Understanding the Linux Kernel

11.3. The SMP Architecture

Symmetrical multiprocessing (SMP ) denotes a multiprocessor architecture in which no CPU is selected as the Master CPU, but rather all of them cooperate on an equal basis, hence the name "symmetrical." As usual, we shall focus on Intel SMP architectures.

How many independent CPUs are most profitably included in a multiprocessor system is a hot issue. The troubles are mainly due to the impressive progress reached in the area of cache systems. Many of the benefits introduced by hardware caches are lost by wasting bus cycles in synchronizing the local hardware caches located on the CPU chips. The higher the number of CPUs, the worse the problem becomes.

From the kernel design point of view, however, we can completely ignore this issue: an SMP kernel remains the same no matter how many CPUs are involved. The big jump in complexity occurs when moving from one CPU (a uniprocessor system) to two.

Before proceeding in describing the changes that had to be made to Linux in order to make it a true SMP kernel, we shall briefly review the hardware features of the Pentium dual-processing systems. These features lie in the following areas of computer architecture:

• Shared memory

• Hardware cache synchronization

• Atomic operations

• Distributed interrupt handling

• Interrupt signals for CPU synchronization

Some hardware issues are completely resolved within the hardware, so we don't have to say much about them.

11.3.1. Common Memory

All the CPUs share the same memory; that ...