Checkpointing

Writing checkpoint files

Long-running simulations commonly write checkpoint files so that runs can be restarted from a recent saved state. In JustPIC, checkpoints are written in JLD2 format.

At the lowest level, you can serialize arrays manually:

jldsave(
    "my_file.jld2";
    particles     = Array(particles),
    phases        = to_cpu(phases),
    phase_ratios  = Array(phase_ratios),
    particle_args = to_cpu.(particle_args),
)

This saves particle information to my_file.jld2, ready to be reloaded later.

If file size matters more than exact restart reproducibility, you can downcast to Float32 before writing:

jldsave(
    "my_file.jld2";
    particles     = Array(Float32, particles),
    phases        = to_cpu(Float32, phases),
    phase_ratios  = Array(Float32, phase_ratios),
    particle_args = to_cpu.(Float32, particle_args),
)

For routine use, prefer the built-in helper:

checkpointing_particles(
    dst,
    particles;
    phases = nothing,
    phase_ratios = nothing,
    chain = nothing,
    t = nothing,
    dt = nothing,
    particle_args = nothing,
    kwargs...,
)

On multiple MPI ranks, pass the rank id to get rank-local filenames:

checkpointing_particles(
    dst,
    particles,
    me;
    phases = nothing,
    phase_ratios = nothing,
    chain = nothing,
    t = nothing,
    dt = nothing,
    particle_args = nothing,
    kwargs...,
)

Any additional keyword arguments are written into the checkpoint as extra fields.

Loading a checkpoint file

To restart a simulation, load the file and cast the stored arrays back to the active backend:

data          = load("my_file.jld2")
particles     = TA(backend)(Float64, data["particles"])
phases        = TA(backend)(Float64, data["phases"])
phase_ratios  = TA(backend)(Float64, data["phase_ratios"])
particle_args = TA(backend).(Float64, data["particle_args"])

TA(backend) selects the backend-appropriate array type, so the same checkpoint can be restored onto CPU or accelerator arrays.

API

checkpointing_particles(dst, particles; phases=nothing, phase_ratios=nothing, chain=nothing, t=nothing, dt=nothing, particle_args=nothing)
checkpointing_particles(dst, particles, me; phases=nothing, phase_ratios=nothing, chain=nothing, t=nothing, dt=nothing, particle_args=nothing)