Chapter 3. Bandwidth and Processing Power

33

But how is it that many different applications can seemingly run at once under Red Hat Linux? The

answer is that Red Hat Linux is a multitasking operating system. In other words, it creates the illusion

that many different things are going on simultaneously when in fact that is not possible. The trick is

to give each process a fraction of a second's worth of time running on the CPU before giving the CPU

to another process for another fraction of a second. If these context switches happen quickly enough,

the illusion of multiple applications running simultaneously is achieved.

Of course, applications do other things than manipulate data using the CPU. They may wait for user

input as well as performing I/O to devices such as disk drives and graphics displays. When these

events take place, the application does not need the CPU. At these times, the CPU can be used for

other processes running other applications without slowing the waiting application at all.

In addition, the CPU can be used by another consumer of processing power: the operating system

itself.

3.2.2.2. The Operating System

It is difficult to determine how much processing power is consumed by the operating system. The

reason for this is that operating systems use a mixture of process level and system level code to

perform their work. While, for example, it is easy to use

top

to see what the process running the

system logging daemon

syslogd

is doing, it is not so easy to see how much processing power is

being consumed by system level I/O related processing.

In general, it is possible to divide this kind of operating system overhead into two types:

Operating system housekeeping

Process related activities

Operating system housekeeping includes activities such as process scheduling and memory manage 

ment, while process related activities include any processes that support the operating system itself

(including system daemons such as

syslogd

,

klogd

, etc.).

3.2.3. Improving a CPU Shortage

When there is insufficient processing power available for the work that needs to be done, you have

two options:

Reducing the load

Increasing the capacity

3.2.3.1. Reducing the Load

Reducing the CPU load is something that can be done with no expenditure of money. The trick is to

identify those aspects of the system load that are under your control and can be cut back. There are

three areas to focus on:

Reducing operating system overhead

Reducing application overhead

Eliminating applications entirely