Chapter 2. Resource Monitoring

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How much free space is available?

How many I/O operations on average does it perform each second?

How long on average does it take each I/O operation to be completed?

How many of those I/O operations are reads? How many are writes?

What is the average amount of data that is read/written with each I/O?

There are more ways of looking at disk drive performance; these points have only scratched the

surface. The main thing to keep in mind is that there are many different types of data for each resource.

The following sections look at the types of utilization information that would be helpful for each of

the major resource types.

2.4.1. Monitoring CPU Power

In its most basic form, monitoring CPU power can be no more difficult than seeing if CPU utilization

ever reaches 100%. If CPU utilization stays below 100%, no matter what the system is doing, there is

additional processing power available for more work.

However, it is a rare system that does not reach 100% CPU utilization at least some of the time. That

is when it becomes important to look at more detailed CPU utilization data. By doing so, it becomes

possible to start determining where the majority of your processing power is being consumed. Here

are some of the more popular CPU utilization statistics:

User Versus System

The percentage of time spent performing user level processing versus system level processing

can point out whether a system's load is primarily due to running applications, or due to op 

erating system overhead. High user level percentages tend to be good (assuming users are not

experiencing unsatisfactory performance), while high system level percentages tend to point to 

ward problems that will require further investigation.

Context Switches

A context switch happens when the CPU stops running one process and starts running another.

Because each context switch requires the operating system to take control of the CPU, excessive

context switches and high levels of system level CPU consumption tend to go hand in hand.

Interrupts

As the name implies, interrupts are situations where the processing being performed by the CPU

is abruptly changed. Interrupts generally occur due to hardware activity (such as an I/O device

completing an I/O operation) or due to software (such as software interrupts that control applica 

tion processing). Because interrupts must be serviced at a system level, high interrupt rates lead

to higher system level CPU consumption.

Runnable Processes

A process may be in different states. For example, it may be:

Waiting for an I/O operation to complete

Waiting for the memory management subsystem to handle a page fault

In these cases, the process has no need for the CPU.

However, eventually the process state changes, and the process becomes runnable. As the name

implies, a runnable process is one that is capable of getting work done as soon as it is scheduled

to receive CPU time. However, if more than one process is runnable at any given time, all but