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Is that a "thread" consists of both "user thread" and "kernel thread" as two parts?
And when I create a "thread", is that I create a "user thread" and another "kernel thread" is created "in the background" by OS also?
I'm confused because of the second picture.
The book I'm reading is "Operating System Concepts, 9th ed., SILBERSCHATZ", and it's at page 169. Thanks a lot
As the given text says, it is not necessary that for every new user thread a new kernel thread is also created. In fact different operating systems can implement this in a different way. Some OS will map every new user thread to a new kernel thread(one to one mapping), some can have a maximum limit on number of kernel threads beyond which a kernel thread will be mapped with more than one user thread(many to one). There can also be an OS which maps a single user thread to more than one kernel thread, and vice versa. This would be a many to many model.
All these models have their own pros and cons. Which model to choose would depend upon what functionality is required from OS, available hardware resources etc.
