On Mon, Oct 01, 2018 at 11:15:32AM +0200, Eric W. Biederman wrote:
In the context of process migration there is a simpler subproblem that I think it is worth exploring if we can do something about.
For a cluster of machines all running with synchronized clocks. CLOCK_REALTIME matches. CLOCK_MONOTNIC does not match between machines. Not having a matching CLOCK_MONOTONIC prevents successful process migration between nodes in that cluster.
Would it be possible to allow setting CLOCK_MONOTONIC at the very beginning of time? So that all of the nodes in a cluster can be in sync?
Here is a question about how to synchronize clocks between nodes. It looks like we will need to have a working network for this, but a network configuration may be non-trivial and it can require to run a few processes which can use CLOCK_MONOTNIC...
No change in skew just in offset for CLOCK_MONOTONIC.
There are also dragons involved in coordinating things so that CLOCK_MONOTONIC gets set before CLOCK_MONOTONIC gets used. So I don't know if allowing CLOCK_MONOTONIC to be set would be practical but it seems work exploring all on it's own.
Dmitry would setting CLOCK_MONOTONIC exactly once at boot time solve your problem that is you are looking at a time namespace to solve?
Process migration is only one of use-cases. Another use-case is restoring from snapshots. It may be even more popular than process migration. We can't guarantee that all snapshots will be done in one cluster. For example, a user meets a bug, does a container snapshot and attaches it to a bug report.
Eric