Source: LinuxFoundationX: LFS101x.2 Introduction to Linux
Other tools: w, uptime
http://alfredoroca.github.io/linux/monitoring/tools/system/load_average/2015/09/15/Monitoring-tools/
First Line of the top Output
The first line of the top output displays a quick summary of what is happening in the system including:
- How long the system has been up
- How many users are logged on
- What is the load average
The load average determines how busy the system is. A load average of 1.00 per CPU indicates a fully subscribed, but not overloaded, system. If the load average goes above this value, it indicates that processes are competing for CPU time. If the load average is very high, it might indicate that the system is having a problem, such as a runaway process (a process in a non-responding state).
Second Line of the top Output
The second line of the top output displays the total number of processes, the number of running, sleeping, stopped and zombie processes. Comparing the number of running processes with the load average helps determine if the system has reached its capacity or perhaps a particular user is running too many processes. The stopped processes should be examined to see if everything is running correctly.
Third Line of the top Output
The third line of the top output indicates how the CPU time is being divided between the users (us) and the kernel (sy) by displaying the percentage of CPU time used for each.
The percentage of user jobs running at a lower priority (niceness - ni) is then listed. Idle mode (id) should be low if the load average is high, and vice versa. The percentage of jobs waiting (wa) for I/O is listed. Interrupts include the percentage of hardware (hi) vs. software interrupts (si). Steal time (st) is generally used with virtual machines, which has some of its idle CPU time taken for other uses.
Fourth and Fifth Lines of the top Output
The fourth and fifth lines of the top output indicate memory usage, which is divided in two categories:
- Physical memory (RAM) – displayed on line 4.
- Swap space – displayed on line 5.
Both categories display total memory, used memory, and free space.
You need to monitor memory usage very carefully to ensure good system performance. Once the physical memory is exhausted, the system starts using swap space (temporary storage space on the hard drive) as an extended memory pool, and since accessing disk is much slower than accessing memory, this will negatively affect system performance.
If the system starts using swap often, you can add more swap space. However, adding more physical memory should also be considered.
Process List of the top Output
Each line in the process list of the top output displays information about a process. By default, processes are ordered by highest CPU usage. The following information about each process is displayed:
- Process Identification Number (PID)
- Process owner (USER)
- Priority (PR) and nice values (NI)
- Virtual (VIRT), physical (RES), and shared memory (SHR)
- Status (S)
- Percentage of CPU (%CPU) and memory (%MEM) used
- Execution time (TIME+)
- Command (COMMAND)
Interactive Keys with top
Besides reporting information, top can be utilized interactively for monitoring and controlling processes. While top is running in a terminal window you can enter single-letter commands to change its behaviour. For example, you can view the top-ranked processes based on CPU or memory usage. If needed, you can alter the priorities of running processes or you can stop/kill a process.
The table lists what happens when pressing various keys when running top:
Command Output
A Sort the process list by top resource consumers
r Renice (change the priority of) a specific processes
k Kill a specific process
t Display or hide summary information (rows 2 and 3)
m Display or hide memory information (rows 4 and 5)
f Enter the top configuration screen
o Interactively select a new sort order in the process list