Note that the mcelog is poorly supported in many distros. An alternative is the EDAC project.
Machine Checked Exceptions (MCE) are errors raised by the CPU to notify problems such as CPU cache faults, bus errors, and ECC parity errors in RAM memory modules. By default, MCE errors are not actually logged anywhere by the kernel. The Linux kernel makes these errors visible in userspace through a character device. For this to be useful a daemon is needed to log exceptions reported through this device. The mcelog package performs the task of copying any MCE errors to a log file.
If you install the mcelog package and see this message, /dev/mcelog not active, it means that you did not create the device. In some systems this is done automatically by udev, but in other systems you must create it manually.
mknod /dev/mcelog c 10 227