In this study, users report that the response time to key presses and mouse movement is occasionally very slow during the working day. They also report that resizing and moving windows is also very slow at certain times. This performance is unacceptable for the interactive applications being run -- word processing, spreadsheets, e-mail, desktop publishing, and graphics processing.
The system's configuration is as follows:
The system administrator is asked to investigate the source of the problem, and suggest means to improve the response time.
The system administrator runs the netstat -i command
on several workstations to record networking statistics
throughout the working day.
A sampling interval of one minute is specified and the output
is written to a file in the /tmp directory:
netstat -i 60 > /tmp/netstat_op
The administrator runs the command on several workstations to try to eliminate the possibility that faulty network interface cards are the cause of the problem.
The recorded output shows occasional short
periods when the network is overloaded
(for clarity, only the statistics for
the network interface
are shown in this example):
input (xxx0) output packets errs packets errs colls ... 110 0 101 0 0 78 0 66 0 0 85 1 75 2 23 180 2 123 1 42 120 1 55 1 18 87 0 67 0 2 67 0 54 0 0 ...At these times, the numbers of input and output errors are non-zero, and the number of collisions approaches 30% of output packets. The same behavior is observed on all the workstations on which statistics were gathered.
If the periods of heavy loading are excluded, the frequency of packet collisions approaches 0%.
From the results of running netstat, the system administrator suspects that some applications must be moving large amounts of data across the network. Careful examination of the figures shows that the network is overloaded approximately 5% of the time. Periods of high loading generally last only a few minutes and seem to occur in bursts. Such behavior is typical if large files are transferred using NFS. It is unlikely to be the result of network traffic caused by remote X clients as these are run locally where possible. Possible culprits are programs used to preview PostScript and graphics image files, DTP packages, and screen-capture utilities.
With the cooperation of several users,
the administrator monitors
network performance using netstat
over a period of 30 minutes.
During this period the users run
the suspect applications to load and manipulate
large files across the network. The outcome of this
investigation is that graphics image previewers and
screen-capture utilities seem to cause the most
network overhead. The files being viewed or created
are often several megabytes in size.
There are several things that can be done to reduce the peak load on the network: