NAME

       mhz - CPU frequency measurement utility

SYNOPSIS

       mhz [-h|-c|-i|-t] [lines [heat [count]]]

DESCRIPTION

       It’s fairly common, especially in embedded systems, to be fooled by a CPU frequency not matching the
       believed one. Most of the time the reason stems from missing frequency operation points, but it can also
       be caused by bogus clock drivers, device tree issues, hardware issues, boot loader issues, firmware
       issues, different silicon quality, thermal throttling and even (long ago) vendors cheating by only
       reporting high values in cpufreq.

       This tool runs long loops of inexpensive arithmetic operations (XOR) that are not optimizable so that
       we’re almost certain that most CPU cores will run them at a rate of one operation per clock cycle, not
       less, not more.

       This has been tested on various cores including old 180 MHz MIPS and less old MIPS32/MIPS64, many 32- and
       64-bit Arm cores of v5/v7/v8/v9 architectures, various i386 and x86_64 up to 6 GHz, RISC-V, PowerPC,
       Sparc64 and Alpha EV6, all of them reporting accurate values.

       The program first runs a self-calibration so that the short loop takes at least 20 milliseconds. Each
       measure is run 5 times and the lowest measure is returned in order to hide as much as possible latency
       spikes caused by interrupt processing and context switches. This delay of 20ms is chosen so that it
       remains accurate on systems with clocks of 100, 250 and 1000 Hz. The longer loop is 5 times more
       instructions, so a full sample cannot take less than 120 ms and since 5 samples are needed, a full test
       takes at least 600ms.

       Various numbers are reported by default, including the timings of the 50- and 250- microsecond loops, and
       the measured TSC (time stamp counter) frequency, when available. Most modern processors have their TSC
       running at a fixed frequency which is often higher than the base frequency but lower than the maximum
       frequency. Knowing the exact frequency is useful when performing clock measurements on code, as the
       frequencies of the instructions and the TSC rarely match nowadays.

OPTIONS

       -h
           Print help message and exit

       -c
           Only report the CPU frequency in MHz, not the rest of the fields

       -t
           Only report the TSC frequency in MHz, not the rest of the fields

       -i
           Truncate the frequency to the nearest integer value. This is essentially used with -c or -t for use
           in scripts.

       lines
           Number of measurements to perform before quitting. By default a single measurement is run. Increasing
           this value allows collecting more values to observe variations, or simply to monitor the CPU
           frequency in real time.

       heat
           Force the CPU to run a busy loop of that many microseconds before starting measurements. This can be
           needed on modern CPUs which switch to low-power mode after a few milliseconds of idle, so as not to
           affect the first measurement by the time it takes to wake from low-power. Default is 0.

       count
           Loop count value to be used as a starting point. There is little point in setting it, as it will
           automatically be adjusted if not sufficient. However it can sometimes be helpful to permit running
           multiple tests under the exact same conditions, e.g. by copying the value from a previous run, or by
           forcing a large value.

EXAMPLES

       Run 3 measures and exit:

           mhz 3

       Run 3 measures after a pre-heating period of 500 ms:

           mhz 3 500000

       Show the integer frequencies of all CPUs on the machine:

           ncpu=$(nproc)
           for ((i=0; i<ncpu;i ++)); do echo -n "$(taskset -c $i mhz -c -i) "; done
           echo

OBSERVATIONS AND HINTS

       When measuring a CPU’s frequency on a multi-core system, please consider always setting the CPU you want
       to measure using taskset -c $CPU. Indeed, it’s very common to see different CPU frequencies and even
       different core models. Running multiple tests without binding to a specific CPU will just report a lot of
       variation caused by the random CPU core that runs the program.

       In order to check the theoretical frequency ranges supported by the various CPUs, using lscpu -e is very
       convenient as it will report for each core the advertised minimum and maximum frequencies. It may happen
       that a CPU will run above the advertised maximum, without being a problem.

       Experience shows that reported values are often off by ~0.1%, sometimes slightly more. Using longer loops
       doesn’t get rid of that. It may be caused by periodic interrupts and other noisy activity. Seeing 1196
       MHz for 1200 MHz is not necessarily an issue and even some vendors will call that "1200".

       Most CPUs can only run at integral multiples of a reference clock. Reference clocks of 24, 25, 33.333 or
       50 MHz are commonly seen. This helps doing the mental round up that may be needed. For example seeing
       1293 MHz could indicate 1296 MHz on a system running on multiples of 24 MHz, or 1300 MHz on a system
       running on multiples of 25/50 MHz. In the Arm world, multiples of 24 MHz are quite common, so that
       advertised frequencies of 1300, 1500, 1600, 1800 and 2000 MHz are in fact more likely 1296, 1512, 1608,
       1800 and 2016.

       Finally, the CPU frequency alone isn’t an indicator of system performance, it’s only one metric. For a
       more realistic system benchmark, please consider using sbc-bench which gathers various utilities
       (including mhz) and monitors many metrics all along the tests in order to detect anomalies.

COPYING

       Copyright © 2016-2023 Willy Tarreau w@1wt.eu. Free use of this software is granted under the terms of the
       MIT/Expat License.

AUTHOR

       Willy Tarreau

mhz 1.0.0                                          2025-07-26                                             MHZ(1)