Tops

Htop

Reading htop

System wide cpu usage

  • low priority processes (nice > 0)

  • normal (user) processes

  • kernel processes

  • IRQ time

  • Soft IRQ time

  • IO Wait time

System wide memory usage

  • Used memory pages

  • Buffer pages

  • Cache pages

Load average

  • 100% utilization is 1 CPU fully loaded

  • Load average: for 1/5/15 minutes

  • Note that loads can exceed 1.0 this just means that processes have to wait longer for the cpu.

Information on processes

  • PID: A process’s process ID number.

  • USER: The process’s owner.

  • PR: The process’s priority. The lower the number, the higher the priority.

  • NI: The nice value of the process, which affects its priority.

  • VIRT: How much virtual memory the process is using.

  • RES: How much physical RAM the process is using, measured in kilobytes.

  • SHR: How much shared memory the process is using.

  • S: The current status of the process (zombied, sleeping, running, uninterruptedly sleeping, or traced).

  • %CPU: The percentage of the processor time used by the process.

  • %MEM: The percentage of physical RAM used by the process.

  • TIME+: How much processor time the process has used.

  • COMMAND: The name of the command that started the process

The difference between VIRT, RES and SHR

  • VIRT: virtual size of a process; a sum of memory it is actually using, memory it has mapped into itself (for instance the video card’s RAM for the X server), files on disk that have been mapped into it (most notably shared libraries), and memory shared with other processes. VIRT represents how much memory the program is able to access at the present moment.

  • RES: resident size; actual physical memory a process is consuming. Corresponds directly to the %MEM column

  • SHR indicates how much of the VIRT size is actually sharable memory or libraries. In the case of libraries, it does not necessarily mean that the entire library is resident. For example, if a program only uses a few functions in a library, the whole library is mapped and will be counted in VIRT and SHR, but only the parts of the library file containing the functions being used will actually be loaded in and be counted under RES.

Internals

Top

Iotop

IOWAIT

Shown by top at wa%: percentage of time that all CPUs are waiting for disk accesses before they can do useful work (e.g on 4 cores max wa% is 400.0). Doesn’t tell much on it’s own.

Iotop

Start iotop with:

sudo iotop -oPa
# short for `sudo iotop --only --processes --accumulated`

Iftop

Example:

                                            __ 2s, 10s and 40s
                                           /
                                          /
foo.example.com  =>  bar.example.com      1Kb  500b   100b
                 <=                       2Mb    2Mb    2Mb

  • TX: Transmit (from this server)

  • RX: Receive (to this server)

Netstat

Nethogs