Commit fb28100d authored by Peter W. Draper's avatar Peter W. Draper
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Merge remote-tracking branch 'origin/master' into repart-by-ticks-with-means

parents 4adcd862 ec378b32
......@@ -7,8 +7,8 @@ General information for adding new schemes
==========================================
The following steps are required for any new options (such as new
:ref:`hydro`, :ref:`chemistry`, :ref:`cooling`,
:ref:`equation_of_state`, :ref:`stars` or :ref:`gravity`)
:ref:`hydro`, chemistry, cooling,
:ref:`equation_of_state`, stars, or gravity)
In order to add a new scheme, you will need to:
......
.. Parameter Files
Matthieu Schaller, 21st October 2018
Josh Borrow 22nd January 2019
.. _Parameter_File_label:
Parameter Files
===============
.. _Parameters_basics:
This section desrcibes the options that are available in the
parameter files.
File format and basic information
---------------------------------
The parameter file uses a format similar to the `YAML format
<https://en.wikipedia.org/wiki/YAML>`_ but reduced to only the
elements required for the SWIFT parameters. Options are given by a
name followed by a column and the value of the parameter:
.. code:: YAML
ICs: santa_barbara.hdf5
dt_max: 1.5
shift: [2., 4., 5.]
Comments can be inserted anywhere and start with a hash:
.. code:: YAML
# Description of the physics
viscosity_alpha: 2.0
dt_max: 1.5 # seconds
A typical SWIFT parameter file is split into multiple sections that
may or may not be present depending on the different configuration
options. The sections start with a label and can contain any number of
parameters:
.. code:: YAML
Cosmology: # Planck13
Omega_m: 0.307
Omega_lambda: 0.693
Omega_b: 0.0455
h: 0.6777
a_begin: 0.0078125 # z = 127
The options can be integer values, floating point numbers, characters
or strings. If SWIFT expects a number and string is given, an error
will be raised. The code can also read an array of values:
.. code:: YAML
shift: [2., 4., 5.]
Some options in the parameter file are optional and
when not provided, SWIFT will run with the default value. However, if
a compulsory parameter is missing an error will be raised at
start-up.
Finally, SWIFT outputs two YAML files at the start of a run. The first one
``used_parameters.yml`` contains all the parameters that were used for this run,
**including all the optional parameters left unspecified with their default
values**. This file can be used to start an exact copy of the run. The second
file, ``unused_parameters.yml`` contains all the values that were not read from
the parameter file. This can be used to simplify the parameter file or check
that nothing important was ignored (for instance because the code is not
configured to use some options).
The rest of this page describes all the SWIFT parameters, split by
section. A list of all the possible parameters is kept in the file
``examples/parameter_examples.yml``.
.. _Parameters_units:
Internal Unit System
--------------------
The ``InternalUnitSystem`` section describes the units used internally by the
code. This is the system of units in which all the equations are solved. All
physical constants are converted to this system and if the ICs use a different
system (see the snapshots' ref:`ICs_units_label` section of the documentation)
the particle quantities will be converted when read in.
The system of units is described using the value of the 5 basic units
of any system with respect to the CGS system. Instead of using a unit
of time we use a unit of velocity as this is more intuitive. Users
hence need to provide:
* a unit of length: ``UnitLength_in_cgs``,
* a unit of mass: ``UnitMass_in_cgs``,
* a unit of velocity ``UnitVelocity_in_cgs``,
* a unit of electric current ``UnitCurrent_in_cgs``,
* a unit of temperature ``UnitTemp_in_cgs``.
All these need to be expressed with respect to their cgs counter-part
(i.e. :math:`cm`, :math:`g`, :math:`cm/s`, :math:`A` and :math:`K`). Recall
that there are no h-factors in any of SWIFT's quantities; we, for instance,
use :math:`cm` and not :math:`cm/h`.
For instance to use the commonly adopted system of 10^10 Msun as a
unit for mass, mega-parsec as a unit of length and km/s as a unit of
speed, we would use:
.. code:: YAML
# Common unit system for cosmo sims
InternalUnitSystem:
UnitMass_in_cgs: 1.98848e43 # 10^10 M_sun in grams
UnitLength_in_cgs: 3.08567758e24 # 1 Mpc in centimeters
UnitVelocity_in_cgs: 1e5 # 1 km/s in centimeters per second
UnitCurrent_in_cgs: 1 # 1 Ampere
UnitTemp_in_cgs: 1 # 1 Kelvin
Note that there are currently no variables in any of the SWIFT physics
schemes that make use of the unit of electric current. There is also
no incentive to use anything else than Kelvin but that makes the whole
system consistent with any possible unit system.
If one is interested in using the more humorous `FFF unit
system <https://en.wikipedia.org/wiki/FFF_system>`_ one would use
.. code:: YAML
# FFF unit system
InternalUnitSystem:
UnitMass_in_cgs: 40823.3133 # 1 Firkin (fir) in grams
UnitLength_in_cgs: 20116.8 # 1 Furlong (fur) in cm
UnitVelocity_in_cgs: 0.01663095 # 1 Furlong (fur) per Fortnight (ftn) in cm/s
UnitCurrent_in_cgs: 1 # 1 Ampere
UnitTemp_in_cgs: 1 # 1 Kelvin
The value of the physical constants in this system is left as an
exercise for the reader [#f1]_.
.. _Parameters_cosmology:
Cosmology
---------
When running a cosmological simulation, the section ``Cosmology`` sets the values of the
cosmological model. The expanded :math:`\Lambda\rm{CDM}` parameters governing the
background evolution of the Universe need to be specified here. These are:
* The reduced Hubble constant: :math:`h`: ``h``,
* The matter density parameter :math:`\Omega_m`: ``Omega_m``,
* The cosmological constant density parameter :math:`\Omega_\Lambda`: ``Omega_lambda``,
* The baryon density parameter :math:`\Omega_b`: ``Omega_b``,
* The radiation density parameter :math:`\Omega_r`: ``Omega_r``.
The last parameter can be omitted and will default to :math:`\Omega_r = 0`. Note
that SWIFT will verify on start-up that the matter content of the initial conditions
matches the cosmology specified in this section.
This section also specifies the start and end of the simulation expressed in
terms of scale-factors. The two parameters are:
* Initial scale-factor: ``a_begin``,
* Final scale-factor: ``a_end``.
Two additional optional parameters can be used to change the equation of
state of dark energy :math:`w(a)`. We use the evolution law :math:`w(a) =
w_0 + w_a (1 - a)`. The two parameters in the YAML file are:
* The :math:`z=0` dark energy equation of state parameter :math:`w_0`: ``w_0``
* The dark energy equation of state evolution parameter :math:`w_a`: ``w_a``
If unspecified these parameters default to the default
:math:`\Lambda\rm{CDM}` values of :math:`w_0 = -1` and :math:`w_a = 0`.
For a Planck+13 cosmological model (ignoring radiation density as is
commonly done) and running from :math:`z=127` to :math:`z=0`, one would hence
use the following parameters:
.. code:: YAML
Cosmology:
a_begin: 0.0078125 # z = 127
a_end: 1.0 # z = 0
h: 0.6777
Omega_m: 0.307
Omega_lambda: 0.693
Omega_b: 0.0455
Omega_r: 0. # (Optional)
w_0: -1.0 # (Optional)
w_a: 0. # (Optional)
When running a non-cosmological simulation (i.e. without the ``-c`` run-time
flag) this section of the YAML file is entirely ignored.
.. _Parameters_gravity:
Gravity
-------
The behaviour of the self-gravity solver can be modified by the parameters
provided in the ``Gravity`` section. The theory document puts these parameters into the
context of the equations being solved. We give a brief overview here.
* The Plummer-equivalent co-moving softening length used for all particles :math:`\epsilon_{com}`: ``comoving_softening``,
* The Plummer-equivalent maximal physical softening length used for all particles :math:`\epsilon_{max}`: ``comoving_softening``,
At any redshift :math:`z`, the Plummer-equivalent softening length used by the
code will be :math:`\epsilon=\min(\epsilon_{max},
\frac{\epsilon_{com}}{z+1})`. This is expressed in internal units.
* The opening angle (multipole acceptance criterion) used in the FMM :math:`\theta`: ``theta``,
* The time-step size pre-factor :math:`\eta`: ``eta``,
The time-step of a given particle is given by :math:`\Delta t =
\eta\sqrt{\frac{\epsilon}{|\overrightarrow{a}|}}`, where
:math:`\overrightarrow{a}` is the particle's acceleration. Power et al. (2003) recommend using :math:`\eta=0.025`.
The last tree-related parameter is
* The tree rebuild frequency: ``rebuild_frequency``.
The tree rebuild frequency is an optional parameter defaulting to
:math:`0.01`. It is used to trigger the re-construction of the tree every time a
fraction of the particles have been integrated (kicked) forward in time.
Simulations using periodic boundary conditions use additional parameters for the
Particle-Mesh part of the calculation. The last three are optional:
* The number cells along each axis of the mesh :math:`N`: ``mesh_side_length``,
* The mesh smoothing scale in units of the mesh cell-size :math:`a_{\rm
smooth}`: ``a_smooth`` (default: ``1.25``),
* The scale above which the short-range forces are assumed to be 0 (in units of
the mesh cell-size multiplied by :math:`a_{\rm smooth}`) :math:`r_{\rm
cut,max}`: ``r_cut_max`` (default: ``4.5``),
* The scale below which the short-range forces are assumed to be exactly Newtonian (in units of
the mesh cell-size multiplied by :math:`a_{\rm smooth}`) :math:`r_{\rm
cut,min}`: ``r_cut_min`` (default: ``0.1``),
For most runs, the default values can be used. Only the number of cells along
each axis needs to be specified. The remaining three values are best described
in the context of the full set of equations in the theory documents.
As a summary, here are the values used for the EAGLE :math:`100^3~{\rm Mpc}^3`
simulation:
.. code:: YAML
# Parameters for the self-gravity scheme for the EAGLE-100 box
Gravity:
eta: 0.025
theta: 0.7
comoving_softening: 0.0026994 # 0.7 proper kpc at z=2.8.
max_physical_softening: 0.0007 # 0.7 proper kpc
rebuild_frequency: 0.01 # Default optional value
mesh_side_length: 512
a_smooth: 1.25 # Default optional value
r_cut_max: 4.5 # Default optional value
r_cut_min: 0.1 # Default optional value
.. _Parameters_SPH:
SPH
---
.. _Parameters_time_integration:
Time Integration
----------------
The ``TimeIntegration`` section is used to set some general parameters related to time
integration. In all cases, users have to provide a minimal and maximal time-step
size:
* Maximal time-step size: ``dt_max``
* Minimal time-step size: ``dt_min``
These quantities are expressed in internal units. All particles will have their
time-step limited by the maximal value on top of all the other criteria that may
apply to them (gravity acceleration, Courant condition, etc.). If a particle
demands a time-step size smaller than the minimum, SWIFT will abort with an
error message. This is a safe-guard against simulations that would never
complete due to the number of steps to run being too large.
When running a non-cosmological simulation, the user also has to provide the
time of the start and the time of the end of the simulation:
* Start time: ``time_begin``
* End time: ``time_end``
Both are expressed in internal units. The start time is typically set to ``0``
but SWIFT can handle any value here. For cosmological runs, these values are
ignored and the start- and end-points of the runs are specified by the start and
end scale-factors in the cosmology section of the parameter file.
Additionally, when running a cosmological volume, advanced users can specify the
value of the dimensionless pre-factor entering the time-step condition linked
with the motion of particles with respect to the background expansion and mesh
size. See the theory document for the exact equations.
* Dimensionless pre-factor of the maximal allowed displacement:
``max_dt_RMS_factor`` (default: ``0.25``)
This value rarely needs altering.
A full time-step section for a non-cosmological run would be:
.. code:: YAML
TimeIntegration:
time_begin: 0 # Start time in internal units.
time_end: 10. # End time in internal units.
dt_max: 1e-2
dt_min: 1e-6
Whilst for a cosmological run, one would need:
.. code:: YAML
TimeIntegration:
dt_max: 1e-4
dt_min: 1e-10
max_dt_RMS_factor: 0.25 # Default optional value
.. _Parameters_ICs:
Initial Conditions
------------------
The ``InitialConditions`` section of the parameter file contains all the options related to
the initial conditions. The main two parameters are
* The name of the initial conditions file: ``file_name``,
* Whether the problem uses periodic boundary conditions or not: ``periodic``.
The file path is relative to where the code is being executed. These
parameters can be complemented by some optional values to drive some
specific behaviour of the code.
* Whether to generate gas particles from the DM particles: ``generate_gas_in_ics`` (default: ``0``),
* Whether to activate an additional clean-up of the SPH smoothing lengths: ``cleanup_smoothing_lengths`` (default: ``0``)
The procedure used to generate gas particles from the DM ones is
outlined in the theory documents and is too long for a full
description here. The cleaning of the smoothing lengths is an
expensive operation but can be necessary in the cases where the
initial conditions are of poor quality and the values of the smoothing
lengths are far from the values they should have.
When starting from initial conditions created for Gadget, some
additional flags can be used to convert the values from h-full to
h-free and remove the additional :math:`\sqrt{a}` in the velocities:
* Whether to re-scale all the fields to remove powers of h from the quantities: ``cleanup_h_factors`` (default: ``0``),
* Whether to re-scale the velocities to remove the :math:`\sqrt{a}` assumed by Gadget : ``cleanup_velocity_factors`` (default: ``0``).
The h-factors are self-consistently removed according to their units
and this is applied to all the quantities irrespective of particle
types. The correct power of ``h`` is always calculated for each
quantity.
Finally, SWIFT also offers these options:
* A factor to re-scale all the smoothing-lengths by a fixed amount: ``smoothing_length_scaling`` (default: ``1.``),
* A shift to apply to all the particles: ``shift`` (default: ``[0.0,0.0,0.0]``),
* Whether to replicate the box along each axis: ``replicate`` (default: ``1``).
The shift is expressed in internal units. The option to replicate the
box is especially useful for weak-scaling tests. When set to an
integer >1, the box size is multiplied by this integer along each axis
and the particles are duplicated and shifted such as to create exact
copies of the simulation volume.
The full section to start a DM+hydro run from Gadget DM-only ICs would
be:
.. code:: YAML
InitialConditions:
file_name: my_ics.hdf5
periodic: 1
cleanup_h_factors: 1
cleanup_velocity_factors: 1
generate_gas_in_ics: 1
cleanup_smoothing_lengths: 1
.. _Parameters_constants:
Physical Constants
------------------
For some idealised test it can be useful to overwrite the value of
some physical constants; in particular the value of the gravitational
constant. SWIFT offers an optional parameter to overwrite the value of
:math:`G_N`.
.. code:: YAML
PhysicalConstants:
G: 1
Note that this set :math:`G` to the specified value in the internal system
of units. Setting a value of `1` when using the system of units (10^10 Msun,
Mpc, km/s) will mean that :math:`G_N=1` in these units [#f2]_ instead of the
normal value :math:`G_N=43.00927`.
This option is only used for specific tests and debugging. This entire
section of the YAML file can typically be left out. More constants may
be handled in the same way in future versions.
.. _Parameters_snapshots:
Snapshots
---------
The ``Snapshots`` section of the parameter file contains all the options related to
the dump of simulation outputs in the form of HDF5 :ref:`snapshots`. The main
parameter is the base name that will be used for all the outputs in the run:
* The base name of the HDF5 snapshots: ``basename``.
This name will then be appended by an under-score and 6 digits followed by
``.hdf5`` (e.g. ``base_name_001234.hdf5``). The 6 digits are used to label the
different outputs, starting at ``000000``. In the default setup the digits
simply increase by one for each snapshot. However, if the optional parameter
``int_time_label_on`` is switched on, then the 6-digits will the physical time
of the simulation rounded to the nearest integer [#f3]_.
The time of the first snapshot is controlled by the two following options:
* Time of the first snapshot (non-cosmological runs): ``time_first``,
* Scale-factor of the first snapshot (cosmological runs): ``scale_factor_first``.
One of those two parameters has to be provided depending on the type of run. In
the case of non-cosmological runs, the time of the first snapshot is expressed
in the internal units of time. Users also have to provide the difference in time
(or scale-factor) between consecutive outputs:
* Time difference between consecutive outputs: ``delta_time``.
In non-cosmological runs this is also expressed in internal units. For
cosmological runs, this value is *multiplied* to obtain the scale-factor of the
next snapshot. This implies that the outputs are equally space in
:math:`\log(a)` (See :ref:`Output_list_label` to have snapshots not regularly spaced in time).
Users can optionally specify the level of compression used by the HDF5 libary
using the parameter:
* GZIP compression level of the HDF5 arrays: ``compression`` (default: ``0``).
The default level of ``0`` implies no compression and values have to be in the
range :math:`[0-9]`. This integer is passed to the i/o library and used for the
lossless GZIP compression algorithm. Higher values imply higher compression but
also more time spent deflating and inflating the data. Note that up until HDF5
1.10.x this option is not available when using the MPI-parallel version of the
i/o routines.
Finally, it is possible to specify a different system of units for the snapshots
than the one that was used internally by SWIFT. The format is identical to the
one described above (See the :ref:`Parameters_units` section) and read:
* a unit of length: ``UnitLength_in_cgs`` (default: ``InternalUnitSystem:UnitLength_in_cgs``),
* a unit of mass: ``UnitMass_in_cgs`` (default: ``InternalUnitSystem:UnitMass_in_cgs``),
* a unit of velocity ``UnitVelocity_in_cgs`` (default: ``InternalUnitSystem:UnitVelocity_in_cgs``),
* a unit of electric current ``UnitCurrent_in_cgs`` (default: ``InternalUnitSystem:UnitCurrent_in_cgs``),
* a unit of temperature ``UnitTemp_in_cgs`` (default: ``InternalUnitSystem:UnitTemp_in_cgs``).
When un-specified, these all take the same value as assumed by the internal
system of units. These are rarely used but can offer a practical alternative to
converting data in the post-processing of the simulations.
For a standard cosmological run, the full section would be:
.. code:: YAML
Snapshots:
basename: output
scale_factor_first: 0.02 # z = 49
delta_time: 1.02
Showing all the parameters for a basic hydro test-case, one would have:
.. code:: YAML
Snapshots:
basename: sedov
time_first: 0.01
delta_time: 0.005
int_time_label_on: 0
compression: 3
UnitLength_in_cgs: 1. # Use cm in outputs
UnitMass_in_cgs: 1. # Use grams in outpus
UnitVelocity_in_cgs: 1. # Use cm/s in outputs
UnitCurrent_in_cgs: 1. # Use Ampere in outputs
UnitTemp_in_cgs: 1. # Use Kelvin in outputs
Some additional specific options for the snapshot outputs are described in the
following pages:
* :ref:`Output_list_label` (to have snapshots not evenly spaced in time),
* :ref:`Output_selection_label` (to select what particle fields to write).
.. _Parameters_statistics:
Statistics
----------
Some additional specific options for the statistics outputs are described in the
following page:
* :ref:`Output_list_label` (to have statistics outputs not evenly spaced in time).
.. _Parameters_restarts:
Restarts
--------
SWIFT can write check-pointing files and restart from them. The behaviour of
this mechanism is driven by the options in the ``Restarts`` section of the YAML
parameter file. All the parameters are optional but default to values that
ensure a reasonable behaviour.
* Whether or not to enable the dump of restart files: ``enable`` (default:
``1``).
This parameter acts a master-switch for the check-pointing capabilities. All the
other options require the ``enable`` parameter to be set to ``1``.
* Whether or not to save a copy of the previous set of check-pointing files:
``save`` (default: ``1``),
* Whether or not to dump a set of restart file on regular exit: ``onexit``
(default: ``0``),
* The wall-clock time in hours between two sets of restart files:
``delta_hours`` (default: ``6.0``).
Note that there is no buffer time added to the ``delta_hours`` value. If the
system's batch queue run time limit is set to 6 hours, the user must specify a
smaller value to allow for enough time to safely dump the check-point files.
* The sub-directory in which to store the restart files: ``subdir`` (default:
``restart``),
* The basename of the restart files: ``basename`` (default: ``swift``)
If the directory does not exist, SWIFT will create it. When resuming a run,
SWIFT, will look for files with the name provided in the sub-directory specified
here. The files themselves are named ``basename_000001.rst`` where the basename
is replaced by the user-specified name and the 6-digits number corresponds to
the MPI-rank. SWIFT writes one file per MPI rank. If the ``save`` option has
been activated, the previous set of restart files will be named
``basename_000000.rst.prev``.
SWIFT can also be stopped by creating an empty file called ``stop`` in the
directory where the code runs. This will make SWIFT dump a fresh set of restart
file (irrespective of the specified ``delta_time`` between dumps) and exit
cleanly. One parameter governs this behaviour:
* Number of steps between two checks for the presence of a ``stop`` file:
``stop_steps`` (default: ``100``).
The default value is chosen such that SWIFT does not need to poll the
file-system to often, which can take a significant amount of time on distributed
systems. For runs where the small time-steps take a much larger amount of time,
a smaller value is recommended to allow for a finer control over when the code
can be stopped.
Finally, SWIFT can automatically stop after a specified amount of wall-clock
time. The code can also run a command when exiting in this fashion, which can be
used, for instance, to interact with the batch queue system:
* Maximal wall-clock run time in hours: ``max_run_time`` (default: ``24.0``),
* Whether or not to run a command on exit: ``resubmit_on_exit`` (default:
``0``),
* The command to run on exit: ``resubmit_command`` (default: ``./resub.sh``).
Note that no check is performed on the validity of the command to run. SWIFT
simply calls ``system()`` with the user-specified command.
To run SWIFT, dumping check-pointing files every 6 hours and running for 24
hours after which a shell command will be run, one would use:
.. code:: YAML
Restarts:
enable: 1
save: 1 # Keep copies
onexit: 0
subdir: restart # Sub-directory of the directory where SWIFT is run
basename: swift
delta_hours: 6.0
stop_steps: 100
max_run_time: 24.0 # In hours
resubmit_on_exit: 1
resubmit_command: ./resub.sh
.. _Parameters_scheduler:
Scheduler
---------
.. _Parameters_domain_decomposition:
Domain Decomposition:
---------------------
This section determines how the top-level cells are distributed between the
ranks of an MPI run. An ideal decomposition should result in each rank having
a similar amount of work to do, so that all the ranks complete at the same
time. Achieving a good balance requires that SWIFT is compiled with either the
ParMETIS or METIS libraries. ParMETIS is an MPI version of METIS, so is
preferred for performance reasons.
When we use ParMETIS/METIS the top-level cells of the volume are considered as
a graph, with a cell at each vertex and edges that connect the vertices to all
the neighbouring cells (so we have 26 edges connected to each vertex).
Decomposing such a graph into domains is known as partitioning, so in SWIFT we
refer to domain decomposition as partitioning.
This graph of cells can have weights associated with the vertices and the
edges. These weights are then used to guide the partitioning, seeking to
balance the total weight of the vertices and minimize the weights of the edges
that are cut by the domain boundaries (known as the edgecut). We can consider
the edge weights as a proxy for the exchange of data between cells, so
minimizing this reduces communication.