Swappiness in Linux governs how aggressively the kernel decides to move inactive pages from RAM to disk-based swap space. This single integer value, ranging from 0 to 100, sits at the heart of memory management and directly impacts system responsiveness, disk longevity, and performance consistency. Understanding its mechanics is essential for administrators tuning servers or developers optimizing applications.
How the Linux Kernel Uses Swappiness
The Linux kernel constantly evaluates which pages to reclaim when free memory becomes tight. A lower swappiness value, such as 10, makes the kernel reluctant to swap, preferring to keep data in RAM as long as possible. Conversely, a higher value, like 60 or 100, encourages the kernel to push idle processes and clean buffers to disk sooner, freeing up RAM for active workloads. This behavior is not a simple on/off switch but a nuanced gradient that balances memory utilization against the latency of disk I/O.
Interaction with the Page Cache
It is crucial to distinguish swapping from the page cache, where the kernel stores recently read disk data in RAM. Even with a swappiness of zero, the kernel can still evict clean page cache entries if applications demand more memory. The setting specifically influences the trade-off between keeping anonymous memory (such as heap and stack data) in RAM versus writing it to swap. Therefore, tuning swappiness primarily affects how the kernel handles process memory rather than file-backed cache.
Default Values and Common Recommendations
Most desktop distributions default to a swappiness of 60, while many server-oriented distributions set it to 10 to prioritize keeping applications in memory. These defaults represent a compromise, but they may not suit every workload. For example, a database server with ample RAM and fast SSDs might perform better with a very low value to prevent unnecessary write amplification. On the other hand, a machine running multiple memory-hungry applications might benefit from a higher setting to reduce the risk of outright memory exhaustion and OOM (Out-Of-Memory) terminations.
Viewing and Changing Swappiness
To inspect the current setting, you can read the /proc/sys/vm/swappiness file using standard command-line tools. The sysctl command provides a more structured way to query kernel parameters, presenting the value in a clean format. Modifying the setting temporarily requires writing a new value to this file or using sysctl with the -w flag, which takes effect immediately but does not survive a reboot. For persistence across restarts, the value must be added to the /etc/sysctl.conf file or a dedicated configuration file within /etc/sysctl.d/ .
Commands to Manage Swappiness
View current value: cat /proc/sys/vm/swappiness
