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fenix command

Syntax

fenix keyword value ...

keyword = restart_file, restart_label, universal, or spares

restart_file arg = file
   file = the file argument to pass to a jump command
restart_label arg = label
   label = the label argument to pass to a jump command
spares arg = N
   N = the number of ranks for Fenix to use as spares
universal arg = none
   initialize Fenix with universal scope

Examples

fenix spares 10
fenix spares 10 restart_label restart
fenix universal restart_label restart

Description

Added in version 2Sep2026.

This command initializes Fenix for online process recovery. From this point on, Fenix claims a number of ranks to be used as spares. The LAMMPS world is rebuilt without the spare ranks. Importantly, this means Fenix should be initialized before you perform any steps that rely on knowing the number of ranks or communicating. In general, the safest approach is to only have variable setup commands before calling this command.

On detecting a process failure, Fenix will automatically regain control. It will rebuild the LAMMPS world to the same number of ranks as before (when possible), using the spare ranks held at initialization. These spares take the place of the failed ranks, leaving surviving processes with the same rank number. Fenix then tears down LAMMPS and restarts according to whatever restart_file and restart_label are configured. The default restart file is “SELF”. Upon restart, the fenix_restarted internal variable is defined and set to 1.

If this command is called more than once, subsequent invocations will update arguments. Fenix is not reinitialized, so changes to the spares argument are ignored.

By default, Fenix initializes assuming LAMMPS is run on a single partition. This is the best option for runs that do not involve partitions, or runs with partitions that run independently and thus have no communication between them. For runs with partitions that communicate, the universal flag is needed to ensure the universe’s uworld is always valid to communicate on.

When the universal flag is not used, communication on the universe’s uworld communicator may cause MPI to abort if any processes have failed. When the universal flag is used, you must not specify a number of spare ranks - instead, all ranks in the last partition are used as spares. When the universal flag is used, shrinking recovery (recovering when all spare ranks have been consumed) is not currently supported.

Restrictions

This command is part of the FENIX package. It is only enabled if LAMMPS was built with that package. See the Build package page for more info.

Default

The default restart_file = SELF.