Linux Memory Management

Also Known As

TL;DR

Explanation

Linux uses virtual memory — each process sees its own address space. The page cache uses free RAM to cache disk reads (cat /proc/meminfo shows Cached). Swap extends RAM to disk — much slower, causes performance degradation. OOM (Out of Memory) killer: when RAM + swap is exhausted, the kernel kills processes to free memory — typically the largest consumer. PHP memory: each PHP-FPM worker uses 20-100MB depending on app; pm.max_children * typical_worker_memory must not exceed available RAM. Memory pressure indicators: free -h, vmstat, /proc/meminfo. PHP memory limit: memory_limit in php.ini controls per-request limit.

Common Misconception

Why It Matters

Common Mistakes

• pm.max_children calculated without accounting for page cache — leave 20% RAM for cache.
• No swap configured — without swap, OOM killer fires immediately when RAM is exhausted.
• memory_limit = -1 (unlimited) in PHP — uncontrolled memory leaks exhaust server RAM.
• Not monitoring RSS (resident set size) per PHP worker — workers grow over time with memory leaks.

Code Examples

; php-fpm.conf — too many workers for available RAM:
pm = static
pm.max_children = 100    ; 100 * 50MB = 5GB needed
; Server has 4GB RAM
; At full load: OOM killer fires, workers killed mid-request
; 500 errors, partial database writes, data corruption

; Calculate safely:
; Available RAM: 4GB - 500MB OS - 500MB MySQL = 3GB for PHP
; Average worker memory: 50MB
; Max children: 3000MB / 50MB = 60 workers
; Leave headroom: use 48
pm = dynamic
pm.max_children = 48
pm.start_servers = 10
pm.min_spare_servers = 5
pm.max_spare_servers = 15

# Monitor OOM kills:
dmesg | grep -i oom
grep -i oom /var/log/syslog

References

• ↗ https://www.kernel.org/doc/Documentation/filesystems/proc.txt

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