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I'm curious what the cost is on modern CPUs in terms of cycles to perform a switch from Kernel and User mode?
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System Call Interfaces (SCI) are the only way to transit from User space to kernel space. Kernel space switching is achieved by Software Interrupt, which changes the processor mode and jump the CPU execution into interrupt handler, which executes corresponding System Call routine.
In kernel mode, the program has direct and unrestricted access to system resources. In user mode, the application program executes and starts. In user mode, a single process fails if an interrupt occurs. Kernel mode is also known as the master mode, privileged mode, or system mode.
Kernel mode is generally reserved for the lowest-level, most trusted functions of the operating system. Crashes in kernel mode are catastrophic; they will halt the entire PC. In User mode, the executing code has no ability to directly access hardware or reference memory.
The hardware allows privileged instructions to be executed only in kernel mode. ... The instruction to switch to kernel mode is an example of a privileged instruction.
Switching from “user mode” to “kernel mode” is, in most existing systems, very expensive.
It has been measured, on the basic request getpid, to cost 1000-1500 cycles on most machines.
Of these just around 100 are for the actual switch (70 from user to kernel space, and 40 back), the rest is "kernel overhead". In the L3 microkernel the minimization of this overhead reduced the overall cost to around 150 cycles.
Cited from here.
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