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Sailfish internals

Architecture overview

Simulation definition

Every Sailfish simulation is defined in terms of a simulation class, typically inheriting from LBFluidSim (or more generally, from LBSim). In this class:

  • additional simulation options can be defined in LBSim.add_options()
  • default values for any simulation parameters can be specified in LBSim.update_defaults()
  • simulation configuration can be altered after command-line parsing in LBSim.modify_config()
  • additional setup (e.g. adding body forces) can be performed in the __init__ method (make sure to call the __init__ method of all base classes at the beginning of your function)
  • boundary and intitial conditions are defined by setting the subdomain attribute to a child of Subdomain (typically Subdomain2D and Subdomain3D)

The global domain of a Sailfish simulation is always a rectangle (2D) or a cuboid (3D), which automatically establishes a global Cartesian coordinate system. The distance between two neighboring lattice nodes is the distance unit. The domain can be sparse (not all nodes in the cuboid exist) and subdivided into smaller units (subdomains). This is accomplished via a child class of LBGeometry (or its specializations LBGeometry2D, LBGeometry3D). This class:

  • specifies domain decomposition into subdomains (LBGeometry.subdomains())
  • defines options specific to the global simulation geometry
  • provides access to the global domain size via the LBGeometry2D.gx,, and LBGeometry3D.gz attributes

LBGeometry.subdomains() returns a list of SubdomainSpec objects. This makes it possible to build a refined, hierarchical grid (to be implemented; by increasing node density in some subdomains) and to define a sparse domain (by returning objects covering only a part of the global coordinate system).

Boundary conditions and initial values of macroscopic fields (density, velocity, etc) are specified in the Subdomain.boundary_conditions() and Subdomain.initial_conditions() methods, respectively. These methods will be called with hx, hy and hz objects, which are numpy coordinate arrays indicating nodes for which values are to be set. The values in these arrays are always in the global coordinate system. The arrays should be used as index objects when accessing field arrays in Sailfish or specifying boundary conditions. Your code in Subdomain.boundary_conditions() and Subdomain.initial_conditions() should always define the global geometry and make no assumptions about its division into subdomains. In particular, the Sailfish framework might arbitrarily subdivide your domain into multiple subdomains to distribute the work among many computational units.

Simulation execution

Sailfish is designed to run fluid simulations in a distributed and hybrid environment, spreading work between multiple machines and GPUs.

The simulation execution begins with an instance of LBSimulationController.

From the command line to a running simulation

This section explains what happens in the first few seconds after you start executing your simulation script and before the simulation is actually running.

Distributed execution

A distributed simulation is started by the controller mapping subdomains to available nodes (as specified in a cluster definition file). This is followed by establishing an SSH connection to all nodes to which at least one block has been assigned. Once the connection is established, the execnet module is used to (optionally) sync files from the controller host to the node, and to execute the _start_cluster_machine_master() function to start a LBMachineMaster on each node. The masters and the controller are then linked by an execnet channel.

Each master starts a LBBlockRunner for each of its subdomains. The runners are executed as subprocesses, and they communicate with the master using zeromq IPC connections. For each connected subdomain pair, one of the subdomains starts a listening zeromq TCP socket with a random port. This port is then communicated to the master, which forwards it to the controller. Once all runners have started, the controller builds a global port map, which is then sent through the masters to all runners, which use it to establish two-way connections between all connected subdomain pairs.

Inside a simulation

This section explains the data structures and data flow of a live simulation.

Template overview and conventions

Specifying configuration options