This repository is a proof-of-concept for using openntpd to keep the Docker for Mac VM in sync with the host time.
This approach could be used as a replacement for the existing sntpc container to fix issues with monotonicity and improve the smoothness of the VM clock. See https://github.com/docker/for-mac/issues/4347 for further details.
The Docker for Mac VM has access to a local ntp server at gateway.docker.internal. Openntpd can use this information to smoothly adjust the VM clock to stay in sync with the host. However this adjustment is limited to 500 ppm which is not sufficient to combat observed clock drift values.
As a workaround, we perform a crude measurement of clock drift on startup and adjust the tick value, then rely on openntpd for fine-grained frequency adjustment on an ongoing basis.
A possible improvement to this implementation would be to periodically check the drift value measured by ntpd and re-adjust the tick as needed. This could help if the drift rate changes over time in excess of 500 ppm.
First, build the Docker image.
docker build -t ntpd-poc .To try it out, stop the existing LinuxKit sntpc container.
docker run --rm --privileged --pid=host justincormack/nsenter1 \
/usr/bin/ctr -n services.linuxkit task pause sntpcThen, start the ntpd container.
docker run --rm --privileged --name ntpd ntpd-pocUse ntpctl to get information about ntpd.
docker exec ntpd ntpctl -s allYou can use sntpc to monitor the offset.
docker exec ntpd sntpc -i 5 -n gateway.docker.internalThe container will adjust the tick on startup. After about 15 minutes, ntpd
will adjust the frequency offset to keep the VM clock more precisely in sync
with the host. You can use the adjtimex command to obtain the current time
parameters.
docker exec ntpd adjtimex