NAMElamssi_checkpoint_restart - overview of LAM's MPI checkpoint / restart SSI modules
DESCRIPTIONThe "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 PhasesLAM defines three phases for checkpoint / restart support in each MPI process:
- When the checkpoint request arrives, before the actual checkpoint occurs.
- After a checkpoint has successfully completed, in the same process as the checkpoint was invoked in.
- 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.
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 ModuleThe Berkeley Lab Checkpoint/Restart (BLCR) single-node checkpointer is a software system from Lawrence Berkeley Labs. See the project web page for more details: www.nersc.gov/research/ftg/checkpoint.
The blcr module has one SSI parameter:
- blcr's default priority is 50.
self CR ModuleThe 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:
- 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.
- Name of the user function to invoke during the Checkpoint phase.
- Name of the user function to invoke during the Continue phase.
- 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:
- self's default priority is 25.