How to Monitor Linux System Performance: CPU, Memory, Disk I/O, and Network

Linux System Environment Monitoring

Linux system environment mainly monitors CPU, memory, disk I/O and network traffic.

1. CPU

(1) View CPU load: uptime

You can view overall load with uptime.

If the server has 1 CPU core, a 1‑minute load average >=3 indicates high load; for 4 cores, load >=12 is high.

root@ubuntu1804:~# uptime
09:57:53 up 7:17, 2 users, load average: 0.10, 0.03, 0.01
# current time
# users
# load average: system load (average number of processes in the run queue) for 1, 5, 15 minutes

(2) View logical CPU count

ehigh@ubuntu:~$ grep -c 'processor' /proc/cpuinfo
8

CPU: central processing unit.

Slots: number of physical CPUs installed
Cores: number of cores per physical CPU
Logical processors: number of logical CPUs (via hyper‑threading)
Hyper‑threading: technique that allows a single CPU core to appear as two logical processors

CPU development history: initially a physical CPU had a single core; later Intel introduced hyper‑threading; then multi‑core architectures placed multiple cores on one die.

Example: a server with a 4‑core, 8‑thread CPU has 4 cores and supports hyper‑threading, resulting in 8 logical CPUs.

(3) Find processes using most CPU: top

In top, press uppercase P to sort by CPU usage.

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
720 root 20 0 473464 22060 19128 S 1.7 0.2 55.14 sunloginclient
3590 emqx 20 0 3483576 201492 74484 S 1.3 2.0 39.37 beam.smp
984 root 20 0 2754328 50696 5192 S 1.0 0.0 32.96 taosd

A process’s CPU usage can exceed 100%; on a 4‑core CPU it can reach 400%.

How to judge if CPU is busy?

Consider both CPU usage and load average; if the 1‑minute load >3 and CPU usage is high, the CPU is busy.

(4) View CPU usage per core: top

Press 1 in top to display each CPU’s details.

top - 10:48:44 up 5 days, 1:15, 1 user, load average: 0.10, 0.14, 0.10
%Cpu0 : 0.0 us, 3.4 sy, 0.0 ni, 96.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu1 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu2 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu3 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu4 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu5 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu6 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu7 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st

2. Memory

(1) View memory size and usage

root@ubuntu:~# free -h
              total        used        free      shared  buff/cache   available
Mem:           9.7Gi       2.1Gi       5.4Gi        49Mi       2.3Gi       7.3Gi
Swap:          0B          0B          0B

Columns: total, used, free, shared, buff/cache, available. Swap is disk space used as virtual memory.

(2) Find processes using most memory: top

In top, press uppercase M to sort by memory usage; press e to change units.

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1377 mysql 20 0 4.8g 0.8g 0.0g S 0.7 7.8 67:01.24 mysqld
977 root 20 0 7.7g 0.3g 0.0g S 0.0 3.4 32:34.83 java
3590 emqx 20 0 3.3g 0.2g 0.1g S 0.7 2.0 64:48.04 beam.smp
446 root 19 -1 0.3g 0.2g 0.2g S 0.0 2.0 1:59.99 systemd-journ

3. Disk I/O

(1) View processes with most disk read/write: iostat

root@ubuntu1804:~# iostat -dkp
Device    tps  kB_read/s  kB_wrtn/s  kB_read  kB_wrtn
sda       0.64   16.17      58.90    487458   1775844
sda1      0.01    0.20       0.00     5893      96
sda6      0.63   15.79      58.90   476129   1775748

kB_read/s = kilobytes read per second; kB_wrtn/s = kilobytes written per second.

When a server feels sluggish despite low CPU and sufficient memory, the bottleneck is often disk I/O; enterprise SSDs can improve performance.

(2) Find processes with highest disk I/O: iotop

root@ubuntu1804:~# iotop -o
Total DISK READ : 5.79 M/s | Total DISK WRITE : 2.28 M/s
TID PRIO USER DISK READ DISK WRITE SWAPIN IO% COMMAND
17238 be/4 tom 5.79 M/s 0.00 B/s 0.00 % 64.19 % find /
17092 be/4 root 0.00 B/s 0.00 B/s 0.00 % 12.71 % [kworker/u256:0]
476  be/3 root 0.00 B/s 2.28 M/s 0.00 % 5.45 % [jbd2/sda6-8]

4. Network

(1) Monitor overall bandwidth with nload

tom@ubuntu1804:~$ nload -u h
Device eth0 [10.0.0.18]:
Incoming: Curr 558.33 kBit/s Avg 297.12 kBit/s Min 1.39 kBit/s Max 894.77 kBit/s Ttl 58.23 MByte
Outgoing: Curr 16.16 kBit/s Avg 15.37 kBit/s Min 4.52 kBit/s Max 32.62 kBit/s Ttl 766.79 kByte

(2) Find processes using most bandwidth with nethogs

tom@ubuntu1804:~$ sudo nethogs
PID USER PROGRAM DEV SENT RECEIVED
1733 root wget eth0 13.352 4317.425 KB/sec
1605 tom sshd: tom@pts/0 eth0 1.025 0.07013.276 KB/sec
TOTAL 0.000 0.000 KB/sec 14.378 4317.701