Multilevel Feedback Queue Scheduling Pdf Free ##HOT##
In computer science, a multilevel feedback queue is a scheduling algorithm. Scheduling algorithms are designed to have some process running at all times to keep the central processing unit (CPU) busy. The multilevel feedback queue extends standard algorithms with the following design requirements:
Multilevel Feedback Queue Scheduling Pdf Free
Whereas the multilevel queue algorithm keeps processes permanently assigned to their initial queue assignments, the multilevel feedback queue shifts processes between queues. The shift is dependent upon the CPU bursts of prior time-slices.
For scheduling, the scheduler always starts picking up processes from the head of the highest level queue. Only if the highest level queue has become empty will the scheduler take up a process from the next lower level queue. The same policy is implemented for picking up in the subsequent lower level queues. Meanwhile, if a process comes into any of the higher level queues, it will preempt a process in the lower level queue.
Also, a new process is always inserted at the tail of the top level queue with the assumption that it will complete in a short amount of time. Long processes will automatically sink to lower level queues based on their time consumption and interactivity level. In the multilevel feedback queue a process is given just one chance to complete at a given queue level before it is forced down to a lower level queue.
FCFS considered to be the simplest of all operating system scheduling algorithms. First come first serve scheduling algorithm states that the process that requests the CPU first is allocated the CPU first and is implemented by using FIFO queue.
Processes in the ready queue can be divided into different classes where each class has its own scheduling needs. For example, a common division is a foreground (interactive) process and a background (batch) process. These two classes have different scheduling needs. For this kind of situation Multilevel Queue Scheduling is used.
Multilevel Feedback Queue Scheduling (MLFQ) CPU Scheduling is like Multilevel Queue Scheduling but in this process can move between the queues. And thus, much more efficient than multilevel queue scheduling.
CPU Scheduling is a process of determining which process will own CPU for execution while another process is on hold. The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at least select one of the processes available in the ready queue for execution. The selection process will be carried out by the CPU scheduler. It selects one of the processes in memory that are ready for execution.
First Come First Serve is the full form of FCFS. It is the easiest and most simple CPU scheduling algorithm. In this type of algorithm, the process which requests the CPU gets the CPU allocation first. This scheduling method can be managed with a FIFO queue.
As the process enters the ready queue, its PCB (Process Control Block) is linked with the tail of the queue. So, when CPU becomes free, it should be assigned to the process at the beginning of the queue.
Round robin is the oldest, simplest scheduling algorithm. The name of this algorithm comes from the round-robin principle, where each person gets an equal share of something in turn. It is mostly used for scheduling algorithms in multitasking. This algorithm method helps for starvation free execution of processes.
Abstract:Software-defined Networking (SDN) and Data Center Network (DCN) are receiving considerable attention and eliciting widespread interest from both academia and industry. When the traditionally shortest path routing protocol among multiple data centers is used, congestion will frequently occur in the shortest path link, which may severely reduce the quality of network services due to long delay and low throughput. The flexibility and agility of SDN can effectively ameliorate the aforementioned problem. However, the utilization of link resources across data centers is still insufficient, and has not yet been well addressed. In this paper, we focused on this issue and proposed an intelligent approach of real-time processing and dynamic scheduling that could make full use of the network resources. The traffic among the data centers could be classified into different types, and different strategies were proposed for these types of real-time traffic. Considering the prolonged occupation of the bandwidth by malicious flows, we employed the multilevel feedback queue mechanism and proposed an effective congestion control algorithm. Simulation experiments showed that our scheme exhibited the favorable feasibility and demonstrated a better traffic scheduling effect and great improvement in bandwidth utilization across data centers.Keywords: data center; dynamic scheduling; congestion control; OpenFlow; software-defined networking
The number of ready queue algorithms between queues and within queues may differ between systems. A round-robin method with various time quantum is typically utilized for such maintenance. Several types of scheduling algorithms are designed for circumstances where the processes can be readily separated into groups. There are two sorts of processes that require different scheduling algorithms because they have varying response times and resource requirements. The foreground (interactive) and background processes (batch process) are distinguished. Background processes take priority over foreground processes.
The ready queue has been partitioned into seven different queues using the multilevel queue scheduling technique. These processes are assigned to one queue based on their priority, such as memory size, process priority, or type. The method for scheduling each queue is different. Some queues are utilized for the foreground process, while others are used for the background process. The foreground queue may be scheduled using a round-robin method, and the background queue can be scheduled using an FCFS strategy.
Let's take an example of a multilevel queue-scheduling (MQS) algorithm that shows how the multilevel queue scheduling work. Consider the four processes listed in the table below under multilevel queue scheduling. The queue number denotes the process's queue.
Each algorithm supports a different process, but some processes require scheduling using a priority algorithm in a general system. There is a different queue for foreground or background operations, but they do not switch between queues or change their foreground or background nature; this type of organization benefits from low scheduling but is inflexible.
The FreeBSD time-share-scheduling algorithm is based on multilevel feedback queues. The system adjusts the priority of a thread dynamically to reflect resource requirements (e.g., being blocked awaiting an event) and the amount of resources consumed by the thread (e.g., CPU time). Threads are moved between run queues based on changes in their scheduling priority (hence the word feedback in the name multilevel feedback queue). When a thread other than the currently running thread attains a higher priority (by having that priority either assigned or given when it is awakened), the system switches to that thread immediately if the current thread is in user mode. Otherwise, the system switches to the higher-priority thread as soon as the current thread exits the kernel. The system tailors this short-term scheduling algorithm to favor interactive jobs by raising the scheduling priority of threads that are blocked waiting for I/O for one or more seconds and by lowering the priority of threads that accumulate significant amounts of CPU time.
One queue is the idle queue, where all idle threads are stored. The other two queues are designated current and next. Threads are picked to run, in priority order, from the current queue until it is empty, at which point the current and next queues are swapped and scheduling is started again. Threads in the idle queue are run only when the other two queues are empty. Realtime and interrupt threads are always inserted into the current queue so that they will have the least possible scheduling latency. Interactive threads are also inserted into the current queue to keep the interactive response of the system acceptable. A thread is considered to be interactive if the ratio of its voluntary sleep time versus its runtime is below a certain threshold. The interactivity threshold is defined in the ULE code and is not configurable. ULE uses two equations to compute the interactivity score of a thread. For threads whose sleep time exceeds their runtime, the following equation is used:
Possibly the most straightforward approach to scheduling processesis to maintain a FIFO (first-in, first-out) run queue. New processesgo to the end of the queue. When the scheduler needs to runa process, it picks the process that is at the head of the queue.This scheduler is non-preemptive. If the process has to block on I/O,it enters the waiting state and the scheduler picksthe process from the head of the queue. When I/O is complete andthat waiting (blocked) process is ready to run again,it gets put at the end of the queue.
Round robin scheduling is a preemptive version of first-come, first-served scheduling.Processes are dispatched in a first-in-first-out sequence but eachprocess is allowed to run for only a limited amount of time.This time interval is known as a time-slice or quantum.If a process does not complete or get blocked because of an I/O operationwithin the time slice, the time slice expires and the process is preempted.This preempted process is placed at the back of the run queuewhere it must wait for all the processes that were already in thequeue to cycle through the CPU.
Advantage: Round robin scheduling is fair in that every processgets an equal share of the CPU. It is easy to implement and, if we knowthe number of processes on the run queue, we can know the worst-caseresponse time for a process.