What is a major drawback of user-level threads managed entirely by a library under the many-to-one model?
Correct answer C. If one thread makes a blocking system call, the whole process blocks
Explanation
In the many-to-one model the kernel sees only one schedulable entity, so a blocking system call by any thread blocks every thread in the process, and the threads cannot run in parallel on a multicore machine. Their advantage is cheap creation and switching with no kernel involvement, which is why the option about system calls is wrong.
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