The GNOME System Monitor
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The Processes window

There is a lot of information displayed in this window, and it is constantly being updated. (If you have the CPU monitor applet running, you will see periodic spikes as the updates occur.) For clarity in this guide, this window's display can be divided into three areas. The top shows menubars, options, and a graphical summary. The central area shows fields of process information. The statusline shows a summary of other machine information at the base of the window.

Figure 1. The processes window of gtop

The top part of the display

At the top of the display are the menubar and the toolbar. These are explained in the menubar section. If gtop is using the 'notebook' display format (this is set in the Preferences dialogue), then beneath the toolbar you will also see a list of the three displays you can view: processes, memory use and filesystem use; with the current display highlighted. Beneath these is a graphical summary of the machine's state.

The graphical summary

The fluctuating graphs visible show:

CPU use;
Memory use;
Swap space use;
The load average.

CPU: The colours for the display are configurable with the Preferences dialogue but default values are:

programs running in user-mode: default colour is yellow
programs at low priority: default colour is pale grey
programs runing in kernel-mode: default colour is dark grey
amount of idle CPU: default colour is black

These vary in quantity as the CPU usage changes, giving you an idea of how busy your CPU is. The actual figures can be seen in percentage form at the base of the gtop display.

MEM: The colours for the display are configurable with the Preferences dialogue but default values are:

free, ie unused, memory: default colour is dark green
buffers, or memory which is holding such things as data which has not yet been written to the disk, and data which has been read from the disk and kept in memory just in case it is needed again: default colour is dark grey
shared memory: memory containing information that more than one program is using: default colour is yellow
other: default colour is mucky yellow-green

These vary in quantity as programs require or yield memory. Because operating systems vary, it is not easy to compare these numbers across operating systems.

SW: The colours for the swap space in use are configurable with the Preferences dialogue but default values are:

swap space in use: default colour is red
swap space not in use: default colour is dark green

On Solaris, the amount of swap space available can change whilst the system is running.

The swap space is a part of the hard drive which is not part of the filesystem. When the kernel decides a program in memory is not being used a lot but might be required soon, it takes parts of the program (called 'pages') out of memory and puts those into the swap space, where it can easily retrieve it. This is called swapping. If the data is put back into memory, the swap space is not immediately freed up, which means its use may look higher than it really is.

LA: The colours for the current load average are configurable with the Preferences dialogue but default values are:

load average: default colour is red
background colour: default colour is green

The load average is a representation of how "busy" your machine is. The figures can be seen at the base of the display in a group of three. The first is the load average over the last minute, the second is the load average of the last 5 minutes, and the third is the load average of the last 15 minutes. When the load average over the last 15 minutes is consistently over 1.0, then the CPU on a single-processor machine is constantly in use at 100% of its capacity. The effect of different load averages will vary by machine. Whilst a single-processor machine may feel horribly slow with a load average of 0.8, a multi-processor machine showing the same load average will feel far less 'busy'. (For the curious wondering why it's called a load average, it is calculated over time as an average from the number of processes which show up as R or D. See Stat below for what these mean.)

The process fields

This is where the most information can be seen. Those familiar with the top program will recognise it easily. The display can be customised with the Preferences dialogue box, but the default settings include all but three information fields. There are two scrollbars: horizontal and vertical. To see everything on the display, you will certainly need to use the vertical scrollbar. You will probably need the horizontal one too.

Figure 2. The gtop processes display

PID: process identification.

This is the unique number of the process. No two processes will have the same number.
user: the name of the user owning the process.

This is normally the user who started the program, but not always. For example, a user can start X running, but the resulting X process is owned and controlled by root. There is also a special user called 'nobody' which runs quite a lot of things. So if 'nobody' is the user, it does not mean that the process is unowned and uncontrolled.
Pri: the priority of the process

Priority controls how much access the process can get to the CPU. The normal priority is zero. See the field called Ni, too.
Ni: the 'nice' value of the process

This field is not shown by default. You can nice a process (see manipulating processes), which then lowers its priority. This is called nice because it is a sociable thing to do for programs which don't need high priority. It is often done with resource-hungry programs which do not need interactive input in order to stop them using all the system's resources. The super-user can nice a process in the opposite direction to give it a higher priority. The range is from -20 (not nice and high-priority) to +19 (nice and low-priority). See the field called Pri, too.
Size: the size of the code, data and stack space (everything it need to run, in other words), in kilobytes.

This is a very rough guide, as it depends on how much memory the process has been allocated, not how much of that it is actually using. It may well not be using it all. So it is a bad way to look at how big a program is. A better way to find out what is in memory is the RSS field.
RSS: the resident memory set size

This is not shown by default. This is the total amount of physical memory (it does not include swap space) in kilobytes being used by the process. If shared libraries are used by the program, that is added into the total.
Resident: the resident memory size.

On BSD and Solaris systems, this field has the same value as the RSS field. See the previous entry for what that means. On Linux, it is the size of the process including what has been swapped out. It is displayed in kilobytes.
Share: shared memory

This field is not shown by default. It is the amount of shared memory used by the process, shown in kilobytes. It includes memory, but not swap space. Shared memory is memory which more than one program is using, saving space.
Stat: the state of the process

This is what the process is doing currently. The symbols you will see here are:
- R = running;
- S = sleeping;
- D = uninterruptible sleep. The process is asleep and waiting for a wake-up call.
- Z = zombie. A zombie process is dead and waiting for its parent process to clean it up;
- T = stopped or traced. Tracing a process means looking at a list of output for every single little thing it does.
Sometimes a second symbol is visible:
- W = the entirety of the program (not just part of it) has been swapped to disk; or the process is currently in the middle of swapping.
- N = niced (positive value), so having less CPU time than normal. See Pri and