Memory considerations with eigenvectors

[martin-ueding]

On the large lattices the reading of all eigenvectors might not fit into memory, we likely have to carefully check how many of them we have in memory at the same time.

[kostrzewa]

An alternative is to generate VdaggerV and use that. In fact, we are now facing this problem and really need to make progress here. @pittlerf this is the place to contribute if you want to take the lead on this.

[kostrzewa]

@martin-ueding Is VdaggerV support still broken or disabled?

[kostrzewa]

@matfischer @urbach This needs to be worked on before the cA2.09.48 rho contractions can be done on Juwels. Also relevant for 3pi. (and anything else, really)

[kostrzewa]

Relatively easy fixes might be to

1. rewrite the VdaggerV code to read the contractions input file, generate all required VdaggerV based on the generated dependencies. This can happen time-slice by time-slice, so memory should not be a problem. One could also trivially MPI-parallelize this, just so as to be able to run larger jobs which might queue more quickly. (or one does MPMD, although that is somewhat less automatic)

2. do the same as above but in the contraction code: read eigenvenctors time slice by time slice and generate the necessary VdaggerV at a constant memory cost of keeping a single time slice of eigenvectors resident. This memory can afterwards of course also be released.