18

Chapter 2. Resource Monitoring

one

1

of the runnable processes must wait for their turn at the CPU. By monitoring the number of

runnable processes, it is possible to determine how CPU bound your system is.

Other performance data that impacts CPU utilization tends to revolve around different services that

the operating system provides to processes. They may include statistics on memory management,

I/O processing, and so on. These statistics also reveal that, when system performance is monitored,

there are no boundaries between the different statistics. In other words, CPU utilization statistics may

end up pointing to a problem in the I/O subsystem, or memory utilization statistics may reveal an

application design flaw.

Therefore, when monitoring system performance, it is not possible to look at any one statistic in com 

plete isolation; only by viewing the overall picture will it be possible to extract meaningful information

from any performance statistics you gather.

2.4.2. Monitoring Bandwidth

Monitoring bandwidth is more elusive than the other resources described here. The reason for this is

due to the fact that performance statistics tend to be device based, while most of the places where

bandwidth is important tend to be the buses that connect devices. In those instances where more than

one device shares a common bus, you might see reasonable statistics for each device, but the aggregate

load those devices place on the bus would be much greater.

Another challenge to monitoring bandwidth is that there can be circumstances where statistics for

the devices themselves may not be available. This is particularly true for system expansion buses

and datapaths

2

. However, even though 100% accurate bandwidth related statistics may not always be

available, there is often enough information to make some level of analysis possible, particularly when

related statistics are taken into account.

Some of the more common bandwidth related statistics are:

Bytes received/sent

Network interface statistics provide an indication of the bandwidth utilization of one of the more

visible buses   the network.

Interface counts and rates

These network related statistics can give indications of excessive collisions, transmit and receive

errors, and more. Through the use of these statistics (particularly if the statistics are available

for more than one system on your network), it is possible to perform a modicum of network

troubleshooting even before the more common network diagnostic tools are used.

Transfers per Second

Normally collected for block I/O devices, such as disk and high performance tape drives, this

statistic is a good way of determining whether a particular device's bandwidth limit is being

reached. Due to their electromechanical nature, disk and tape drives can only perform so many

I/O operations every second; their performance rapidly degrades as this limit is reached.

2.4.3. Monitoring Memory

If there is one area where a wealth of performance statistics can be found, it is in the area of moni 

toring memory utilization. Due to the inherent complexity of today's demand paged virtual memory

1. Assuming a single processor computer system.

2. More information on buses, datapaths and bandwidth is available in Chapter 3 Bandwidth and Processing

Power.