Users, permissions, and processes / Stage 12 of 13 · step 7 of 9

The server is sluggish

~20 min

Not running Starting usually ~20 s

Situation

In the morning users complain that the site opens slowly. The nightly report export, report-sync, is still running on the server as the www-data account, and it must not be interrupted.

Task

Remove the cause so that:

no stray work keeps the CPU busy;
the load does not come back a few seconds later;
the report-sync export completes as the same process, neither stopped nor started again.

The tools you need are already installed. There is no internet on this machine.

Lab goals

0 of 3

The CPU is free of stray work — not checked yet
The load does not come back — not checked yet
The report-sync export runs on as the same process — not checked yet

Hints

0 of 3 open · they open one at a time

Hint 1

Show the solution after 3 failed checks

Processes · picked for this lab

top — processes and load in real time

Cheat sheet

$_ — insert into the terminal

top a live process list; q to quit
top -o %MEM sorted by memory from the start
top -u student one user's processes
top -p 1234,1240 only these PIDs
top -d 5 refresh every 5 seconds
top -b -n 1 | head -n 15 one plain-text snapshot — for a report
inside: P — by CPU, M — by memory, 1 — per core, c — full
command, k — send a signal, h — help, q — quit

Most settings are easier to change with keys inside top; startup options are for scripts and snapshots. Full list — man top.

Find what is loading the CPU

Open top, press P (sort by CPU) and c (full command line). The top row of the table is the main consumer. %CPU over 100 means the process is using several cores.

Find what ate the memory

Press M to sort by %MEM. Look at RES, not VIRT: VIRT includes memory that is reserved but not used. What to do next — memory-swap.

Stop a process right from top

Press k, enter the PID (the top process is offered by default) and a signal: Enter sends 15 (SIGTERM). More on signals — kill.

A snapshot for a report or ticket

bash

top -b -n 1 -o %MEM | head -n 15 > top.txt

-b (batch) prints plain text without redrawing the screen; -n 1 takes one snapshot.

htop

htop is the same idea with color, mouse and scrolling. If it's installed, the keys are labeled at the bottom: F6 — sort, F9 — signal, F10 — quit.

Load average is high, but the CPU is nearly idle

The queue holds processes in state D waiting on the disk. Check wa in the %Cpu(s) line and processes with S = D. Step-by-step analysis — system-monitoring.

free is almost zero — out of memory?

No, if buff/cache and avail Mem are large: Linux keeps spare memory as file cache and hands it to programs on demand. The warning signs are a small avail Mem and a growing used in Swap.

A process shows 150 %CPU

The percentage is per core: a multithreaded program on several cores goes over 100. The 1 key shows each core's load separately.

How to quit top

q. If top is waiting for input (after k, o or u), press Esc first.

Garbage and terminal codes when piping

Without -b, top draws the screen with control codes. For > file and | grep always use top -b -n 1.