---
name: "lamssi_cr"
section: "7"
platform: "linux"
package: "lam-runtime"
version: "7.1.4-7.2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/lamssi_cr"
---

# lamssi\_cr(7)

overview of LAM's MPI checkpoint / restart SSI modules

## NAME

lamssi\_checkpoint\_restart - overview of LAM's MPI checkpoint / restart SSI modules

## DESCRIPTION

The "kind" for checkpoint / restart SSI modules is "cr". Specifically, the string "cr" (without the quotes) is the prefix that should be used with the *mpirun* command line with the *-ssi* switch.  For example:

- mpirun -ssi cr blcr C my\_mpi\_program

  LAM/MPI can involuntarily checkpoint and restart parallel MPI jobs. Doing so requires that LAM/MPI was compiled with thread support and that back-end checkpointing systems are available at run-time.  MPI jobs will have to run with at least MPI\_THREAD\_SERIALIZED support.  If a job elects to run with checkpoint/restart support and an available cr module is found, the job's thread level will automatically be promoted to MPI\_THREAD\_SERIALIZED.  See the User's Guide for more details.

### Checkpoint Phases

LAM defines three phases for checkpoint / restart support in each MPI process:

- Checkpoint.

  When the checkpoint request arrives, before the actual checkpoint occurs.

- Continue.

  After a checkpoint has successfully completed, in the same process as the checkpoint was invoked in.

- Restart

  After a checkpoint has successfully completed, in a new / restarted process.

The Continue and Restart phases are identical except for the process in which they are invoked -- the Continue phase is invoked in the same process as the Checkpoint phase was invoked.  The Restart phase is only invoked in newly restarted processes.

## AVAILABLE MODULES

LAM currently has two cr modules:  *blcr* and *self*. In order for an MPI job to be able to be checkpointed and restarted, all of its MPI SSI modules must support checkpoint/restart. Currently, this means using the *crtcp* RPI module or the *gm* RPI module when compiled with  *gm\_get()* support (see the User's Guide for more details).

### blcr CR Module

The Berkeley Lab Checkpoint/Restart (BLCR) single-node checkpointer is a software system from Lawrence Berkeley Labs.  See the project web page for more details: *http://www.nersc.gov/research/ftg/checkpoint/*.

The  *blcr* module has one SSI parameter:

- cr\_blcr\_priority

  *blcr*'s default priority is 50.

### self CR Module

The *self* module, when used with checkpoint/restart SSI modules, will invoke the user-defined functions to save and restore checkpoints. It is simply a mechanism for user-defined functions to be invoked at LAM's Checkpoint, Continue, and Restart phases. Hence, the only data that is saved during the checkpoint is what is written in the user's checkpoint function. No MPI library state is saved at all.

As such, the model for the *self* module is slightly different than, for example, the *blcr* module. Specifically, the Restart function is not invoked in the same process image of the process that was checkpointed. The Restart phase is invoked during **MPI\_INIT** of a new instance of the application (i.e., it starts over from main()).

Multiple SSI parameters are available:

- cr\_self\_user\_prefix

  Specify a string prefix for the name of the checkpoint, continue, and restart functions that should be invoked by LAM.  That is,  specifying "-ssi cr\_self\_user\_prefix foo" means that LAM expects to find three functions at run-time:  int foo\_checkpoint(),  int foo\_continue(), and int foo\_restart(). This is a convenience parameter that can be used instead of the three parameters listed below.

- cr\_self\_user\_checkpoint

  Name of the user function to invoke during the Checkpoint phase.

- cr\_self\_user\_continue

  Name of the user function to invoke during the Continue phase.

- cr\_self\_user\_restart

  Name of the user function to invoke during the Restart phase.

If none of these parameters are specified and the *self* module is selected, it will use the default prefix *lam\_cr\_self*

Finally, the usual priority SSI parameter is also available:

- cr\_self\_priority

  *self*'s default priority is 25.

## SEE ALSO

[lamssi(7)](/lamssi/7.md), [mpirun(1)](/mpirun/1.md), LAM User's Guide