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Updated on 2025-03-03

Java implements thread scheduler and time sharding

1. Thread Scheduler

1.1 What is a thread scheduler?

A thread scheduler is part of an operating system or Java virtual machine (JVM) that determines which thread should get CPU time and execute in a multi-threaded environment. Because CPU resources are limited, and modern computing environments usually require multiple threads to run at the same time, the thread scheduler allocates CPU time between different threads, so that all threads can obtain execution opportunities within a reasonable time.

1.2 Types of thread scheduling

Thread scheduling is usually divided into two main types:

  • Preemptive Scheduling：

    • In preemptive scheduling, the thread scheduler has the right to interrupt the currently running thread at any time and assign the CPU to other threads with higher priority. The scheduler can force interrupt it even if the thread does not complete its execution.
    • Most modern operating systems use preemptive scheduling because it can better respond to the system's real-time requirements and scheduling in multi-task environments.

  • Cooperative Scheduling：

    • In collaborative scheduling, thread switching is achieved by the currently running thread actively releasing CPU resources. The thread returns control to the scheduler at the appropriate time (such as completing a task or entering a waiting state), and the scheduler allocates the CPU to the next thread.
    • The efficiency of this scheduling method depends on the design and implementation of threads. If a thread does not release the CPU for a long time, other threads may be unable to execute.

1.3 The role of thread scheduler in Java

In Java, thread scheduling is implemented by the underlying operating system or JVM. Java programmers cannot directly control how thread schedulers work, but can affect scheduler decisions by setting thread priorities.

Thread priority：

  • Each Java thread has a priority (Thread.MIN_PRIORITYarriveThread.MAX_PRIORITY) The scheduler will usually prioritize execution of threads with higher priority. However, priority does not guarantee that threads will definitely obtain CPU time first, it is just a reference factor for the scheduler.
  • The scheduler may not allocate CPU time strictly according to priority in actual execution, especially on multi-core processors or in specific operating system implementations.
Thread thread1 = new Thread(() -> {
    ("Thread 1 is running");
});
Thread thread2 = new Thread(() -> {
    ("Thread 2 is running");
});

(Thread.MAX_PRIORITY);
(Thread.MIN_PRIORITY);

();
();

In this example,thread1Set the highest priority, andthread2The lowest priority is set. However, priority does not necessarily affect the actual execution order, as it depends on the implementation of the underlying thread scheduler.

1.4 How thread scheduler works

A thread scheduler usually determines which thread gets the CPU through the following steps:

  • Thread status check: The scheduler first checks the status of all threads to ensure that only threads in runnable states are scheduled.

  • Priority check: The scheduler usually refers to the priority of the thread to decide which thread to schedule first. Threads with high priority usually get CPU time first.

  • Time slice allocation: For preemptive scheduling, the scheduler allocates a time slice to each thread. If the thread does not complete the task within the time slice, the scheduler forces the interrupt and assigns the CPU to other threads.

2. Time Slicing

2.1 What is time sharding?

Time sharding is a key concept in thread scheduling. It refers to dividing CPU time into several small time periods (i.e. time slices) and distributing it to each thread in turn for execution. Under the time sharding mechanism, each thread obtains a fixed-length time slice to perform the task. When the time slice is used up, the scheduler will pause the thread and allocate CPU resources to the next runnable thread.

2.2 How time sharding works

The time sharding mechanism is usually used in preemptive scheduling, and its working principle is as follows:

  • Time slice allocation: The scheduler allocates a fixed-length time slice to each runnable thread, such as 10 milliseconds. In this time slice, threads can exclusively occupy CPU resources and perform tasks.

  • Time slices run out: If the thread does not complete the task within the time slice, the scheduler forces the thread to be interrupted and put it back into the runnable queue.

  • Context Switching: The scheduler selects the next thread from the runnable queue and assigns the CPU to that thread. This process is called Context Switch. Context switching involves saving the state of the current thread and loading the state of the next thread, which brings some performance overhead.

  • Loop execution: The scheduler continuously loops through the above process to ensure that all runnable threads can obtain execution time fairly.

2.3 Advantages of time sharding

  • Fairness: The time sharding mechanism ensures that all runnable threads can obtain CPU execution time, avoiding that some threads exclusively occupy CPU resources and other threads do not get execution opportunities for a long time.

  • Responsiveness: Time sharding allows multiple threads to execute alternately in a short time, improving the system's responsiveness and allowing users to feel the smoothness of the application.

  • Parallel simulation: On a single-core processor, time sharding can simulate parallel execution of multiple threads, making multiple tasks "look" like they are executed simultaneously.

2.4 Disadvantages of time sharding

  • Context Switching Overhead: Each time the time slice is exhausted, the scheduler will switch context, save the status of the current thread and restore the status of the next thread. This process involves the storage and recovery of CPU registers, program counters, memory management information, etc., which brings additional performance overhead.

  • The trade-off of time slice size: The size of the time slice needs to be weighed. If the time slice is too short, the time the scheduler spends on context switching will increase, reducing system efficiency; if the time slice is too long, the system's responsiveness will decrease and it will not be able to respond to the needs of other threads in a timely manner.

3. The relationship between thread scheduler and time sharding

Thread schedulers and time sharding are closely related in multitasking operating systems:

  • Thread schedulers use time sharding to manage thread execution order. In preemptive scheduling, the thread scheduler determines which thread should be executed and how long it takes to execute by allocating a time slice.

  • Effective management of time sharding is the key to efficient work of thread schedulers. Through reasonable time sharding, thread schedulers can improve overall performance and response speed of the system while ensuring fairness.

4. Implementation and application scenarios

4.1 Implementation in the operating system

  • Linux: In Linux system, thread scheduler is implemented based on CFS (Completely Fair Scheduler), adopting a priority-based scheduling algorithm, combining time sharding and process priority to determine the scheduling strategy. Linux's CFS scheduler allocates time slices based on the virtual run time of each task to ensure fairness.

  • Windows: The Windows operating system uses preemptive scheduling and combines the priority-driven time-shatter scheduling mechanism. High-priority threads will get the time slice first, while low-priority threads may be preempted.

4.2 Application scenarios

  • Desktop Applications: In graphical user interface (GUI) applications, thread scheduler and time sharding ensure a balance between user input, interface updates and background tasks, so that the application can still respond to user operations in a timely manner when executing time-consuming tasks.

  • Real-time system: In real-time systems such as industrial control, aerospace, thread scheduling and time sharding need to be strictly managed to ensure that critical tasks are completed within a specific time.

  • Server and multitasking systems: In a multitasking server environment, thread scheduler and time sharding ensure that all client requests can be processed in a timely manner, avoiding some tasks from exclusive resources for a long time.

5. Challenges of thread schedulers and time sharding

Although thread schedulers and time sharding play an important role in multitasking systems, they also face some challenges:

  • Complexity: Designing an efficient thread scheduler requires consideration of multiple factors, such as priority, time slice size, real-time task, etc., which makes the adjustment

The design of the degree algorithm is very complicated.

  • Performance overhead: Frequent context switching can lead to performance overhead, especially in high concurrency environments, how to balance fairness and performance is a difficult problem.

  • System responsiveness: The setting of time slice size directly affects the system's responsiveness and task execution efficiency. Finding a suitable time slice size requires detailed analysis and tuning.

6. Conclusion

Thread schedulers and time sharding are core concepts in multithreaded programming and operating system design, which ensure that multiple threads can efficiently and reasonably share CPU resources. In practical applications, the thread scheduler manages the execution order of threads by allocating time slices to ensure the fairness and responsiveness of the system. However, the complexity of scheduling and the performance overhead of context switching also present challenges.

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