Linux - the top command

linux_top.md

Linux - top

• Monitor the system performance live over time.

First Line of the top Output

• 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

load average

• Determines how busy the system is.
  • 1.00 per CPU: a fully subscribed, but not overloaded.
  • above 1.00 per CPU: processes are competing for CPU time.
• Note: 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 number of processes:
  • total
  • running
  • sleeping
  • stopped
  • zombie
• 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

• 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

• memory usage:
  • Physical memory (RAM)
  • Swap space

swap space

• Temporary storage space on the hard drive
• Once the physical memory is exhausted, the system starts using swap space as an extended memory pool
• Note: since accessing disk is much slower than accessing memory, this will negatively affect system performance.
• Note: 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

• Information about a process
• By default, processes are ordered by highest CPU usage.
  • 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.
• e.g., 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.

command	output
t	Display or hide summary information (rows 2 and 3)
m	Display or hide memory information (rows 4 and 5)
A	Sort the process list by top resource consumers
r	Renice (change the priority of) a specific processes
k	Kill a specific process
f	Enter the top configuration screen
o	Interactively select a new sort order in the process list