Master Linux Memory: Understanding Swap, Buffers, and How to Free RAM

Linux memory management involves physical RAM, virtual memory (swap), and caching mechanisms (buffers and cache) to optimize performance and handle limited physical memory.

1. What is Linux's memory mechanism?

Physical memory is the actual RAM provided by hardware, while virtual memory extends it using disk space called swap. When RAM is insufficient, the kernel moves rarely used pages to swap, freeing RAM for active processes. Linux uses a paging system and a "recently used" algorithm to decide which pages to swap.

Buffers store metadata of block devices (e.g., directory entries, permissions) and dentries/inodes, whereas cache holds copies of file contents that have been read, speeding up subsequent accesses.

2. When does Linux start using virtual memory (swap)?

The swappiness parameter controls swap usage. A value of 60 means swap is used when free RAM falls below 40% of total memory. Setting it to 0 minimizes swap usage, while 100 makes the system aggressively use swap.

cat /proc/sys/vm/swappiness
60

Typical recommendation: set swap size to twice the RAM if RAM ≤ 4 GB; otherwise, swap should be larger than RAM. Lowering swappiness generally improves performance.

Temporarily change swappiness:

sysctl vm.swappiness=10
vm.swappiness = 10
cat /proc/sys/vm/swappiness
10

Permanently change swappiness:

vim /etc/sysctl.conf   # add line: vm.swappiness = 35
sysctl -p /etc/sysctl.conf
cat /proc/sys/vm/swappiness   # should show 35

3. How to free memory?

Linux does not automatically free cached memory. The /proc/sys/vm/drop_caches file controls cache release:

0 – Do not free

1 – Free page cache

2 – Free dentries and inodes

3 – Free all caches

Example command (run as root):

echo 3 > /proc/sys/vm/drop_caches

After executing, you should see a noticeable increase in free memory.

4. How to free swap?

Ensure that free RAM is at least equal to the amount of swap you plan to release; otherwise the system may crash. To release swap, deactivate the swap partition and reactivate it:

swapoff -a
swapon -a

Or manually unmount and remount a specific swap device (e.g., /dev/sda5):

swapoff /dev/sda5
swapon /dev/sda5

Properly managing memory and swap helps maintain system stability and performance.