Monitoring.

Memory Monitoring in Linux, Understanding calloc(), Thrashing, Thrashing and Working Set Model,

Paging and Segmentation, Memory Management

Memory Monitoring in Linux

Memory monitoring means observing how RAM, swap, cache, and processes use memory in a Linux system.

It helps detect:

Memory leaks

High RAM usage

Thrashing

Swap overuse

Performance bottlenecks

Common Linux Memory Monitoring Tools

Tool	Purpose
free	Overall memory usage
top	Real-time process monitoring
htop	Interactive process monitor
vmstat	Virtual memory statistics
sar	Historical memory stats
ps	Process memory usage
/proc/meminfo	Detailed kernel memory info
pmap	Process memory map
smem	Accurate proportional memory
iotop	Detect swap/thrashing I/O

1. Using free

Command:

free -h

Example output:

              total   used   free   shared  buff/cache  available
Mem:           7.7G   2.1G   1.2G      200M       4.4G       5.0G
Swap:          2.0G   100M   1.9G

Important Columns

Column	Meaning
used	Currently used memory
free	Completely unused RAM
buff/cache	Kernel cache
available	Memory available to apps

2. Using top

Command:

top

Shows:

CPU usage

RAM usage

Running processes

Important fields:

Field	Meaning
%MEM	Process memory percentage
RES	Resident memory
VIRT	Virtual memory
SHR	Shared memory

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📌 3. Using htop

Interactive version of top.

Install:

sudo apt install htop

Run:

htop

Features:

Colorful UI

Process tree

Easy sorting/filtering

4. Using vmstat

Command:

vmstat 1

Updates every second.

Important columns:

Column	Meaning
si	Swap in
so	Swap out
free	Free memory
us	User CPU
id	Idle CPU

Detecting Thrashing

High:

si

so

indicates excessive swapping.

5. /proc/meminfo

Command:

cat /proc/meminfo

Provides detailed memory statistics.

Useful fields:

MemAvailable

Cached

Buffers

SwapFree

6. Monitoring Specific Process Memory

Using ps

ps aux --sort=-%mem | head

Shows top memory-consuming processes.

Using pmap

pmap <PID>

Displays:

Memory map of process

7. Using smem

Better memory accounting.

Install:

sudo apt install smem

Run:

smem

Provides:

USS

PSS

RSS

📌 8. Historical Monitoring with sar

Install:

sudo apt install sysstat

View memory statistics:

sar -r

Useful for:

Performance analysis over time

9. Monitoring Swap Usage

Command:

swapon --show

or:

cat /proc/swaps

10. Detecting Memory Leaks

Signs:

Memory usage continuously increases

System slows over time

Swap usage rises

Tools:

valgrind

top

smem

Example:

valgrind ./program

Important Memory Metrics

Metric	Meaning
RSS	Physical RAM used
VSZ/VIRT	Virtual memory size
PSS	Shared memory proportion
Swap usage	Disk-backed memory
Page faults	Missing pages loaded

Understanding Linux Cache

Linux aggressively uses RAM for caching.

So:

High memory usage is often normal.

Cached memory can be reclaimed when needed.

Warning Signs

Symptom	Possible Cause
High swap activity	Low RAM
System freezes	Thrashing
OOM kills	Memory exhaustion
Constant page faults	Insufficient working set

Example Monitoring Workflow

Check free memory
        ↓
Identify heavy processes
        ↓
Check swap usage
        ↓
Inspect page faults/swapping
        ↓
Find leaks or bottlenecks

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📌 Useful Real-Time Commands

Watch Memory Continuously

watch -n 1 free -h

Top Memory Processes

ps aux --sort=-rss | head

Page Fault Statistics

vmstat -s

Summary

Linux memory monitoring helps:

Optimize performance

Detect memory leaks

Prevent thrashing

Analyze swap activity

Core tools:

free

top

htop

vmstat

/proc/meminfo

sar

smem

are essential for system administration and debugging.