Mode switches

The use of a system call by a user-mode process allows a kernel function to be called from user mode. Access to functions that directly or indirectly invoke system calls is typically provided by programming libraries, which provide access to operating system functions.

Mode switches should be differentiated from the context switches seen in the output of the vmstat (cs column) and sar (cswch/s) commands. A context switch occurs when the currently running thread is different from the previously running thread on that processor.

The scheduler performs a context switch when any of the following occurs:

• A thread must wait for a resource (voluntarily), such as disk I/O, network I/O, sleep, or locks
• A higher priority thread wakes up (involuntarily)
• The thread has used up its time slice (usually 10 ms).

Context switch time, system calls, device interrupts, NFS I/O, and any other activity in the kernel is considered as system time.