How to implement thread in kernel space?

Kernel-level threads are threads that are created and managed directly by the operating system kernel. Unlike user-level threads, which are managed by thread libraries in user space, kernel threads are scheduled by the kernel's thread scheduler and have direct access to system resources.

Functions of Kernel

The kernel performs several critical functions in the operating system −

• Memory management − Allocating and deallocating memory for processes and threads

• Process and thread scheduling − Determining which processes/threads get CPU time

• File system management − Managing file operations and storage

• Interrupt handling − Processing hardware and software interrupts

• Device management − Controlling access to hardware devices

• System call interface − Providing services to user applications

Execution Modes

Programs execute in two distinct modes −

• User mode − Limited access to system resources; cannot directly access hardware or privileged instructions

• Kernel mode − Full access to all system resources including memory, CPU, and hardware devices

The transition from user mode to kernel mode occurs through system calls, interrupts, or exceptions. When a thread requires kernel services (like I/O operations), it must switch to kernel mode through a system call.

Implementing Thread in Kernel Space

Kernel-level threads are implemented through the following mechanism −

Step-by-Step Implementation

Step 1 − The kernel maintains a thread table in kernel space to track all threads in the system. Each entry contains thread-specific information like thread ID, state, priority, and register values.

Step 2 − When a thread wants to create a new thread or terminate an existing one, it makes a system call to the kernel. The kernel handles the creation/destruction by updating its internal thread table.

Step 3 − The kernel maintains a Thread Control Block (TCB) for each thread, storing registers, stack pointer, program counter, state information, and scheduling data.

Step 4 − The kernel scheduler directly schedules kernel threads, giving it complete control over thread execution and the ability to preempt threads when necessary.

Advantages

• True parallelism − Multiple kernel threads can run simultaneously on different CPU cores

• Better for blocking operations − When one thread blocks on I/O, other threads can continue execution

• Fair scheduling − Kernel can allocate CPU time fairly among threads from different processes

• System-wide visibility − Kernel has complete knowledge of all threads for better resource management

Disadvantages

• High overhead − Thread operations require system calls, making them significantly slower than user-level threads

• Kernel complexity − Requires the kernel to maintain Thread Control Blocks and manage thread scheduling

• Context switching cost − Switching between kernel threads involves more overhead than user-level thread switching

• Limited scalability − Kernel resources limit the total number of threads that can be created

Conclusion

Kernel-level threads provide true parallelism and better handling of blocking operations but come with higher overhead costs. They are ideal for applications that require genuine concurrent execution and can tolerate the performance penalty of kernel-managed threading.