Linux tune the VM subsystem.

Tuning the memory subsystem in Linux is a powerful but delicate task. The right settings can boost your system’s performance, but incorrect changes may cause instability or slowdowns. Always adjust one parameter at a time and monitor your system before making further changes.

Exploring /proc/sys/vm

The /proc/sys/vm directory contains files that represent kernel parameters for the virtual memory subsystem. You can read and write to these files to tune system behavior.

To view the files, use:
cd /proc/sys/vm
ls -l

Sample output:
-rw-r--r-- 1 root root 0 Oct 16 04:21 block_dump
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_background_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_expire_centisecs
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_ratio
-rw-r--r-- 1 root root 0 Oct 16 04:21 dirty_writeback_centisecs
-rw-r--r-- 1 root root 0 Oct 16 04:21 drop_caches
-rw-r--r-- 1 root root 0 Oct 16 04:21 swappiness
-rw-r--r-- 1 root root 0 Oct 16 04:21 vfs_cache_pressure
...

Key Parameters and Their Effects

1. dirty_background_ratio
   Purpose: Sets the percentage of system memory filled with “dirty” pages (pages to be written to disk) before the background writeback daemon (pdflush) starts writing them out.

Check current value:
sysctl vm.dirty_background_ratio

Default example:
vm.dirty_background_ratio = 10

Tuning:
Increasing this value (for example, to 20) means less frequent flushes, which may benefit systems with fast disks but can cause larger flushes at once.
sysctl -w vm.dirty_background_ratio=20

2. swappiness
   Purpose: Controls how aggressively the kernel swaps memory pages to disk.

Check current value:
sysctl vm.swappiness

Default example:
vm.swappiness = 60

Tuning:
Lower values reduce swapping (good for desktops), higher values increase swapping (can benefit workloads with long-sleeping processes).
sysctl -w vm.swappiness=100

3. dirty_ratio
   Purpose: Sets the percentage of system memory that can be filled with dirty pages before processes generating writes must themselves start writing data to disk.

Check current value:
sysctl vm.dirty_ratio

Default example:
vm.dirty_ratio = 40

Tuning:
Lowering this value (for example, to 25) causes data to be written to disk more frequently, reducing the risk of large data loss but possibly impacting performance.
sysctl -w vm.dirty_ratio=25

Best Practices for VM Tuning

• Change one setting at a time.

• Monitor system performance after each change using tools like vmstat, top, or free.

• If performance improves, keep the new setting. If not, revert to the previous value.

• Document your changes for future reference and troubleshooting.