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 favors anonymous memory or the page cache. A high value improves file-system performance, while aggressively swapping less active processes out of physical memory. A low value avoids swapping processes out of memory, which usually decreases latency, at the cost of I/O performance. The default value is 60.

Warning

Since RHEL 6.4, setting swappiness=0 more aggressively avoids swapping out, which increases the risk of OOM killing under strong memory and I/O pressure.

A low swappiness value is recommended for database workloads. For example, for Oracle databases, Red Hat recommends a swappiness value of 10.

vm.swappiness=10

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.

Warning

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

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.

Red Hat recommends a slightly lower value of 15 for database workloads.

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. For database workloads, Red Hat recommends a lower value of 3.

dirty_expire_centisecs

Specifies the number of centiseconds (hundredths of a second) dirty data remains in the page cache before it is eligible to be written back to disk. Red Hat does not recommend tuning this parameter.

dirty_writeback_centisecs

Specifies the length of the interval between kernel flusher threads waking and writing eligible data to disk, in centiseconds (hundredths of a second). Setting this to 0 disables periodic write behavior. Red Hat does not recommend tuning this parameter.

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. For further information, refer to the Deployment Guide, available from http://access.redhat.com/site/documentation/Red_Hat_Enterprise_Linux/.