5.4. Tuning Virtual Memory

Virtual memory is typically consumed by processes, file system caches, and the kernel. Virtual memory utilization depends on a number of factors, which can be affected by the following parameters:

swappiness

A value from 0 to 100 which controls the degree to which the system swaps. A high value prioritizes system performance, aggressively swapping processes out of physical memory when they are not active. A low value prioritizes interactivity and avoids swapping processes out of physical memory for as long as possible, which decreases response latency. The default value is 60.

min_free_kbytes

The minimum number of kilobytes to keep free across the system. This value is used to compute a watermark value for each low memory zone, which are then assigned a number of reserved free pages proportional to their size.

Extreme values can break your system

Be cautious when setting this parameter, as both too-low and too-high values can be damaging.

Setting min_free_kbytes too low prevents the system from reclaiming memory. This can result in system hangs and OOM-killing multiple processes.

However, setting this parameter to a value that is too high (5-10% of total system memory) will cause your system to become out-of-memory immediately. Linux is designed to use all available RAM to cache file system data. Setting a high min_free_kbytes value results in the system spending too much time reclaiming memory.

dirty_ratio

Defines a percentage value. Writeout of dirty data begins (via pdflush) when dirty data comprises this percentage of total system memory. The default value is 20.

dirty_background_ratio

Defines a percentage value. Writeout of dirty data begins in the background (via pdflush) when dirty data comprises this percentage of total memory. The default value is 10.

drop_caches

Setting this value to 1, 2, or 3 causes the kernel to drop various combinations of page cache and slab cache.

1: The system invalidates and frees all page cache memory.
2: The system frees all unused slab cache memory.
3: The system frees all page cache and slab cache memory.

This is a non-destructive operation. Since dirty objects cannot be freed, running sync before setting this parameter's value is recommended.

Important

Using the drop_caches to free memory is not recommended in a production environment.

To set these values temporarily during tuning, echo the desired value to the appropriate file in the proc file system. For example, to set swappiness temporarily to 50, run:

# echo 50 > /proc/sys/vm/swappiness

To set this value persistently, you will need to use the sysctl command. Refer to The proc File System appendix of the Red Hat Enterprise Linux 6 Deployment Guide for further information.