Linux Virtual Memory, Paging, Swap & The Out-of-Memory (OOM) Killer

Lab 06 of 20 · Memory & OOM · 5 hands-on activities · bilingual (English + සිංහල)

පිටුව 06: Virtual Memory, Paging, Swap සහ Out-of-Memory (OOM) Killer මෙහෙයුම

RAM, Swap, Virtual Memory, 4KB Paging, Buffers/Cache කියන්නේ මොනවද? RAM එක පිරුණම Linux Kernel එකේ OOM Killer ක්‍රියාත්මක වන හැටි.

Architectural Foundation & Systems Diagram

Memory management is one of the most critical responsibilities of the Linux kernel. Applications never access physical RAM chips directly; they work in a Virtual Address Space divided into fixed-size Pages (typically 4 KB).

• 🧠 RAM vs Swap Space — RAM is high-speed physical memory. Swap is emergency overflow space on disk. When RAM is exhausted, inactive memory pages are paged out to swap to keep the server alive.

• 📦 Buffers & Page Cache — Linux aggressively uses unused RAM to cache recently read disk files. If an application requests memory, the kernel instantly frees cache. "Free RAM is wasted RAM!"

• 🎯 The OOM Killer Heuristic — When physical RAM and swap are 100% full, the kernel invokes the Out-Of-Memory (OOM) Killer. It computes an oom_score for every process and terminates the biggest memory hog to rescue the OS!

Step-by-step practical lab

Step 01: List All Active System Processes

සිංහලෙන් — Server එකේ දුවන සියලුම Processes පරීක්ෂා කරමු.

Display a comprehensive snapshot of every process running on the system across all users, with CPU and memory consumption.

ps aux

• a : සියලුම පරිශීලකයින්ගේ processes පෙන්වයි.
• u : User-friendly format එකෙන් CPU% සහ RAM% පෙන්වයි.
• x : Terminal එකකට සම්බන්ධ නොවූ background daemons ද ඇතුළත් කරයි.

Expected output

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1  22584 11420 ?        Ss   Sep14   0:18 /sbin/init
nginx     1042  1.3  0.2 145280 12480 ?        S    Sep14   4:12 nginx: worker process

Step 02: Gracefully Terminate a Process via SIGTERM

සිංහලෙන් — Process එකක් ක්‍රමවත්ව නවතා දැමීමට Signal 15 (SIGTERM) යවමු.

Send signal 15 (SIGTERM) requesting the target process to flush its memory buffers, close file handles, and cleanly exit.

kill -15 1042

• -15 : SIGTERM: Application එකට දත්ත save කර ක්‍රමවත්ව exit වීමට ඉඩ දෙයි.

Expected output

Signal SIGTERM (15) sent to PID 1042.

Hands-on practice activities

Work through these yourself — tick them off as you go.

• Activity 26: Inspect Physical RAM & Swap Allocation (Drill) — Display total, used, free, shared, buff/cache, and available memory in megabytes.
• Activity 27: Read Kernel Memory Telemetry (Drill) — Query detailed virtual memory statistics directly from kernel instrumentation.
• Activity 28: Search Kernel Ring Buffer for OOM Kill Events (Drill) — Search dmesg kernel logs for Out of Memory killer process terminations.
• Activity 29: Differentiate 'Free' vs 'Available' Memory (Checkpoint) — Never alert on low 'Free' memory! Always alert on low 'Available' memory, which accounts for reclaimable page cache.
• Activity 30: Linux Memory Page Size Quiz (Quiz) — What is the standard architecture page size for Linux on x86_64 systems? Answer: 4 KB (4096 bytes).

Keep going: ← Lab 05 · Lab 07 → · Track your progress in the Self Guide