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1. What is the thread model?
The method used for thread creation, management, scheduling, etc. is called the thread model. Thread models are generally divided into the following three types: 内核级线程模型, 用户级线程模型, 两级线程模型. The difference between them lies in the correspondence between user threads and kernel threads. User-level threads are managed by applications. Kernel-level threads are managed by the operating system.
User threads will only be mapped to designated kernel threads before they can be scheduled and executed by the CPU. The three thread models introduced below refer to the three ways in which user threads are mapped to kernel threads.
2. Three thread models
Kernel-level threading model (1:1)
One user thread corresponds to one kernel thread. This is the easiest to implement. The scheduling of coroutines is completed by the CPU.
advantage:
Easiest to implement
Ability to utilize multiple cores
If one thread in the process is blocked, it will not block other threads and can switch to other threads in the same process to continue execution.
shortcoming:
The cost of context switching is high, and creation, deletion and switching are all completed by the CPU (to create a user thread, you need to create a kernel thread and make a system call)
User-level threading model (N: 1)
All threads in 1 process correspond to 1 kernel thread 
. Advantages:
The cost of context switching is low, and coroutine switching can be completed in user mode.
shortcoming:
Unable to take advantage of multiple cores
Once the coroutine blocks, causing the thread to block, other coroutines of this thread cannot execute.
Two-level thread model (M: N)
M threads correspond to N kernel threads
advantage:
Ability to utilize multiple cores
Context switching is cheap
If one thread in the process is blocked, other threads will not be blocked, and other threads in the same process will be switched to continue execution.
shortcoming:
The most complex to implement
3. Summary
What Go implements is the two-level thread model (M:N), to be precise, the GMP model, which is an improved implementation of the two-level thread model, allowing it to more flexibly schedule between threads.
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