Unlike Windows, which offers various "one-click optimization" utilities to free up memory, Linux provides powerful command-line tools for analyzing memory consumption. This article explores three essential commands for checking memory usage: free, top, and vmstat.

The free Command

The free command displays information about physical and swap memory. The -m flag outputs values in megabytes:

$ free
             total       used       free     shared    buffers     cached
Mem:       1008504     547364     461140      12836      53984     254508
-/+ buffers/cache:     238872     769632
Swap:      1046524          0    1046524

$ free -m
             total       used       free     shared    buffers     cached
Mem:           984        534        450         12         52        248
-/+ buffers/cache:        233        751
Swap:         1021          0       1021

Understanding these columns requires knowing how Linux manages memory:

Column	Meaning
total	Total physical memory available
used	Memory currently allocated (includes buffers and cache)
free	Memory not in use
shared	Shared memory (rarely used in modern systems)
buffers	Memory reserved for disk write operations
cached	Memory holding file data read from disk

The second row (-/+ buffers/cache) is particularly important as it shows actual memory usage. The "used" value here represents memory actively consumed by applications, while "free" indicates truly available memory.

Understanding Buffers vs. Cache

Both buffers and cache serve as memory caches, but they differ in purpose:

• buffers: Stores data pending writes to disk. A buffer is "something that has yet to be written" to disk.
• cache: Holds recently read data for faster access. A cache is "something that has been read" from disk and stored for later use.

From a systems perspective, page cache caches file data while buffer cache caches raw disk blocks. When working with files, data goes to page cache; when using tools like dd directly on block devices, data uses buffer cache.

Understanding Swap

Swap (virtual memory) acts as overflow storage when physical memory is exhausted. When RAM runs low, Linux moves "anonymous memory" (heap and stack allocasions from malloc or new that have no disk backing) to swap space. This allows the system to continue running even when physical memory is fully utilized.

The top Command

The top command provides a real-time view of system processes, similar to Windows Task Manager:

$ uptime
 10:05:01 up 34 min,  1 user,  load average: 0.00, 0.11, 0.16

The first line displays system uptime and load averages. The second line shows process states:

Field	Description
total	Total number of processes
running	Processes currently executing (R state)
sleeping	Processes waiting for events (S state)
stopped	Processes halted (T state)
zombie	Terminated processes not yet reaped (Z state)

The CPU usage breakdown shows:

Field	Description
us	User space CPU usage percentage
sy	Kernel space CPU usage percentage
ni	User processes with altered priority
id	Idle CPU percentage
wa	CPU time waiting for I/O
hi	Hardware interrupt CPU time
si	Software interrupt CPU time
st	Steal time (virtual machines only)

The process table columns include:

Column	Description
PID	Process ID
USER	Process owner
PR	Priority
NI	Nice value (negative = higher priority)
VIRT	Total virtual memory used
RES	Resident (non-swappped) physical memory
SHR	Shared memory
S	Process state
%CPU	CPU usage since last update
%MEM	Physical memory percentage
TIME+	Total CPU time consumed
COMMAND	Command name

Press f within top to customize displayed columns.

The vmstat Command

vmstat (virtual memory statistics) provides compact system performance data:

$ vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0     76 187932 154112 372228    0    0   143    32   36   90  1  1 94  3  0

Column descriptions:

Field	Description
r	Processes in run queue (waiting for CPU)
b	Processes in uninterruptible sleep (blocked)
swpd	Virtual memory used (KB)
free	Idle memory (KB)
buff	Buffer memory (KB)
cache	Cache memory (KB)
si	Swap in (KB/s) - pages swapped from disk
so	Swap out (KB/s) - pages swapped to disk
bi	Blocks received from disk (blocks/s)
bo	Blocks sent to disk (blocks/s)
in	Interrupts per second
cs	Context switches per second
us	User mode CPU percentage
sy	Kernel mode CPU percentage
id	Idle CPU percentage
wa	CPU idle with pending I/O
st	Stolen CPU time (virtual environments)

These three commands provide comprehansive visibility into Linux memory management. Use free for quick memory summaries, top for process-level detail with real-time updates, and vmstat for system-wide performance metrics including I/O and CPU statistics.