1.4 KiB
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. If you are using SLURM, you may want to
use something like:
% srun ./application
% srun 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