Page Cache is a disk cache which holds data of files and executable programs, for example pages with actual contents of files or block devices. Page Cache (disk cache) is used to reduce the number of disk reads. To control the percentage of total memory used for page cache in Red Hat Enterprise Linux 3, the following kernel parameter can be changed:

# cat /proc/sys/vm/pagecache
1       15      30

The above three values are usually good for database systems. It is not recommended to set the third value very high, around 100, as it used to be with older Red Hat Enterprise Linux 3 kernels. This can cause significant performance problems for database systems. If you upgrade to a newer kernel like 2.4.21-37, then these values will automatically change to "1 15 30" unless its set to different values in /etc/sysctl.conf. For information on tuning the pagecache kernel parameter, see Understanding Virtual Memory. Note this kernel parameter does not exist in Red Hat Enterprise Linux 4.

The pagecache parameters can be changed in the proc file system without reboot:

# echo "1 15 30" > /proc/sys/vm/pagecache

Alternatively, you can use sysctl(8) to change it:

# sysctl -w vm.pagecache="1 15 30"

To make the change permanent, add the following line to the file /etc/sysctl.conf. This file is used during the boot process.

# echo "vm.pagecache=1 15 30" >> /etc/sysctl.conf

With Red Hat Enterprise Linux 4 and 5, the pagecache is dynamically adjusted. You can adjust the minimum free pages using the following command:

# echo 1024 > /proc/sys/vm/min_free_kbytes

To make the change permanent, add the following line to the file /etc/sysctl.conf:

# echo vm.min_free_kbytes=1024 >> /etc/sysctl.conf

The pagecache pages can be reclaimed by adjusting the swappiness percentage as described in the next section.

 

CentOS 6.X

强制回收缓存脚本: