Performance considerations

Author:

Jacek Dziedzic, University of Southampton

How quickly ONETEP runs your calculations depends on a number of factors. Choosing the number of nodes, processes and threads has already been described in Running ONETEP in parallel environments.

Here we mention techniques that can be used in addition to an efficient parallel decomposition to improve performance.

Fast sparse-to-dense and dense-to-sparse conversions

In many scenarios, ensemble DFT calculations being a notable example, ONETEP needs to convert between two matrix representations – a distributed sparse one (“SPAM3”), and a distributed dense one (“BLACS”). These conversions happen multiple (hundreds) times in a run, and involve substantial data communication. There are two ways in which ONETEP can do these – conveniently termed “slow” and “fast”.

The slow approach is the only approach available until ONETEP 6.1.21. Starting from ONETEP 6.1.22, a fast approach is also available, and is the default. This means that no action is required on your part to use the fast approach. The fast approach is up to several times faster (for this part of the calculation).

You can check which approach you are using by examining the timings printed out at the end of your calculation (provided you are not using timings_level 0). If your timings include sparse_spam3toblacs_real_fast and sparse_blacstospam3_real_fast – you are using the fast approach. If instead you have sparse_spam3toblacs_real_slow and sparse_blacstospam3_real_slow – you are using the slow approach.

To switch between the approaches use:

  • fast_dense_to_sparse T and fast_sparse_to_dense T – for the fast approach,

  • fast_dense_to_sparse F and fast_sparse_to_dense F – for the slow approach.

The fast approach is currently incompatible with the image comms subsystem needed by NEB. This means you cannot use the fast approach with NEB. Please add fast_dense_to_sparse F and fast_sparse_to_dense F to your input file when using NEB.

Fast density calculation (for users)

This is a user-level explanation – for developer-oriented material, see Fast density calculation (for developers).

Calculating the electronic charge density is one of the more time-consuming operations in ONETEP. In a typical calculation it has to be performed hundreds of times. There are two ways in which ONETEP can calculate the density – conveniently termed “slow” and “fast”.

The slow approach is the only approach available until ONETEP 7.1.7. Starting from ONETEP 7.1.8, a fast approach is also available, but it is not the default. This means that action is required on your part to use the fast approach. The fast approach is up to several times faster (for this part of the calculation).

The fast approach works best for “serious” systems, it’s not meant to address scenarios with KE cutoffs below 700-800 eV or NGWFs smaller than 8.0 a0.

To switch between the approaches use:

  • fast_density T – for the fast approach,

  • fast_density F – for the slow approach.

Be aware that the fast approach is an approximation. The approximation is well-controllable, meaning you can get as close with the accuracy to the slow (exact) approach as you like, albeit sacrificing performance as you do so. Conversely, you can make the fast approach as fast as you like, but you will be sacrificing accuracy as you do so, up to a point of making your results worthless. This means care must be taken when changing the parameters of this approach – non-expert users are advised to use the defaults, or even “safe settings” described below.

You can check which approach you are using by examining the timings printed out at the end of your calculation (provided you are not using timings_level 0). If your timings include density_on_dbl_grid_fast and density_fast_new_ngwfs – you are using the fast approach. If instead you have density_on_dbl_grid and density_batch_interp_deposit – you are using the slow approach.

The main idea behind the fast density approach is trimming interpolated NGWFs, that is, ignoring the points where their values are below a prescribed threshold. The value of this threshold, set by fast_density_trim_threshold is the main parameter controlling the balance between accuracy and efficiency. It is also independent of NGWF radii. The default is 2E-6. The parameter already includes grid weights, so you do not need to adjust it when changing psinc_spacing or cutoff_energy. To make your calculation more accurate, decrease the threshold – probably not below 1E-7. To make your calculation faster, increase the threshold – probably not above 1E-5.

If you use fast_density_output_detail VERBOSE (or higher), ONETEP will print out an estimate of the accuracy of the approximation every time NGWFs change. It will look like something like Fig. 11, see the accuracy of approximation line. This tells you to how many digits the approximated NGWF charge is equal to the exact (double FFT-box) NGWF charge, in the root-mean-square sense over all NGWFs in the system. In this example our approximated charge is no further from 1.0 (a correctly normalized NGWF) than by 1E-7 (and is slightly closer, because we got 7.16, not 7.0).

As your calculation progresses, this value will fluctuate, and is likely go down slightly, as the NGWFs become more diffuse. As a rule of thumb, if it gets below 5.0-6.0, you will have difficulty converging NGWFs to the default threshold. If it is above 9.0, you are probably using too much accuracy, losing efficiency as you do that.

Fast density -- information on accuracy and memory use.

Fig. 11 The summary printed by fast density every time the NGWFs change. Of main interest are: accuracy of approximation (shown in red) and estimated high-memory watermark per MPI rank (shown in yellow).

Another notable quantity in Fig. 11 is the estimated high-memory watermark per MPI rank (shown in yellow). This is a reminder that the fast density approach uses significantly more memory than the slow approach. The value in the printout is the expected maximum memory that fast density uses per MPI rank. If your printout is truncated before you reached this line, you most likely already ran out of memory. At this stage, we use an all-or-nothing approach – there is no way to give the algorithm a memory allowance and tell it that it should not consume more. Work on this is in progress. The best way to reduce memory load is to use fewer processes per node and more threads. If this is not sufficient, you can reduce the memory load by using more nodes, but this is not a linear dependence – i.e. you will not reduce the load by a factor of two if you add twice as many nodes. Finally, note that what is printed out is the amount of memory consumed by the fast density approach, not by all of ONETEP.

When is fast density used?

Fast density is only used for energy evaluations done from hamiltonian_mod – via hamiltonian_lhxc_calculate() and hamiltonian_energy_components(). These are the costly density calculations, because they are done hundreds of times in the course of a calculation. All other density calculations (done in forces, properties, eigenstates, linear response, lr_tddft, population, dma, dmft, EDA, implicit solvent restarts) are always done using the exact (slow) method. The rationale is that these are done much less often and possibly require more accuracy.

If you want to know when the fast and slow routines are called, specify fast_density_output_detail PROLIX or higher.

More accuracy

The default settings should give you sufficient accuracy to converge NGWFs to the default threshold and to get energies and forces that are negligibly different from those obtained with the slow approach. However, for more difficult systems, particularly if using low kinetic energy cutoffs (say, below 700 eV – like would probably be used with PAW), you might need to adjust the parameters to get desired accuracy.

In addition to adjusting fast_density_trim_threshold down (to perhaps 1E-6 or 5E-7), you may want to use fast_density_off_for_last T (the default is F). This will tell ONETEP to use the slow (but exact) approach for the final energy evaluation. You will know this happened by examining the output file and looking for:

! Looks like the last energy evaluation.
! The fast density calculation will now be disabled in the interest of accuracy.

Note that this will not be printed if fast_density_output_detail is BRIEF or if fast density would already have been switched off by fast_density_elec_energy_tol (see below). This setting resets any time you start a new NGWF convergence loop – that means that in auto solvation, geometry optimisation, MD, etc. each optimisation will start with fast density turned on.

Also note that this switching is done in the NGWF convergence loop. If you are working with fixed NGWFs (maxit_ngwf_cg 0 (or negative)), this switching will not take place.

Furthermore, particularly if your calculation struggles to converge to the default NGWF threshold, you can set fast_density_elec_energy_tol. This is the energy change per atom between NGWF steps below which ONETEP will switch to the slow (but exact) approach. It’s the same quantity that is used as the energy convergence criterion in elec_energy_tol. The default is 1E-50, effectively turning this off. Setting it to 1E-7 will typically have ONETEP switch to the slow approach for the last few NGWF iterations. The higher you set this, the sooner ONETEP will switch to the slow approach. This, of course, eats into your efficiency gain. You will know if and when this happened by examining the output file and looking for:

! Energy change per atom: 0.30287E-07 Eh < 0.10000E-06.
! The fast density calculation will now be disabled in the interest of accuracy.

Note that this will not be printed if fast_density_output_detail is BRIEF. This setting resets any time you start a new NGWF convergence loop – that means that in auto solvation, geometry optimisation, MD, etc. each optimisation will start with fast density turned on.

Note that you need at least two NGWF iterations to have a meaningful energy change to examine, so this setting has no effect if you take fewer than two NGWF iterations.

Remaining options

The default output detail of fast density is the same as specified for output_detail. You can set it separately by specifying fast_density_output_detail. The available options are the same as for all ONETEP output details: BRIEF, NORMAL, VERBOSE, PROLIX and MAXIMUM.

If, in the future, other methods of trimming NGWFs than by using a fixed threshold become available, you will be able to use fast_density_trim_by to control these. Currently the only supported option is VALUE.

Example settings

For a quick-and-dirty calculation use:

  • fast_density T

  • fast_density_trim_threshold 2E-5.

For a typical calculation just use:

  • fast_density T (which will use the default of fast_density_trim_threshold 2E-6).

For an accurate, but slower calculation use:

  • fast_density T

  • fast_density_trim_threshold 1E-6

  • fast_density_off_for_last T

  • fast_density_elec_energy_tol 1E-7.

For very safe settings that should provide a modest gain in efficiency, try:

  • fast_density T

  • fast_density_trim_threshold 5E-7

  • fast_density_off_for_last T

  • fast_density_elec_energy_tol 3E-7.

Compatilibity

Fast density is known to work (to the best of our knowledge) with the following additional functionalities:

  • extended NGWFs,

  • PBCs and OBCs,

  • implicit solvation,

  • hybrid functionals and Hartree-Fock exchange,

  • fine_grid_scale larger than 2.0,

  • PAW,

  • DFT+U,

  • conduction,

  • MD,

  • geometry optimisation,

  • TS search,

  • NEB,

  • EDFT and LNV.

Fast density is known not to work (this we know with certainty) with the following additional functionalities:

  • complex NGWFs,

  • TD-DFT (mixed bases are not supported at this point).

  • EMFT (regions).

ONETEP will stop with an error if either of these is used with fast_density T.