Multiple FFTs at once about dftk.jl

[mfherbst]

Good idea. I would expect that for a large number of bands, using a "half-band" version is also reasonable, since it probably will take quite some iterations to go from 1 locked to half locked and in between you need to use the 1-band version for almost the complete band. But I agree, we should look at the other codes.

[antoine-levitt]

So I did a bit of digging.

If I understand correctly QE does not do multiple FFTs at once (PW/src/vloc_psi.F90 calls fwfft, defined in FFTXlib/fft_interfaces.F90, without the optional argument howmany).

Abinit... has a m_fft.F90 file that is 4940 lines long (so about twice the size of DFTK right now), which as far as I understand is . That's just the high-level interface though; if you add the low-level wrapper around FFTW3, it adds 15k lines. But of course there's also the FFT routines from Goedecker, that are still used by default when running under MPI, which add another 10k lines. So the only way to know what is actually going on is to run it through a debugger.

[antoine-levitt]

Just while I'm at it, here are some fun LOC stats. Counting just the "core" part (as best as I could find out what's core and what's not) of what DFTK does right now, excluding parallelisation etc. as far as I could:

Abinit: sloccount 28_numeric_noabirule/ 29_kpoints/ 32_util/ 41_xc_lowlevel/ 42_parser/ 44_abit* 46_diago/ 53_ffts/ 55_abiutil/ 56* 57* 56* 61* 62* 66_nonlocal/ 66_wfs/ 67_common/ 69_wfdesc/ 94_scfcv/ 98_main/ 83_cut3d/ => 190k

QE: sloccount LAXlib/ FFTXlib/ UtilXlib/ KS_Solvers/ PW/src/ => 85k (excluding the copy of an '97 version of FFTW they have in there)

[antoine-levitt]

GPAW is comparatively pretty good: the core part is about 30k lines of python, supports three different basis sets, with about 10k of C code for performance-critical parts.

[antoine-levitt]

So, reading the FFTW manual more closely, there is a "wisdom" mechanism for reusing measurements to create new plans; they say it's pretty smart, and so this might negate the problem of the changing nband (ie do the FFTW_MEASURE with the full nband, and reuse the wisdom to do a smaller number of bands)

[mfherbst]

Ah that can be done quite easily in fact.

[mfherbst]

This issue is related to the TODO given in https://github.com/mfherbst/DFTK.jl/blob/master/src/core/PlaneWaveBasis.jl#L76

[antoine-levitt]

Closing in favour of #15