Understanding Linux Memory: Read free Output and Manage Buffers/Cache

1. Viewing Linux Memory with the free Command

Total : total physical memory size.

Used : memory currently in use (including cached, buffers, and shared portions).

Free : amount of idle memory.

Shared : memory shared between processes (generally can be ignored).

Buffers : data that has been written to memory and is cached for quick response before being flushed to disk.

Cached : memory occupied by cached read data to speed up subsequent accesses.

-/+ buffers/cache is considered as two parts:

-buffers/cache : memory actively used (note this is not the same as the used column because buffers and cache can be reclaimed); value = used - buffers - cached.

+buffers/cache : memory available for use (also not the same as the free column); value = free + buffers + cached.

Swap: the amount of swap space used on disk.

The design purpose is that when the +buffers/cache memory is exhausted, new read/write requests trigger the kernel to write some memory data to disk, using disk space as virtual memory.

2. Buffer and Cache Overview

Cache is designed to accelerate data exchange between CPU and memory, while Buffer improves data exchange between memory and storage devices.

Cache mainly targets read operations. Modern CPUs have multiple levels of cache (L1, L2, L3) to store frequently used instructions and data, reducing latency. In Linux, the cached column represents data cached in RAM for faster future reads.

Buffer focuses on write operations, aggregating writes between memory and disk to reduce fragmentation and improve performance. The kernel periodically flushes buffers to disk, and the sync command can trigger this manually.

3. Common Symptoms

Symptom 1: Frequent file access causes physical memory to be quickly consumed while the cached value keeps growing.

Explanation: Linux caches data that has been read to speed up subsequent accesses, leveraging the CPU's faster processing speed compared to memory.

Symptom 2: Swap space becomes occupied.

Explanation: When physical memory is insufficient, the system starts using swap; monitoring swap usage can indicate memory pressure.

4. Manually Clearing Swap and Buffers/Cache

(1) Clear Swap swapoff -a && swapon -a Explanation: If swap is in use and there is enough free memory (+buffers/cache), executing swapoff -a moves swap contents back to RAM without data loss.

(2) Clear Buffers/Cache

sync; sync; sync && echo 3 > /proc/sys/vm/drop_caches

sleep 2

echo 0 > /proc/sys/vm/drop_caches

Explanation:

sync          -> flushes dirty pages to disk
echo 3 > /proc/sys/vm/drop_caches -> tells the kernel to drop pagecache, dentries and inodes
sleep 2       -> pause to ensure the previous command completes
echo 0 > /proc/sys/vm/drop_caches -> restores the default setting

5. Summary

When free physical memory is low, it does not necessarily mean poor system health because buffers and cache can be reclaimed on demand, effectively acting as additional free memory.

Frequent swap usage, or non‑zero values for buffers and cache over extended periods, indicates genuine memory pressure; focusing on the -/+ buffers/cache line from free provides a clearer picture of usable memory.