[Users] PreSync and recovery from checkpoint

[Roland Haas]

Hello Gabriele,

Sorry for the delay. Your email was flagged as "spam" by the University
of Illinois and I forgot to respond after noting that your email was an
"unanswered question" week before last week.

Steve responded to you as well in the meantime I believe.

> In this specific case, I can either turn on checkpointing for the BSSN
> variables
> in ProcaEvolve, or I can recompute them from the ADM ones after recovery.
> What is the correct way to re-initialize a derived variable after recovery?
If you want to be consistent with eg ML_BSSN then checkpointing and
recovering is the way to go.

Recomputing would reduce the checkpoint size a bit. Usually that is not
a problem (speed or size wise). One has to be *very* careful though
that one can exactly (to the last bit) recompute the variable though
(eg there must not be anything that runs after BSSNtoADM that modifies
the BSSN variables [eg boundary routines, constraint projections,
whatnot]). If one cannot guarantee that, then recovering from
checkpoints (and not touching the recovered variables) would be
preferable. For historic reasons there is (at least) one thorn in ET
(GRHydro) that violates this, namely it runs con2prim in
post-recover-variables (since it runs in mol_poststep and mol_poststep
runs in that bin) but con2prim is not going to produce the same results
due to its result depending on the initial guess which is the values of
the primitive variables in hydrobase ("previous" values during
evolution, "just recovered values" during recovery). This behavious
occasionally causes headaches when a run fails / succeeds depending on
which checkpoint one recovers from.

Yours,
Roland

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