1.4 Process  (Page 5/9)

Non-preemptive: user thread executing ® system call to perform I/O ® user thread state is saved for restart ® OS code to perform system call is invoked ® I/O operation started (by invoking I/O driver) ® set thread status to waiting ® move thread’s TCB from run queue to wait queue associated with specific device ® call context switching code

Software vs hardware context switching

Intel 80386 and higher CPUs contain hardware support for context switches. However, most modern operating systems perform software context switching, which can be used on any CPU, rather than hardware context switching in an attempt to obtain improved performance. Software context switching was first implemented in Linux for Intel-compatible processors with the 2.4 kernel.

One major advantage claimed for software context switching is that, whereas the hardware mechanism saves almost all of the CPU state, software can be more selective and save only that portion that actually needs to be saved and reloaded. However, there is some question as to how important this really is in increasing the efficiency of context switching. Its advocates also claim that software context switching allows for the possibility of improving the switching code, thereby further enhancing efficiency, and that it permits better control over the validity of the data that is being loaded.

The cost of context switching

Context switching is generally computationally intensive. That is, it requires considerable processor time, which can be on the order of nanoseconds for each of the tens or hundreds of switches per second. Thus, context switching represents a substantial cost to the system in terms of CPU time and can, in fact, be the most costly operation on an operating system.

Consequently, a major focus in the design of operating systems has been to avoid unnecessary context switching to the extent possible. However, this has not been easy to accomplish in practice. In fact, although the cost of context switching has been declining when measured in terms of the absolute amount of CPU time consumed, this appears to be due mainly to increases in CPU clock speeds rather than to improvements in the efficiency of context switching itself.

One of the many advantages claimed for Linux as compared with other operating systems, including some other Unix-like systems, is its extremely low cost of context switching and mode switching.

Entering and exiting the kernel

User and kernel address spaces

In a modern operating system, each user process runs in its own address space, and the kernel operates in its protected space. At the processor level (machine code level), the main distinction between the kernel and a user process is the ability to access certain resources such as executing privileged instructions, reading or writing special registers, and accessing certain memory locations.

The separation of user process from user process insures that each process will not disturb each other. The separation of user processes from the kernel insures that user processes will not be able to arbitrarily modify the kernel or jump into its code. It is important that processes cannot read the kernel's memory, and that it cannot directly call any function in the kernel. Allowing such operations to occur would invalidate any protection that the kernel wants to provide.