13.1.
Symmetrical Multi-Processing
One of the easiest and cheapest ways to improve hardware
performance is to put more than one CPU on the board. This can be
done either making the different CPU's take on different jobs
(asymmetrical multi-processing) or by making them all run in
parallel, doing the same job (symmetrical multi-processing, a.k.a.
SMP). Doing asymmetrical multi-processing effectively requires
specialized knowledge about the tasks the computer should do, which
is unavailable in a general purpose operating system such as Linux.
On the other hand, symmetrical multi-processing is relatively easy
to implement.
By relatively easy, I mean exactly that: not that it's
really easy. In a symmetrical multi-processing
environment, the CPU's share the same memory, and as a result code
running in one CPU can affect the memory used by another. You can
no longer be certain that a variable you've set to a certain value
in the previous line still has that value; the other CPU might have
played with it while you weren't looking. Obviously, it's
impossible to program like this.
In the case of process programming this normally isn't an issue,
because a process will normally only run on one CPU at a
time. The kernel, on the other hand, could be
called by different processes running on different CPU's.
In version 2.0.x, this isn't a problem because the entire kernel
is in one big spinlock. This means that if one CPU is in the kernel
and another CPU wants to get in, for example because of a system
call, it has to wait until the first CPU is done. This makes Linux
SMP safe, but
inefficient.
In version 2.2.x, several CPU's can be in the kernel at the same
time. This is something module writers need to be aware of.
