What is CPU Scheduling Criteria?

Scheduling is an essential concept that serves in the multitasking, multiprocessor and distributed systems. There are several schedulers available for this purpose. But these schedulers also require a criterion up on which they can decide how to schedule the processes. In this article we discuss about these scheduling criteria. Today a number of scheduling algorithms are available and all these have different properties. This is why these may work up on different scheduling criteria. Also the chosen algorithm may favor one class of processes more than the other.

What Criteria is used by algorithms for Scheduling?

Below mentioned are some of the criteria used by these algorithms for scheduling:
1. CPU utilization:
- It is a property of a good system to keep the CPU as busy as possible all the time.
- Thus, this utilization ranges from 0 percent to 100 percent.
- However, in the systems that are loaded lightly, the range is around 40 percent and for the systems heavily loaded it ranges around 90 percent.

2. Throughput:
- The work is said to be done if the CPU is busy with the execution of the processes.
- Throughput is one measure of CPU performance and can be defined as the number of processes being executed completely in a certain unit of time.
- For example, in short transactions throughput might range around like 10 processes per second.
- In longer transactions this may range around only one process being executed in one hour.

3. Turnaround time:
- This is an important criterion from the point of view of a process.
- This tells how much time the processor has taken for execution of  a processor.
- The turnaround time can be defined as the time duration elapsed from the submission of the process till its completion.

4. Waiting time:
- The amount of time taken for the process for its completion is not affected by the CPU scheduling algorithms.
- Rather, these algorithms only affects the time when the process is in waiting state.
- The time for which the process waits is called the waiting time.

5. Response time:
- The turnaround is not a good criterion in all the situations.
- The response time is favorable in the case of the interactive systems.
- It happens many a times that a process is able to produce the output in a fairly short time compared to the expected time.
- This process then can continue with the next instructions.
- The time taken for a process from its submission till production of the first response is calculated as the response time and is another criterion for the CPU scheduling algorithms.

All these are the primary performance criteria out of which one or more can be selected by a typical CPU scheduler. These criteria might be ranked by the scheduler depending up on their importance. One common problem in the selection of performance criteria is the possibility of conflict ion between them.
For example, increasing the number of active processes will increase the CPU utilization but at the same time will decrease the response time. This is often desirable to produce reduction in waiting time and turnaround time also. In a number of cases the average measure is optimized. But there are certain cases also where it is more beneficial to optimize the maximum or the minimum values.
It is not necessary that a scheduling algorithm that maximizes the throughput will decrease the turnaround time. Out of a mix of short and long jobs, if a scheduler runs only the short jobs, it will produce the best throughput. But at the same time the turnaround time for the long jobs will be so high which is not desirable.

What is a thread? What is meant by multi-threading?

About Thread

- A thread is a smaller instance of a process i.e., a sequence of instructions and an operating system scheduler can manage it independently. 

- A thread is sometimes also called as the light weight process. 

- The way these threads and processes are implemented is different for different operating systems. 

- But in majority of the cases threads are contained within a process. 

- The same process can have more than one thread. 

- These threads have to share the resources including memory while different processes may not share these. 

- In simple words, we can say that the instructions or code and the context or the values of the process are shared by its constituting threads. 

In this article we focus on threads and multi-threading.

- Multi-threading is a task of multiprocessor systems.

- But even single processor systems can do it by time division multiplexing just like the multitasking. 

- In TDM context switch occurs between the many threads. 

- This happens many times and so it seems to the user that a number of processors are being executed concurrently.

- However, in multiprocessor systems concurrency can be truly achieved since every processor processes one thread and so many threads are executed simultaneously. 

- Both multiprocessor and time threading is supported by most of the modern operating systems with help from the process scheduler. 

- The threads can only be manipulated through a system and this all is facilitated by the kernel of the operating system. 

- This is why some implementations have been named as the kernel thread. 

- An example of kernel thread is the LWP or the lightweight process and it shares same state as well as info. 

- Some programs even use user space threads when threading with the help of signals and timers etc. 

- These programs perform a kind of ad hoc time slicing. 

- Some may take threads and processes to be the same but there is a considerable difference between the two:

1. Processes are independent whereas the threads are a subset of the processes.
2. More state information is contained in threads whereas the same process state, resources and memory are shared by all the threads contained in that process.
3. Address spaces of different processes are different whereas the same address space is also shared by the threads.
4. IPC or the inter–process communication is the only medium for the processes to communicate with each other.
5. The threads within the same process are switched with context switch which is faster than that of the processes.

Features of Multi-threading

- Multi-threading is now among the widespread programming models. 

- The major characteristic feature of this model is that multiple threads can execute within the same process context. 

- Even though the resources of the process are shared by the threads, they execute independently. 

- The most widespread application of this model is in parallel computing.

- Full advantage of this technology can be taken only when it is applied to a multiprocessor system or a distributed system. 

- This is so because the program threads have a natural tendency to heed to the true concurrent execution. 

- But in these cases, necessary precautions must be taken for avoiding the race conditions and other undesirable behavior. 

- For the correct manipulation of data thread synchronization is also important. - Mutually exclusive operations are another requirement of the threads for preventing the simultaneous modification of the common data. 

- If these primitives are used carelessly, then it can lead the system to a deadlock. 

- Another feature of multi-threading is that it is always responsive to I/P. 

- This can be contrasted with the single threaded applications where if one block occurs, the whole program freezes.