# Distributed traces (MPI)

The ovni trace is designed to support concurrent programs running in different
nodes in a cluster. It is often the case that the monotonic clock
(`CLOCK_MONOTONIC`) are not synchronized between machines (in general they
measure the time since boot).

To generate a coherent Paraver trace, the offsets of the clocks need to be
provided to the emulator too. To do so, run the `ovnisync` program using MPI on
the same nodes your workload will use.

!!! warning

    Run only one MPI process of ovnisync per node.

If you are using SLURM, you may want to use something like:

	% srun ./application
	% srun --ntasks-per-node=1 ovnisync

!!! warning

    Beware that you cannot launch two MPI programs inside the same srun session,
	you must invoke srun twice.

By default, it will generate the `ovni/clock-offsets.txt` file, with the
relative offsets to the rank 0 of MPI. The emulator will automatically pick the
offsets when processing the trace. Use the ovnisync `-o` option to select a
different output path (see the `-c` option in ovniemu to load the file).

Here is an example table with three nodes, all units are in nanoseconds. The
standard deviation is less than 1 us:

```
rank       hostname             offset_median        offset_mean          offset_std
0          xeon01               0                    0.000000             0.000000
1          xeon04               1165382584           1165382582.900000    135.286341
2          xeon05               3118113507           3118113599.070000    180.571610
```