Timestamp hook

The OCI specifications do not define any requirement on exposing information about the inner workings of runtimes and hooks to the user. This can make determining the startup overhead of a standard container runtime difficult.

Sarus bundles a hook which leaves a timestamp on a logfile, accompanied by a configurable message. Since the OCI Runtime Specification mandates hooks to be executed in the order they are entered during configuration, interleaving this hook between other hooks generates useful data to measure the time spent by each component.

The timestamp has the following format:

[<UNIX time in seconds.nanoseconds>] [<hostname>-<hook PID>] [hook] [INFO] Timestamp hook: <optional configurable message>

The following is an actual timestamp example:

[1552438146.449463] [nid07641-16741] [hook] [INFO] Timestamp hook: After-runtime

This hook does not alter the container in any way, and is primarily meant as a tool for developers and system administrators working with OCI hooks.

Hook installation

The hook is written in C++ and it will be compiled when building Sarus without the need of additional dependencies. Sarus's installation scripts will also automatically install the hook in the $CMAKE_INSTALL_PREFIX/bin directory. In short, no specific action is required to install the Timestamp hook.

Sarus configuration

The Timestamp hook can be configured to run at any of the container lifecycle phases supported for hook execution (prestart, poststart, poststop), since it is not tied to the workings of other hooks or the container application.

The hook optionally supports the following environment variable:

  • TIMESTAMP_HOOK_MESSAGE: String to display as part of the timestamp printed to the target file. This variable is optional, and it is meant to differentiate between subsequent invocations of the timestamp hook for the same container.

The following is an example of OCI hook JSON configuration file enabling the Timestamp hook:

    "version": "1.0.0",
    "hook": {
        "path": "/opt/sarus/bin/timestamp_hook",
        "env": [
    "when": {
        "always": true
    "stages": ["prestart"]

As mentioned above, the real value of the Timestamp hook lies in interleaving it between other hooks in order to have a measurement of the elapsed time. For example, using other hooks described in this documentation and creating multiple Timestamp hook JSON configuration files:

$ ls /opt/sarus/etc/hooks.d

The previous example could produce an output in the logfile like the following:

[775589.671527655] [hostname-12385] [hook] [INFO] Timestamp hook: After-runtime
[775589.675871678] [hostname-12386] [hook] [INFO] Timestamp hook: After-glibc-hook
[775589.682727735] [hostname-12392] [hook] [INFO] Timestamp hook: After-NVIDIA-hook
[775589.685961371] [hostname-12393] [hook] [INFO] Timestamp hook: After-MPI-hook
[775589.690460309] [hostname-12394] [hook] [INFO] Timestamp hook: After-SSH-hook
[775589.693946863] [hostname-12396] [hook] [INFO] Timestamp hook: After-SLURM-global-sync-hook

Sarus support at runtime

The hook is activated by setting the TIMESTAMP_HOOK_LOGFILE variable in the container environment to the absolute path to the logfile where the hook has to print its timestamp. Note that the target logfile does not need to exist in the container's filesystem, since the OCI Runtime Specification mandates hooks to execute in the runtime namespace. If the variable is not set, the hook exits without performing any action.

When launching jobs with many containers (e.g. for an MPI application), it is advisable to point the Timestamp hook to a different file for each container, in order to avoid filesystem contention and obtain cleaner measurements. The following example shows one way to achieve this in a batch script for the Slurm Workload Manager.

#!/usr/bin/env bash
#SBATCH --job-name="sarus"
#SBATCH --nodes=<NNODES>
#SBATCH --ntasks-per-node=<NTASKS_PER_NODE>
#SBATCH --output=job.out
#SBATCH --time=00:05:00


echo "START_TIME=`date +%s`"

srun bash -c 'file=<JOB_DIR>/out.procid_${SLURM_PROCID}; TIMESTAMP_HOOK_LOGFILE=${file}.timestamp-hook sarus --verbose run --mpi <image> <application> &>${file}.sarus'

echo "END_TIME=`date +%s`"

The Timestamp hook does not require any direct support from the Sarus engine, but it relies on the presence of the TIMESTAMP_HOOK_LOGFILE variable in the container in order to work. Given the way Sarus creates the container environment, the variable can be set in two ways:

  • in the host environment (like in the example above), having it automatically transferred inside the container by Sarus;

  • directly in the container by using the -e/--env option of sarus run.