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Chapter 5. Memory Usage and Page Cache
5.1. Checking the Memory Usage
To determine the size and usage of memory, you can enter the following command:
grep MemTotal /proc/meminfo
You can find a detailed description of the entries in
/proc/meminfo at http://www.redhat.com/advice/tips/meminfo.html.
Alternatively, you can use the
free(1) command to check the memory:
$ free
total used free shared buffers cached
Mem: 4040360 4012200 28160 0 176628 3571348
-/+ buffers/cache: 264224 3776136
Swap: 4200956 12184 4188772
$
In this example the total amount of available memory is 4040360 KB. 264224 KB are used by processes and 3776136 KB are free for other applications. Do not get confused by the first line which shows that 28160KB are free! If you look at the usage figures you can see that most of the memory use is for buffers and cache. Linux always tries to use RAM to speed up disk operations by using available memory for buffers (file system metadata) and cache (pages with actual contents of files or block devices). This helps the system to run faster because disk information is already in memory which saves I/O operations. If space is needed by programs or applications like Oracle, then Linux will free up the buffers and cache to yield memory for the applications. If your system runs for a while you will usually see a small number under the field "
free" on the first line.
