Top Command Explanation

                                                         Top Command Explanation 

As a Linux system admin top command is a frequently used command to view resource utilization (Memory and CPU) by processes on server. This command helps us to find which process is utilizing what resources of system and nail down the process which is hogging all memory or churning CPU.

Although there are much better and user friendly tools then top like htop but i would consider this to be used as a first level command and later use more specific commands to further drill down to RC the issue . Below post i am writing on how to use and read results of top command.

Reading Linux Top Command Output:

When we execute top command on linux, it shows a lot of results, here i am trying to show you to how to read it row by row.

Result Row #1:

Row 1 results shows about server up time from last reboot, currently logged in users and cpu load on server. The same output you can find using linux uptime command.

top - 21:56:08 up 62 days,  6:38,  3 users,  load average: 0.08, 0.04, 0.00

Result Row #2:

Row 2 shows the number of process running on server and there state.

Tasks: 187 total,   1 running, 186 sleeping,   0 stopped,   0 zombie

Zombie process is a process that has completed execution but still has an entry in the process table. This entry is still needed to allow the parent process to read its child’s exit status. Zombies are basically the leftover bits of dead processes that haven’t been cleaned up properly. A program that creates zombie processes isn’t programmed properly – programs aren’t supposed to let zombie processes stick around.

Result Row #3:

Row three shows the cpu utilization status on server, you can find here how much cpu is free and how much is utilizing by system.

Cpu(s):  0.3%us,  0.5%sy,  0.0%ni, 98.8%id,  0.3%wa,  0.0%hi,  0.0%si,  0.0%st

0.3% us : %CPU used by User processes

0.5%sy : %CPU used by System Processes

0.0%ni : %CPU used by setting Nice value
98.8%id : %CPU in Idle state
0.3%wa : %CPU Waiting on I/O
0.0%hi / 0.0%si  :  %CPU used by Hardware/Software Interrupts 
0.0%st : Steal time is the time that a virtual CPU waits for a real CPU while the hypervisor is servicing another virtual processor.

Result Row #4:

Row 4 shows the memory utilization on server, you can find here how much memory is used, the same results you can find using free command.

            Total Memory    Used Memory      Free Memory    Buffered Memory

Mem:   8193720k total,  6317764k used,   1875956k free,   493816k buffers

Result Row #5:

Row 5 shows the swap memory utilization on server, you can find here how much swap is being used, the same results you can find using free command ( last line ). Usually if you system is swapping / using swap memory it indicated system is under memory pressure and will most likely cause system to run very slow

            Total Swap Mem   Used Swap      Free Swap        Cached Swap

Swap:  9215992k total,      3920k used,  9212072k free,  1742556k cached

Result Row #6 ( Running Processes ):

In this steps you will see all running process on servers and details about each process as below.

PID USER      PR  NI  VIRT   RES  SHR S %CPU %MEM     TIME+   COMMAND                                                                                                             4706 mapr        10 -10  2706m   2.4g  17m  S    2.0       30.7      641:03.31    mfs    

PID - Process ID of process           USER - User who's running the process

PR - Priority of process                 NI - Nice value of process

VIRT - Virtual Memory used by process    RES - Physical Memory used by process ( Actual Memory )

SHR - Shared Memory used by process     S -  Current status of the process

%CPU / %MEM - % CPU/MEM used by the process

TIME+   -  Total time process is been running for.         COMMAND - Name of process

By default TOP sorts the output by %CPU usage (k), If you would want to sort output on basis of any fields, you can use SHIFT+F and select appropriate Alphabet to sort field via field as below and press Enter.

 a: PID        = Process Id

  b: PPID       = Parent Process Pid

  c: RUSER      = Real user name

  d: UID        = User Id

  e: USER       = User Name

  f: GROUP      = Group Name

  g: TTY        = Controlling Tty

  h: PR         = Priority

  i: NI         = Nice value

  j: P          = Last used cpu (SMP)

* K: %CPU       = CPU usage

  l: TIME       = CPU Time

  m: TIME+      = CPU Time, hundredths

  n: %MEM       = Memory usage (RES)

  o: VIRT       = Virtual Image (kb)

  p: SWAP       = Swapped size (kb)

  q: RES        = Resident size (kb)

  r: CODE       = Code size (kb)

  s: DATA       = Data+Stack size (kb)

  t: SHR        = Shared Mem size (kb)

  u: nFLT       = Page Fault count

  v: nDRT       = Dirty Pages count

  w: S          = Process Status

  x: COMMAND    = Command name/line

  y: WCHAN      = Sleeping in Function

 z: Flags      = Task Flags <sched.h>

Note :-  When top is running and you press 1 it will show you per CPU (different for each CPU) resource utilization .