Linux – CentOS only detecting 50% of ram

The cache memory in Linux-based machines is often used to cache disk blocks into memory to prevent writing to the harddisk and speed up I/O operations. Also, a lot of free memory is cached and released when it is actually required.

When interpreting the output of free, you should look at the second row for the actual memory usage.

The above numbers can be interpreted as:

• 9534176 kilobytes cached memory
• 737752 actually used memory
• 15701956 actually free memory

For more information, take a look at http://www.faqs.org/docs/linux_admin/buffer-cache.html.

The problem is that a lot of the kernel data structures such as the page descriptors (one struct for every 4KB page in the system) need to be in low memory. So as the total memory in the machine goes up, more and more low memory is also needed, and eventually low memory becomes a very scarce resource.

IIRC the usual rule of thumb is that 16 GB total is about the upper sane limit for a 32-bit kernel. There's not very much you can do about it.

You can try to boot with less memory (mem= command line parameter to the kernel). But the real solution is to switch to a 64-bit kernel.