Output List

In the sections Snapshots, Statistics and StructureFinding, you can specify the options output_list_on and output_list which receive an int and a filename. The output_list_on enable or not the output list and output_list is the filename containing the output times. With the file header, you can choose between writing redshifts, scale factors or times.

Example of file containing with times (in internal units):

# Time
0.5
1.5
3.0
12.5

Example of file with scale factors:

# Scale Factor
0.1
0.2
0.3

Example of file with redshift:

# Redshift
20
15
10
5

If an output list is specified, the basic values for the first snapshot (time_first, scale_factor_first) and difference (delta_time) are ignored.

When an output list is used SWIFT will not write a “0th” snapshot straight after having read the ICs. Similarly, SWIFT will also not write a snapshot at the end of a simulation unless a snapshot at the final time is specified in the list.

Output Selection

Users can generate a yaml file containing all the possible fields available for a given configuration of SWIFT by running ./swift --output-params output.yml or equivalently ./swift -o output.yml. The file generated contains the list of fields that a simulation running with this config would output in each snapshot. It also lists the description string of each field and the unit conversion string to go from internal comoving units to physical CGS. Entries in the file look like:

Default:
  # Particle Type Gas
  Coordinates_Gas: off  # Co-moving positions of the particles : a U_L  [ cm ]
  Velocities_Gas: on  # Peculiar velocities of the stars. This is (a * dx/dt) where x is the co-moving positions of the particles : U_L U_t^-1  [ cm s^-1 ]
  Masses_Gas: on  # Masses of the particles : U_M  [ g ]
  SmoothingLengths_Gas: on  # Co-moving smoothing lengths (FWHM of the kernel) of the particles : a U_L  [ cm ]
  ...

For cosmological simulations, users can optionally add the --cosmology flag to generate the field names appropriate for such a run.

Users can select the particle fields to output in snapshot using a (separate) YAML parameter file. By default, you can define a section Default at the top level of this file (in the exact same way as the file dumped by using the -o option in SWIFT). By default, all fields are written, but by using the “off” string, you can force the code to skip over unwanted outputs.

You must then, in the regular SWIFT parameter file, select the following options:

Snapshots:
  select_output_on: 1
  select_output: your_select_output_yaml.yml

This field is mostly used to remove unnecessary output by listing them as “off”. A classic use-case for this feature is a DM-only simulation (pure N-body) where all particles have the same mass. Outputting the mass field in the snapshots results in extra i/o time and unnecessary waste of disk space. The corresponding section of the YAML file would look like:

Default:
  Masses_DM: off

Entries can simply be copied from the output.yml generated by the -o runtime flag.

For convenience, there is also the option to set a default output status for all fields of a particular particle type. This can be used, for example, to skip an entire particle type in certain snapshots (see below for how to define per-snapshot output policies). This is achieved with the special Standard field for each particle type:

BlackHolesOnly:
  Standard_Gas: off
  Standard_DM: off
  Standard_DMBackground: off
  Standard_Stars: off
  Standard_BH: on  # Not strictly necessary, on is already the default

Combining Output Lists and Output Selection

It is possible to combine the behaviour of the output list and the select output file. To do so, you will need to enable both the select_output and output_list options in your main parameter_file.yml as follows:

Snapshots:
  output_list_on: 1
  output_list: "output_list.txt"
  select_output_on: 1
  select_output: "select_output.yml"

A typical use case for such a scenario is the dumping of ‘snapshots’ and so-called ‘snipshots’, containing less information than their full snapshot cousins. To do this, we will define two top-level sections in our select_output.yml file as follows:

# Only turn off DM masses in snapshots, everything else is turned on
Snapshot:
  Masses_DM: off

# Turn off lots of stuff in snipshots!
Snipshot:
  Metal_Mass_Fractions_Gas: off
  Element_Mass_Fractions_Gas: off
  ...

To then select which outputs are ‘snapshots’ and which are ‘snipshots’, you will need to add the Select Output column to the output_list.txt as follows:

# Redshift, Select Output
100.0, Snapshot
90.0, Snipshot
80.0, Snipshot
70.0, Snipshot
60.0, Snapshot
...

This will enable your simulation to perform partial dumps only at the outputs labelled as Snipshot. The name of the output selection that corresponds to your choice in the output list will be written to the snapshot header as Header/SelectOutput.