clockspeed 1 user commands djbwares
clockspeed speed up or slow down the local clock, continuously
`clockspeed`

## Description

`clockspeed` resets the local UNIX clock every three seconds according to (1) an internal hardware counter on the central processing unit and (2) occasional real-time measurements from a reliable source.

On older x86 and amd64 processors, the CPU's internal counter does not run at a constant rate, as its update frequency is determined by CPU frequency, which the operating system can vary dynamically according to system load and system power management policy. The same is true for the virtual count register of arm64 processors. Only use `clockspeed` on processors that have what the amd64 world calls an "Invariant Time-Stamp Counter", whose update frequency is fixed.

`clockspeed` reads the real-time measurements from `/usr/local/clockspeed/adjust`, a FIFO which it creates and reads from. Each real-time measurement must be a single 16-byte packet, expressed as a TAI64NA time adjustment to the local UNIX clock. This is the output of programs such as taiclock 1 and sntpclock 1.

`clockspeed` does not place any limits on the possible adjustments. It is your responsibility to make sure that the measurements are reliable.

After two real-time measurements, `clockspeed` can figure out the number of real attoseconds per CPU tick. It saves this number in TAI64NA format in `/usr/local/clockspeed/etc/atto`, overwriting `/usr/local/clockspeed/etc/atto.tmp` for reliability. It reads `/usr/local/clockspeed/etc/atto` when it starts up again.

`clockspeed` must be run by `root`.

## History

`clockspeed` was originally part of Daniel J. Bernstein's clockspeed toolset in 1998.

## Author

Original code and documentation by Daniel J. Bernstein. Documentation modernizations by Jonathan de Boyne Pollard.