# Quick Start¶

In this quick start guide, we assume that you already have a basic understanding of SCR and how it works on HPC systems. We also assume you have access to a SLURM cluster with a few compute nodes and a working MPI environment. We walk through a bare bones example to quickly get you started with SCR. For more in-depth information, see subsequent sections in this user’s guide.

## Building SCR¶

SCR has a number of dependencies. To simplify the install process, one can use a release tarball with CMake or use Spack. For full details on building SCR, please see Section Build SCR.

### CMake¶

SCR requires CMake version 2.8 or higher. The SCR build uses the CMake FindMPI module to link with MPI. This module looks for the standard mpicc compiler wrapper, which must be in your PATH.

wget https://github.com/LLNL/scr/releases/download/v3.0rc2/scr-v3.0rc2.tgz
tar -zxf scr-v3.0rc2.tgz
cd scr-v3.0rc2

mkdir build install
cd build
cmake -DCMAKE_INSTALL_PREFIX=../install ..
make -j install


There are a number of CMake options to configure the build. For more details, see Section CMake.

### Spack¶

If you use the Spack package manager, SCR and many of its dependencies have corresponding packages.

Before installing SCR with Spack, one should first configure packages.yaml. In particular, SCR depends on the system resource manager and MPI library, and one should define entries for those in packages.yaml. Examples for configuring common resource managers and MPI libraries are listed in Section Spack.

SCR can then be installed for SLURM systems with:

spack install scr@3.0rc2


## Building the SCR test_api Example¶

In this quick start guide, we use the test_api.c program.

If you install SCR with CMake, test_api.c is compiled as part of the make install step. You can find it in the examples subdirectory within the CMake build directory:

cd examples


If you still have this direcotry, then skip ahead to the next section to run test_api.c.

Alternatively, you will find source files for example programs in the <install>/share/scr/examples directory, where <install> is the path in which SCR was installed.

If you install SCR with Spack, you can identify the SCR install directory with the following command:

spack location -i scr


Then build test_api.c by executing:

cd <install>/share/scr/examples
make test_api


Upon a successful build, you will have a test_api executable.

## Running the SCR test_api Example¶

A quick test of your SCR installation can be done by running test_api in an interactive job allocation. The following assumes you are running on a SLURM-based system. If you are not using SLURM, then modify the node allocation and run commands as appropriate for your resource manager.

First, obtain compute nodes for testing. Here we allocate 4 nodes:

salloc -N 4


Once you have the compute nodes you can run test_api. Here we execute a 4-process run on 4 nodes:

srun -n 4 -N 4 ./test_api


This example program writes 6 checkpoints using SCR. Assuming all goes well, you should see output similar to the following

>>: srun -n 4 -N 4 ./test_api
Init: Min 0.033856 s    Max 0.033857 s  Avg 0.033856 s
No checkpoint to restart from
At least one rank (perhaps all) did not find its checkpoint
Completed checkpoint 1.
Completed checkpoint 2.
Completed checkpoint 3.
Completed checkpoint 4.
Completed checkpoint 5.
Completed checkpoint 6.
FileIO: Min   52.38 MB/s        Max   52.39 MB/s        Avg   52.39 MB/s       Agg  209.55 MB/s


If you do not see output similar to this, there may be a problem with your environment or your build of SCR. Please see the detailed sections of this user guide for more help or email us (see Support and Additional Information.)

One can use test_api to conduct more interesting tests. In the SCR source directory, the testing directory includes scripts to demonstrate different aspects of SCR. Depending on your shell preference, TESTING.csh or TESTING.sh are good for getting started. Each script contains a sequence of additional configurations and commands for running test_api. One can find those TESTING scripts in a clone of the repo, e.g.:

git clone git@github.com:llnl/scr.git
cd scr/testing


Here we provide an example of integrating the SCR API into an application to write checkpoints.

int main(int argc, char* argv[]) {
MPI_Init(argc, argv);

/* Call SCR_Init after MPI_Init */
SCR_Init();

for (int t = 0; t < TIMESTEPS; t++) {
/* ... Do work ... */

/* Ask SCR if a checkpoint should be saved (optional) */
int need_ckpt;
SCR_Need_checkpoint(&need_ckpt);
if (need_ckpt)
checkpoint(t);
}

/* Call SCR_Finalize before MPI_Finalize */
SCR_Finalize();

MPI_Finalize();

return 0;
}

void checkpoint(int timestep) {
/* Define a name for our checkpoint */
char name[256];
sprintf(name, "timestep.%d", timestep);

/* Tell SCR that we are starting a checkpoint phase */
SCR_Start_output(name, SCR_FLAG_CHECKPOINT);

int rank;
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

/* Define our checkpoint file name */
char file[256];
sprintf(file, "%s/rank_%d.ckpt", name, rank);

/* Register our checkpoint file with SCR,
* and obtain path we should use to open it */
char scr_file[SCR_MAX_FILENAME];
SCR_Route_file(file, scr_file);

/* Each process will inform SCR whether it wrote
* its checkpoint successfully */
int valid = 1;

/* Use path from SCR to open checkpoint file for writing */
FILE* fs = fopen(scr_file, "w");
if (fs != NULL) {
int rc = fwrite(state, ..., fs);
if (rc == 0)
/* Failed to write, mark checkpoint as invalid */
valid = 0;

fclose(fs);
} else {
/* Failed to open file, mark checkpoint as invalid */
valid = 0;
}

/* Tell SCR that we have finished our checkpoint phase */
SCR_Complete_output(valid);

return;
}


Further sections in the user guide give more details and demonstrate how to perform restart with SCR. For a description of the API, see SCR API, and for more detailed instructions on integrating the API, see Integrate SCR.

It may also be instructive to examine the source of the test_api.c program and other programs in the examples directory.

## Final Thoughts¶

This was a quick introduction to building and running with SCR. For more information, please look at the more detailed sections in the rest of this user guide.