Manual Reference Pages - MCELOG (8)
mcelog - Decode kernel machine check log on x86 machines
mcelog [options] [device]
mcelog [options] --daemon
mcelog [options] --client
mcelog [options] --ascii
mcelog [options] --is-cpu-supported
X86 CPUs report errors detected by the CPU as
machine check events (MCEs). These can be data corruption detected in the CPU caches,
in main memory by an integrated memory controller, data
transfer errors on the front side bus or CPU interconnect or other internal
Possible causes can be cosmic radiation, instable power supplies,
cooling problems, broken hardware, running systems out of specification,
or bad luck.
Most errors can be corrected by the CPU by internal error correction
mechanisms. Uncorrected errors cause machine check exceptions which
may kill processes or panic the machine. A small number of corrected
errors is usually not a cause for worry, but a large number can indicate
When a corrected or recovered error happens the x86 kernel writes a record describing
the MCE into a internal ring buffer available through the
mcelog retrieves errors from
/dev/mcelog, decodes them into a human readable format and prints them
on the standard output or optionally into the system log.
Optionally it can also take more options like keeping statistics or
triggering shell scripts on specific events. By default mcelog
supports offlining memory pages with persistent corrected errors,
offlining CPU cores if they developed cache problems,
and otherwise logging specific events to the system log after
they crossed a threshold.
The normal operating modi for mcelog are running
as a regular cron job (traditional way, deprecated),
running as a trigger directly executed by the kernel,
or running as a daemon with the
When an uncorrected machine check error happens that the kernel
cannot recover from then it will usually panic the system.
In this case when there was a warm reset after the panic
mcelog should pick up the machine check errors after reboot.
This is not possible after a cold reset.
In addition mcelog can be used on the command line to decode the kernel
output for a fatal machine check panic in text format using the
--ascii option. This is typically used to decode the panic console output of a fatal
machine check, if the system was power cycled or mcelog didnt
run immediately after reboot.
When the panic triggers a kdump kexec crash kernel the crash
kernel boot up script should log the machine checks to disk, otherwise
they might be lost.
Note that after mcelog retrieves an error the kernel doesnt
store it anymore (different from
dmesg(1)), so the output should be always saved somewhere and mcelog
not run in uncontrolled ways.
When invoked with the
--is-cpu-supported option mcelog exits with code 0 if the current CPU is supported, 1 otherwise.
--syslog option is specified redirect output to system log. The
--syslog-error option causes the normal machine checks to be logged as
--syslog ). Normally only fatal errors or high level remarks are logged with error level.
High level one line summaries of specific errors are also logged to the syslog by
default unless mcelog operates in
--logfile=file option is specified append log output to the specified file. With the
--no-syslog option mcelog will never log anything to the syslog.
--cpu=cputype option is specified set the to be decoded CPU to
mcelog --help for a list of valid CPUs.
Note that specifying an incorrect CPU can lead to incorrect decoding output.
Default is either the CPU of the machine that reported the machine check (needs
a newer kernel version) or the CPU of the machine mcelog is running on, so normally
this option doesnt have to be used. Older versions of mcelog had separate
options for different CPU types. These are still implemented, but deprecated
and undocumented now.
--dmi option mcelog will look up the DIMMs reported in machine
checks in the
SMBIOS/DMI tables of the BIOS and map the DIMMs to board identifiers.
This only works when the BIOS reports the identifiers correctly.
Unfortunately often the information reported
by the BIOS is either subtly or obviously wrong or useless.
This option requires that mcelog has read access to /dev/mem
(normally requires root) and runs on the same machine
in the same hardware configuration as when the machine check
--ignorenodev is specified then mcelog will exit silently when the device
cannot be opened. This is useful in virtualized environment
with limited devices.
--filter is specified
mcelog will filter out known broken machine check events (default on). When the
--no-filter option is specified mcelog does not filter events.
--raw is specified
mcelog will not decode, but just dump the mcelog in a raw hex format. This
can be useful for automatic post processing.
When a device is specified the machine check logs are read from
device instead of the default
--ascii option mcelog decodes a fatal machine check panic generated
by the kernel ("CPU n: Machine Check Exception ...") in ASCII from standard input
and exits afterwards.
Note that when the panic comes from a different machine than
where mcelog is running on you might need to specify the correct
cputype on older kernels. On newer kernels which output the
PROCESSOR field this is not needed anymore.
--file filename option is specified
mcelog --ascii will read the ASCII machine check record from input file
filename instead of standard input.
--config-file file option mcelog reads the specified config file.
/etc/mcelog/mcelog.conf See also
CONFIG FILE below.
--daemon option mcelog will run in the background. This gives the fastest reaction
time and is the recommended operating mode.
If an output option isnt selected (
--syslog-error ), this option implies
--logfile=/var/log/mcelog. Important messages will be logged as one-liner summaries to syslog
--no-syslog is given.
--foreground will prevent mcelog from giving up the terminal in daemon mode. This
is intended for debugging.
--client option mcelog will query a running daemon for accumulated errors.
--cpumhz=mhz option assume the CPU has
mhz frequency for decoding the time of the event using the CPU time stamp
counter. This also forces decoding. Note this can be unreliable.
on some systems with CPU frequency scaling or deep C states, where
the CPU time stamp counter does not increase linearly.
By default the frequency of the current CPU is used when mcelog
determines it is safe to use. Newer kernels report
the time directly in the event and dont need this anymore.
--pidfile file option writes the process id of the daemon into file
file. Only valid in daemon mode.
Mcelog will enable extended error reporting from the memory
controller on processors that support it unless you tell it
not to with the
--no-imc-log option. You might need this option when decoding old logs
from a system where this mode was not enabled.
--version displays the version of mcelog and exits.
mcelog supports a config file to set defaults. Command line options override
the config file. By default the config file is read from
/etc/mcelog/mcelog.conf unless overridden with the
The general format is
optionname = value White space is not allowed in value currently, except at the end where it is dropped
Comments start with #.
All command line options that are not commands can be specified in the config file.
For example t to enable the
--no-syslog option use
no-syslog = yes (or no to disable). When the option has a argument
logfile = /tmp/logfile
For more information on the config file please see
The kernel prefers old messages over new. If the log buffer overflows
only old ones will be kept.
The exact output in the log file depends on the CPU, unless the --raw option is used.
mcelog will report serious errors to the syslog during decoding.
mcelog runs in daemon mode and receives a
SIGUSR1 it will close and reopen the log files. This can be used to rotate logs without
restarting the daemon.
/dev/mcelog (char 10, minor 227)
AMD x86-64 architecture programmers manual, Volume 2, System programming
Intel 64 and IA32 Architectures Software Developers manual, Volume 3, System programming guide
Chapter 15 and 16. http://www.intel.com/sdm
Datasheet of your CPU.
Visit the GSP FreeBSD Man Page Interface.
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