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I have read about the differences in them, but my concepts are messed up on these points

  • Are User Level threads schedulable? Can it perform CPU Operations (no system calls) without being mapped to Kernel Threads?

    I know about the one-to-one, many-to-one mapping but didn't really understood the need for User Threads if all map to Kernel threads eventually

  • Do Kernel Level threads run in Kernel Mode or User Mode? (with and without system calls)

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  • The first question ("schedulability" of the user-level threads) is answered here: https://stackoverflow.com/a/39185831/3440745. As for the second question, yes, kernel-level threads are run in the kernel mode. On Stack Overflow we are strongly against asking **several questions** in the single question post. This defeats the purpose of SO. – Tsyvarev Sep 19 '18 at 19:53
  • Unclear what you mean by "user level" and "kernel level." There can be threads that belong to the operating system or, to device drivers, that run in kernel mode/supervisor mode/privileged mode; and there can be threads that are part of a user-mode process, that are known to, and scheduled by the operating system; and there can be so-called _[green threads](https://en.wikipedia.org/wiki/Green_threads)_, that run in a user-mode process, and which are unknown to the operating system. – Ohm's Lawman Sep 19 '18 at 20:24

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