Admin at 09:26, 17 January 2026

2026-01-17T09:26:27Z

← Older revision		Revision as of 09:26, 17 January 2026
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	* available: estimated memory available for new applications without swapping		* available: estimated memory available for new applications without swapping

	=== Available Memory Heuristic ===		== Available Memory Heuristic : Output Modes and Formats ==
	The available field is calculated by the kernel to reflect reclaimable memory, including page cache and slabs.		The available field is calculated by the kernel to reflect reclaimable memory, including page cache and slabs. This value is more meaningful than free for capacity planning.

	~~<nowiki>~~
	~~free -h</nowiki>~~

	This value is more meaningful than free for capacity planning.

	~~== Output Modes and Formats ==~~

	=== Human Readable Output ===		=== Human Readable Output ===

Admin: Created page with "Category:Wiki '''''[https://it-arts.net/index.php/Category:Wiki Return to Wiki Index]''''' == Memory Accounting Model == === Key components === * total: total installed RAM * used: memory actively used by processes or the kernel * free: completely unused memory pages * shared: memory used by tmpfs and shared memory * buff/cache: memory used by kernel buffers and page cache * available: estimated memory available for new applications without swapping === Available..."

2026-01-17T09:24:43Z

Created page with "Category:Wiki '''''[https://it-arts.net/index.php/Category:Wiki Return to Wiki Index]''''' == Memory Accounting Model == === Key components === * total: total installed RAM * used: memory actively used by processes or the kernel * free: completely unused memory pages * shared: memory used by tmpfs and shared memory * buff/cache: memory used by kernel buffers and page cache * available: estimated memory available for new applications without swapping === Available..."

New page

[[Category:Wiki]]

'''''[https://it-arts.net/index.php/Category:Wiki Return to Wiki Index]'''''

== Memory Accounting Model ==

=== Key components ===
* total: total installed RAM
* used: memory actively used by processes or the kernel
* free: completely unused memory pages
* shared: memory used by tmpfs and shared memory
* buff/cache: memory used by kernel buffers and page cache
* available: estimated memory available for new applications without swapping

=== Available Memory Heuristic ===
The available field is calculated by the kernel to reflect reclaimable memory, including page cache and slabs.

<nowiki>
free -h</nowiki>

This value is more meaningful than free for capacity planning.

== Output Modes and Formats ==

=== Human Readable Output ===
<nowiki>
free -h</nowiki>

Automatically scales values using powers of 1024.

=== Byte-Level Precision ===
<nowiki>
free -b</nowiki>

Used for scripting and exact measurements.

=== SI Units ===
<nowiki>
free -H</nowiki>

Uses powers of 1000 instead of 1024.

== Time-Based Monitoring ==

=== Continuous Sampling ===
<nowiki>
free -s 2</nowiki>

Refreshes output every two seconds.

=== Limited Iterations ===
<nowiki>
free -s 1 -c 5</nowiki>

Useful for observing memory pressure spikes.

== Swap Memory Analysis ==

=== Swap Utilization ===
<nowiki>
free -h --swap</nowiki>

Displays swap usage alongside RAM.

Key indicators:
* High swap usage with low CPU usage may indicate memory leaks
* Active swap with free RAM often indicates kernel tuning issues

=== Swap-Free Systems ===
On systems without swap, free reports zero swap usage but still computes available memory correctly.

== Kernel Buffers and Cache ==

=== Page Cache Behavior ===
Linux aggressively uses free memory for caching filesystem data.

<nowiki>
free -h</nowiki>

High buff/cache values are normal and desirable.

=== Droppable Memory ===
Cached memory can be reclaimed instantly under pressure without performance penalties.

== NUMA Awareness ==

=== Node-Level Reporting ===
On NUMA systems, free reflects aggregated memory only.

For node-specific analysis, free must be correlated with:
<nowiki>
numactl --hardware</nowiki>

== Control Groups and Containers ==

=== cgroup Memory Limits ===
Inside containers, free reports memory visible to the cgroup, not the host.

<nowiki>
free -h</nowiki>

This reflects:
* memory.max
* memory.current
* memory.swap.max

Misinterpretation may occur if host memory is assumed.

== Security Concepts ==

=== Information Disclosure ===
free exposes memory size, swap presence, and pressure indicators.

Mitigations:
* Restrict shell access
* Avoid exposing metrics to untrusted users
* Mask memory info in containers when necessary

=== Side-Channel Considerations ===
Memory pressure patterns may leak workload characteristics in shared environments.

Used in combination with:
* cache timing attacks
* allocation pressure observation

== Performance and Capacity Planning ==

=== Memory Saturation Detection ===
Indicators of saturation:
* available approaching zero
* rising swap usage
* increased page faults

<nowiki>
free -s 1</nowiki>

=== Overcommit Awareness ===
free does not reflect vm.overcommit settings directly and must be correlated with:
* /proc/meminfo
* vmstat

== Common Misinterpretations ==

=== Low Free Memory ===
Low free memory does not indicate a problem.

Correct interpretation relies on available memory.

=== Cache vs Used ===
buff/cache is not wasted memory and should not be forcibly cleared in production systems.

== Troubleshooting ==

=== System Feels Slow but Free Memory Exists ===
Possible causes:
* Swap thrashing
* I/O wait
* CPU contention

Check:
<nowiki>
free -h</nowiki>

Then correlate with:
* vmstat
* iostat

=== OOM Killer Triggered Unexpectedly ===
Possible reasons:
* cgroup memory limits
* overcommit misconfiguration
* memory fragmentation

Validate available memory and swap:
<nowiki>
free -h</nowiki>

=== Memory Usage Differs from top or ps ===
Reason:
* free shows system-wide accounting
* top/ps show process RSS/VSZ

This discrepancy is expected.

== Useful Links ==

* https://man7.org/linux/man-pages/man1/free.1.html
* https://man7.org/linux/man-pages/man5/proc_meminfo.5.html
* https://www.kernel.org/doc/html/latest/admin-guide/mm/index.html
* https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html

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'''''[https://it-arts.net/index.php/Category:Wiki Return to Wiki Index]'''''

FREE - Documentation - Revision history

Admin at 09:26, 17 January 2026