back. Examples are file space, terminal, and maybe memory.

OS makes two related kinds of decisions about resources:

• Allocation: who gets what. Given a set of requests for resources, which processes should be given which resources in order to make most efficientuse of the resources? Implication is that resources are not easily preemptible.
• Scheduling: how long can they keep it. When more resources are requested than can be granted immediately, in which order should they beserviced? Examples are processor scheduling (one processor, many processes), memory scheduling in virtual memory systems. Implication is that resource ispreemptible.

Cpu scheduling

Processes may be in any one of three general scheduling states:

• Running.
• Ready. That is, waiting for CPU time. Scheduler and dispatcher determine transitions between this and running state.
• Blocked. Waiting for some other event: disk I/O, message, semaphore, etc. Transitions into and out of this state are caused by variousprocesses.

There are two parts to CPU scheduling:

• The dispatcher provides the basic mechanism for running processes.
• The scheduler is a piece of OS code that decides the priorities of processes and how long each will run.

This is an example of policy/mechanism separation.

Goals for Scheduling Disciplines

• Efficiency of resource utilization (keep CPU and disks busy).
• Minimize overhead (context swaps).
• Minimize response time. (Define response time.)
• Distribute cycles equitably. What does this mean?

Fcfs (also called fifo)

Run until finished.

• In the simplest case this means uniprogramming.
• Usually, "finished" means "blocked". One process can use CPU while another waits on a semaphore. Go to back of run queue when ready.
• Problem: one process can monopolize CPU.

Solution: limit maximum amount of time that a process can run without a context switch. This time is called a time slice.

Round robin

Run process for one time slice, then move to back of queue. Each process gets equal share of the CPU. Most systems use somevariant of this. What happens if the time slice is not chosen carefully?