In Java, the thread scheduler is responsible for determining which thread should execute next when multiple threads are competing for CPU time. The scheduler uses a priority-based algorithm to decide which thread should be executed first, based on the priority assigned to each thread.
By default, all Java threads have a priority of 5, which means they have the same priority and are treated equally by the scheduler. However, you can change the priority of a thread using the setPriority() method of the Thread class.
Here is an example of setting the priority of a thread:
Thread t1 = new Thread(); t1.setPriority(Thread.MAX_PRIORITY); // set the priority to the highest possible value
The thread priorities range from 1 to 10, where 1 is the lowest priority and 10 is the highest priority. You can also use the predefined constants Thread.MIN_PRIORITY, Thread.NORM_PRIORITY, and Thread.MAX_PRIORITY to set the priority.
The thread scheduler is responsible for allocating CPU time to threads based on their priority. Higher priority threads are given more CPU time compared to lower priority threads. However, the scheduler is not guaranteed to follow the priority order strictly. It is possible for a lower priority thread to execute before a higher priority thread if the lower priority thread is currently holding a resource that the higher priority thread needs.
In summary, the thread scheduler in Java is responsible for determining which thread should execute next based on their priority. You can set the priority of a thread using the setPriority() method, and the scheduler will allocate CPU time to threads based on their priority.
Thread Scheduler Algorithms:
There are several thread scheduling algorithms used by operating systems and programming languages to determine the order in which threads are executed. In Java, the thread scheduler uses a priority-based scheduling algorithm, where each thread is assigned a priority, and the scheduler executes threads in order of their priority.
Other common thread scheduling algorithms include:
- First-Come, First-Serve (FCFS): This algorithm schedules threads in the order they arrive. The first thread to arrive gets executed first, followed by the next thread in the queue. This algorithm is simple and easy to implement, but it can lead to long waiting times for higher-priority threads.
- Round Robin (RR): This algorithm assigns a time slice to each thread, allowing it to execute for a fixed amount of time before being preempted and replaced by the next thread in the queue. The preempted thread is added back to the end of the queue. This algorithm is good for time-sharing systems, but it can lead to poor performance if the time slice is too short.
- Shortest Job First (SJF): This algorithm schedules threads based on their expected execution time. The thread with the shortest expected execution time is executed first, followed by the next shortest thread, and so on. This algorithm is good for minimizing waiting times, but it requires accurate estimates of execution time, which can be difficult to obtain.
- Priority Scheduling: This algorithm assigns a priority to each thread, allowing higher-priority threads to be executed first. Within each priority level, the threads are scheduled using another algorithm, such as FCFS or RR. This algorithm is flexible and can be adapted to different types of systems, but it can lead to starvation if lower-priority threads are never executed.
- Multi-level Feedback Queue (MLFQ): This algorithm assigns threads to multiple queues based on their priority and execution time. The threads in each queue are scheduled using a different algorithm, such as RR or SJF. The threads can move between queues based on their behavior, such as if they use a lot of CPU time or wait for I/O operations. This algorithm is complex but can provide good performance in systems with varying workloads.
In summary, thread scheduling algorithms determine the order in which threads are executed, and each algorithm has its strengths and weaknesses. Java uses a priority-based scheduling algorithm, but other algorithms such as FCFS, RR, SJF, Priority Scheduling, and MLFQ are commonly used in other systems.
First Come First Serve Scheduling:
First Come First Serve (FCFS) is a non-preemptive CPU scheduling algorithm that schedules threads in the order they arrive. The first thread to arrive is the first thread to be executed, followed by the next thread in the queue.
In FCFS scheduling, once a thread starts executing, it will continue until it completes its execution or is blocked by an I/O operation. The scheduler does not interrupt the thread unless it completes or blocks. Once a thread is blocked, it is removed from the CPU and added to a waiting queue until the I/O operation is completed.
FCFS scheduling is simple and easy to understand, but it can lead to long waiting times for threads with higher priority or shorter execution times. If a long-running thread arrives before a short-running thread with a higher priority, the short-running thread will have to wait until the long-running thread completes, leading to increased waiting times.
FCFS scheduling is commonly used in batch processing systems or systems with a low volume of interactive users. In batch processing, jobs are submitted in advance and are executed in the order they are submitted. FCFS scheduling is also used in single-user systems where only one user can use the system at a time.
Overall, FCFS scheduling is a simple and fair scheduling algorithm that works well in certain types of systems but may not be suitable for systems with varying workloads or time-sensitive tasks
Time-slicing scheduling:
Time-slicing scheduling, also known as round-robin scheduling, is a preemptive CPU scheduling algorithm that allows multiple threads to share the CPU by switching between them at regular intervals called time slices.
In time-slicing scheduling, each thread is assigned a fixed time slice, usually a few milliseconds, during which it is allowed to execute. Once the time slice is over, the scheduler interrupts the thread and switches to the next thread in the queue. The interrupted thread is then added to the end of the queue and waits for its next turn. This process repeats until all threads have been executed.
Time-slicing scheduling provides a fair way of allocating CPU time to threads, as each thread is given a fixed amount of time to execute, and no thread is allowed to monopolize the CPU. It also allows for quick response times to interactive tasks since threads can be switched quickly.
However, time-slicing scheduling can lead to increased overhead due to frequent context switching between threads, which can negatively impact performance, especially in systems with high CPU usage. It can also lead to poor response times for long-running tasks since each task has to wait for its turn to execute.
Overall, time-slicing scheduling is a useful scheduling algorithm in systems where fairness and quick response times are important, such as interactive systems. It is commonly used in operating systems, virtual machines, and web servers.
Preemptive-Priority Scheduling:
Preemptive-priority scheduling is a CPU scheduling algorithm that assigns priorities to threads and allows higher-priority threads to preempt lower-priority threads. The threads are executed based on their priorities, with higher-priority threads being executed before lower-priority threads.
In preemptive-priority scheduling, each thread is assigned a priority value, usually an integer value, with higher values indicating higher priority. The scheduler uses the priority values to decide which thread to execute next. If a higher-priority thread arrives while a lower-priority thread is executing, the scheduler interrupts the lower-priority thread and starts executing the higher-priority thread. The interrupted thread is then added back to the queue with its remaining execution time.
Preemptive-priority scheduling ensures that higher-priority threads get more CPU time than lower-priority threads, which is useful in systems where some threads have higher priority than others. For example, real-time systems, where timely response is critical, often use this scheduling algorithm.
However, preemptive-priority scheduling can lead to starvation, where lower-priority threads never get a chance to execute, especially if many higher-priority threads are constantly arriving. To avoid this, some implementations use aging, where the priority of a thread is increased as it waits in the queue, ensuring that eventually, it will get a chance to execute.
Overall, preemptive-priority scheduling is a useful CPU scheduling algorithm in systems where different threads have different levels of priority, and timely execution is important. However, it requires careful management to avoid starvation and ensure fair allocation of CPU time to all threads.
Working of the Java Thread Scheduler:
The Java Thread Scheduler is responsible for managing and scheduling the execution of threads in a Java program. When a program creates a new thread, the thread is added to the scheduler’s queue, and the scheduler decides when to start executing the thread based on various scheduling algorithms.
In Java, the thread scheduler uses a preemptive-priority scheduling algorithm, where each thread is assigned a priority value, and the thread with the highest priority value is executed first. The priority values range from 1 to 10, with 1 being the lowest priority and 10 being the highest priority. The thread scheduler uses time-slicing to allow multiple threads to share the CPU and switch between them at regular intervals.
When a thread is created, it is assigned a priority value by default, which is usually the same as the priority of the parent thread. The program can also explicitly set the priority of a thread using the setPriority() method. The thread scheduler uses these priority values to decide which thread to execute next.
If multiple threads have the same priority value, the thread scheduler uses time-slicing to allocate CPU time to each thread. Each thread is given a fixed time slice during which it is allowed to execute, and the scheduler switches to the next thread after the time slice is over.
The thread scheduler also uses other factors to determine the scheduling of threads, such as thread state, waiting time, and synchronization. For example, a thread that is waiting for I/O or a synchronized resource is not executed until the resource is available.
Overall, the Java Thread Scheduler plays a crucial role in managing the execution of threads in a Java program and ensures that threads are executed in a fair and efficient manner.