Master Linux Performance Monitoring: Top, Free, and Vmstat Commands Explained

Top Command

The top command is a real‑time Linux performance analysis tool that displays each process's resource usage, similar to Windows Task Manager.

Usage:

top [options]

Key options include:

-b batch mode

-c show full command line

-I ignore idle processes

-s secure mode

-S cumulative mode

-i<interval> set refresh interval

-u<user> filter by user

-p<pid> filter by PID

-n<count> number of iterations

Example – display all processes:

<code>top</code>

Sample output (truncated) shows system time, uptime, load average, task counts, CPU usage percentages, memory statistics, and a list of processes with columns such as PID, USER, PR, NI, VIRT, RES, SHR, S, %CPU, %MEM, TIME+, COMMAND.

Interpretation of the first five lines:

Current time, uptime, number of users, and load average (1‑, 5‑, 15‑minute averages).

Task summary: total, running, sleeping, stopped, zombie processes.

CPU states: user (%us), system (%sy), nice (%ni), idle (%id), I/O wait (%wa), hardware IRQ (%hi), software IRQ (%si), steal (%st).

Memory: total, used, free, buffers, cached.

Swap: total, used, free, cached.

Additional tips:

Press 1 to toggle per‑CPU view.

Press b to highlight the currently running process.

Press x to toggle column highlighting; default sort column is %CPU .

Use Shift+> or Shift+< to change the sort column (e.g., to %MEM).

Free Command

The free command shows the amount of free and used physical memory, swap space, and buffers/cache.

Usage:

free [options]

Important options:

-b display in bytes

-k display in kilobytes (default)

-m display in megabytes

-g display in gigabytes

-o omit the buffer/cache line

-s <seconds> continuously monitor

-t show total memory line

-V show version

Example – show memory in default units:

<code>free</code>

Sample output:

<code>              total       used       free     shared    buffers     cached
Mem:      32940112   30841684    2098428          0    4545340   11363424
-/+ buffers/cache:   14932920   18007192
Swap:     32764556    1944984   30819572</code>

Key fields:

total – total physical memory.

used – memory in use (including kernel buffers and cache).

free – memory not yet allocated.

shared – memory used by multiple processes.

buffers – memory used for block device buffers.

cached – page cache.

From an application perspective, usable memory = free + buffers + cached.

Continuous monitoring example:

<code>free -s 10</code>

Vmstat Command

vmstat (Virtual Memory Statistics) reports information about processes, memory, paging, block I/O, traps, and CPU activity.

Usage examples:

vmstat

– one‑time snapshot.

vmstat 5 5

– sample every 5 seconds, 5 times.

vmstat -a

– show active/inactive memory.

vmstat -s

– display memory statistics in human‑readable form.

vmstat -d

– show disk I/O statistics.

vmstat -m

– show slab allocator info.

vmstat -f

– show number of forks since boot.

Key fields in the default output:

procs : r (running queue), b (blocked for I/O).

memory : swpd, free, buff, cache.

swap : si (swap in), so (swap out).

io : bi (blocks received), bo (blocks sent).

system : in (interrupts), cs (context switches).

cpu : us, sy, id, wa, st.

Interpretation tips:

High r (>4) and low id (<40%) indicate CPU overload.

Non‑zero si / so suggest memory pressure causing swapping.

Frequent disk activity with high b (>3) points to I/O bottlenecks.

Example – display virtual memory usage:

<code>vmstat</code>

Example – show active/inactive memory:

<code>vmstat -a 2 5</code>

Example – count forks since boot:

<code>vmstat -f</code>

Example – detailed memory statistics:

<code>vmstat -s</code>

Example – disk I/O statistics:

<code>vmstat -d</code>

Example – slab allocator details:

<code>vmstat -m</code>

Understanding these commands helps administrators diagnose performance issues, plan capacity, and keep Linux systems running smoothly.