11. Kernel Level vs User Level Threads

User Level Threads (ULT): managed entirely by a user-space library (old Java green threads, early Go). The kernel sees only one process; the library switches threads without any syscall.

Kernel Level Threads (KLT): managed directly by the OS kernel, which schedules each individually.

Aspect User Level Kernel Level
Who manages	Thread library	OS kernel
Kernel awareness	Invisible	Fully visible
Context switch	No syscall — fast	Syscall — slower
Parallelism	No (single core)	Yes
I/O blocking	Blocks all threads	Blocks only that thread
Crash impact	One crash kills all	Isolated
Example	Old green threads	Linux pthreads

Threading models: Many-to-One (pure ULT, fast switching, no parallelism), One-to-One (pure KLT, true parallelism, expensive switches — Linux pthreads), Many-to-Many (hybrid — M user threads over N kernel threads). Go's goroutines are the classic Many-to-Many example: thousands of goroutines multiplexed over a handful of OS threads.

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