SCHEDULING IN SYMBIAN OS

ByHasib Shaikh

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3. Scheduling Strategies• The goal of any scheduling strategy is to maximize

CPU usage and throughput while minimizing turnaround time, waiting time, and response time.

• Here we focus on the problems of deciding which process should use the CPU and when a process should be removed from using the CPU.

• Scheduling Strategies:1. First-Come-First-Served Strategy2. Shortest-Job-First Strategy3. Round-Robin Strategy4. Priority Strategy5. Multiple-Queuing Strategy6. Real-time Strategy

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1. First-Come-First-Served Strategy

• Provides easiest way to schedule a CPU• It allows the first process that requests a CPU to

use the CPU until the process is completed.

• When one process is using the CPU, other processes that need the CPU simply queue up in the ready queue.

• This allows the head of the ready queue to be used as the next process to be scheduled.

• It is non pre-emptive. Processes are removed from the CPU only when they are in the waiting state or they have terminated.

1. First-Come-First-Served Strategy contd…

• Consider the following set of processes that arrive to use a CPU in the order stated:

• An FCFS scheduler would schedule them as shown in

Figure 5.1.

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•Consider the following set of processes that arrive to use a CPU in the order stated:

•The throughput of our imaginary system is 4 processes in 53 time units•the average waiting time is 28 time units.

1. First-Come-First-Served Strategy contd…

•Consider a different ordering of processes.• In this situation, we can measure time in

the following way:

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•In this scenario, the throughput of our system is the same: 4 processes in 53 time units.• But the waiting time is much better: the average waiting time for this example is 9.5 time units.

1. First-Come-First-Served Strategy contd..

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•FCFS strategy does not guarantee minimal criteriaand measures may vary substantially depending on process execution times and the order of execution. •Fairness issues hurt the consideration of this scheduling strategy.•FCFS is inherently unpredictable and may verylikely produce unfair schedules.

2. Shortest-Job-First Strategy

•If we always chose the process with the shortest running time first, it would seem that we could improve the measurements we are watching.

•As an example, consider a new set of processes:

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2. Shortest-Job-First Strategy contd…

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•The shortest-job-first (SJF) scheduling strategy is illustrated in Figure given below •This ordering produces the following measurements:

•This order of processing has an average wait time of 10.75 time units.

•If we had used the FCFS strategy, the average waiting time would be12.25 time units.

2. Shortest-Job-First Strategy contd…

•While it is possible to prove that an SJF strategy is optimal for average times, the strategy has several issues.1. it penalizes long processes simply for

being long.2. Secondly, it becomes possible to starve a

process. Starvation occurs when a process is waiting

in the ready queue but never makes it to the running state. As long as processes enter the queue with running times shorter than it, that process is never run on the CPU.

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3. Round-Robin Strategy

• Both FCFS and SJF are usually used as non-pre-emptive strategies. However, we still have the criterion of fairness to consider.▫Fairness is a measure of how much time each

process spends on the CPU.▫ If we schedule processes to run to completion or we

depend on processes to give up the CPU when they can, we can make measurements but we can make no statement about fairness.

• Fairness can only be assured when we use a pre-emptive strategy.▫ In a round-robin strategy, all processes are given the

same time slice and are pre-empted and placed on the ready queue in the order they ran on the CPU.

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3. Round-Robin Strategy contd…19

• For example, consider the processes below:

•Let’s say the time slice in this system is 5 time units.

• A roundrobin scheduling strategy would produce a timeline like that shown in above Figure 5.4.

3. Round-Robin Strategy contd…• There is very little to manage about a round-

robin strategy. ▫The only variable in the scheme is the length of the

time slice – the amount of time spent on the processor.

▫ Setting this time to be too short – say close to the time it takes to perform a context switch – is counterproductive. It lets the context-switching time dominate performance and lowers CPU efficiency.

• However, making a time slice too long gets away from the benefits of a round-robin strategy.

• Response time decreases for short requests.

• A round-robin strategy puts all processes on an equal footing.

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4. Priority Strategy

•A priority-scheduling strategy takes into account that processes have different importance placed upon them.

•In priority scheduling, the process in the ready queue with the highest priority is chosen to run.

• This type of scheduling can be either pre-emptive or non-pre-emptive, as it is the choice of the next process that defines a priority-scheduling strategy.

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4. Priority Strategy contd…

•Priority scheduling makes certain requirements of the operating system.▫The operating system must employ the concept

of process priority. The priority is an attribute of a process that

measures the importance of the process in relationship to others and allows the operating system to make decisions about scheduling and the amount of time to keep a process on a processor.

In a pre-emptive scheduling environment, process priority is a very useful attribute to assign to a process.

The priority of the process is usually given by a number, which is kept in the process’s PCB.

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4. Priority Strategy contd…• It is usually required that the operating system be allowed

to manipulate priorities somehow. ▫ Priorities are set by the user or the process creator but the

operating system is usually allowed to change priorities dynamically.

▫The operating system requires the ability to adjust the priority of such a process as it moves through its execution.

• Priorities themselves usually take the form of numbers, quantities that can be easily compared by the operating system.

• Microsoft Windows assigns values from 0 to 31; various Unix implementations assign negative as well as positive values to reflect user (negative) and system (positive) priority assignment.

• As that shows, there is no general agreement on assigning priority values.

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4. Priority Strategy contd…•example of priority scheduling. Let’s say

that requests for the processor are made in the following order:

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• For the purposes of this example, higher numbers mean higher priority

5. Multiple-Queuing Strategy•We have described priority scheduling as a

matter of choice: choosing the process with the highest priority to schedule on the processor.▫ In many operating systems, processes do not have

unique priorities. There may be many processes with the same priority.

▫ Many system processes have the same high priority. This means that the scheduler is eventually going to have to choose between processes with the same priority.

• Priority scheduling is often implemented with priority queues.

• A priority queue holds processes of a certain priority value or range of values.

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5. Multiple-Queuing Strategy contd…

•The multiple-queuing scheduling strategy could even use multiple strategies:▫different scheduling strategies for different

queues.▫ Processes are

either permanently assigned to a specific queue, based on their characteristics upon entering the system,

Or they can move between queues, based on their changing characteristics as they are executed in the system.

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6. Real-time Strategy

• real-time systems can be classified as one of two different system types, each with different scheduling needs.▫ Hard real-time systems guarantee that time constraints

are met. there is usually a specific amount of time that is specified

along with the process-scheduling request. The system guarantees that the process is run in the

specified amount of time or that the process is not run at all.

▫ Soft real-time systems place a priority on time-critical processes and are less restrictive. It do not guarantee performance; they give real-time

processes favorable treatment and keep certain latency times to a minimum.

A real-time operating system must be able to assume that scheduling overhead is restricted to a certain time.

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4.Scheduling in Symbian OS• It is a mobile phone operating system that is

intended to have the functionality of a general-purpose operating system.

• It can load arbitrary code and execute it at run time;

• It can interact with users through applications.

• At the same time, the operating system must support real-time functionality, especially where communication functions are concerned.▫Because of the real-time requirements, Symbian OS

is implemented as a real-time operating system.

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4.Scheduling in Symbian OS contd…

•It is built to run on multiple phone platforms, without specialized hardware, so the operating system is considered to be a soft real-time system.

•It needs enough real-time capabilities to run the protocols for mobile protocol stacks, such as GSM and 3G.

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4.Scheduling in Symbian OS contd…

• The combination of general-purpose functionality with real-time system requirements means that the best choice for implementation is a system that uses a static, monotonic scheduling strategy, augmented by time slices.

• Static, monotonic scheduling is a simple strategy to use-▫ It organizes processes with the shortest deadline

first and the introduction of time slices means that

processes with the same deadline (or no deadline) can be assigned time slices and scheduling using a priority-scheduling scheme.

▫There are 64 levels of priority in Symbian OS.

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4.Scheduling in Symbian OS contd…

•As we discussed before, a key to soft real-time performance is predictable execution time.

• If an operating system can predict how long a process will run, then a static, monotonic scheduling strategy will work, since it makes some big assumptions about run time.

•Predicting execution time is based on the process and several system characteristics.

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4.Scheduling in Symbian OS contd…

• There are several important characteristics that must be predictable, including:

• latency times: an important benchmark is the latency of handling interrupts: the time from an interrupt to a user thread and from an interrupt to a kernel thread.

• the time to get information about processes: for example, the time it takes to find the highest priority thread in the ready state.

• the time to move threads between queues and the CPU: manipulating scheduling queues – for example, moving processes to and from the ready queue – must be bounded.▫ This functionality is used all the time and it must have a

bound on it or the system cannot predict performance.

• Predicting these quantities is important and is reflected in the design of the scheduler.

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