Understanding top Command
This post helps understanding top command on Linux. top is a tool that outputs a list of the currently running processes of the system and the output updates in every 3 seconds by default. It also shows CPU load, currently used and free RAM as well as a lot of other useful information.
This tool is installed by default in most Linux distributions. I have provided an example output below:
top - 12:14:43 up 92 days, 6:28, 1 user, load average: 0.00, 0.10, 0.03 Tasks: 22 total, 1 running, 21 sleeping, 0 stopped, 0 zombie Cpu(s): 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 1048576k total, 48376k used, 1000200k free, 0k buffers Swap: 0k total, 0k used, 0k free, 22128k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 19232 784 536 S 0.0 0.1 0:00.04 init 2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 [kthreadd/324] 3 root 20 0 0 0 0 S 0.0 0.0 0:00.00 [khelper/324] 130 root 16 -4 10644 604 248 S 0.0 0.1 0:00.00 /sbin/udevd -d 554 root 20 0 17372 1284 268 S 0.0 0.1 0:00.03 /sbin/udevd
Before proceeding to explain every field in the output of top command, please read about the interactive commands used in top.
|u||User||Filter a user’s processes|
|r||renice||Set process Nice value|
|d||delay||Set update interval in seconds|
|n||number||Maximum number of displayed processes|
|N||Number||Sort by PID|
|P||Percent||Sort by CPU usage|
|M||Memory||Sort by RAM usage|
Also, pressing the space bar refreshes the screen output.
Understanding top command: Summary Area
1st Row — top
This line shows the time since the system was online. It also shows the load averages of last 1, 5 and 15 minutes.
top - 12:14:43 up 92 days, 6:28, 1 user, load average: 0.00, 0.10, 0.03
Understand each sections in 1st row in order:
- Current time – 12:14:43
- Uptime – 92 days, 6:28
- Currently logged in Users – 1 user
- Average load – load average: 0.00, 0.10, 0.03 (The 3 values refer to the last minute, five minutes and 15 minutes)
2nd Row – task
Displays the number of processes active at the time of the last update. It also shows how many processes are running, sleeping, stopped or undead
Tasks: 22 total, 1 running, 21 sleeping, 0 stopped, 0 zombie
Understand each sections in 2nd row in order:
- Total Processes – 22 total
- Running Processes – 1 running
- Sleeping Processes – 21 sleeping
- Stopped Processes – 0 stopped
- Zombie Processes – 0 zombie
Zombie Processes: Zombie process also known as defunct process is a process that finished the execution but it still has an entry in the process table to report to its parent process.
3rd Row – Cpu(s)
This section displays the CPU utilization in percent.
Cpu(s): 0.0%us, 0.0%sy, 0.0%ni,100.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Understand each sections in 3rd row in order:
- Percentage of CPU for user processes – 0.0%us
- Percentage of CPU for system processes – 0.0%sy
- Percentage of CPU processes that have no default priority (nice) – 0.0%ni
- Percentage of CPU not used – 100.0%id
- Percentage of CPU processes waiting for I/O operations – 0.0%wa
- Percentage of CPU serving hardware interrupts – 0.0% hi – Hardware IRQ
- Percentage of CPU serving software interrupts – 0.0% si – Software Interrupts
- Percentage of Steel Time – 0.0%st – Steal Time
Steal time is the percentage of time a virtual CPU waits for a real CPU while the hypervisor is servicing another virtual processor.
4th and 5th Row – Memory usage
This section shows the statistics on the utilization of the main memory and also displays the swap utilization.
Mem: 1048576k total, 48376k used, 1000200k free, 0k buffers Swap: 0k total, 0k used, 0k free, 22128k cached
The above rows shows the usage of RAM and swap in the order Total memory, Memory in use, Free memory and buffers cached.
All Other rows: Processes list
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 19232 784 536 S 0.0 0.1 0:00.04 init 2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 [kthreadd/324] 3 root 20 0 0 0 0 S 0.0 0.0 0:00.00 [khelper/324] 130 root 16 -4 10644 604 248 S 0.0 0.1 0:00.00 /sbin/udevd -d
- PID – This is the ID of a process.
- USER – The owner of the process
- PR – priority of the process
- NI – NICE value of the process
- VIRT – Virtual memory used by process
- RES – Physical memory used by a process
- SHR – Shared memory of the process
- S – Status of the process. The Different status’ are given below:
S - sleeping D - uninterruptible sleep R - running W - swapped out process Z - zombies T - stopped or traced N - process with positive nice value < - process with negative nice value
- %CPU – Percentage of CPU used a process
- %MEM – Percentage of RAM used a process
- TIME+ – The total time of activity of process (0:17.75)
- COMMAND – It shows the exact command or name used by a process.
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