Avoid atomic_cas in open boundaries and add gpu tests (#849)

* make activating particle thread save

* fix assertion

* fix gpu

* use bool-vector

* add gpu tests

* fix unit test

* fix

* implement suggestions

---------

Co-authored-by: LasNikas <niklas.nehe@web.de>

Author: LasNikas

src/general/buffer.jl

@@ -7,12 +7,7 @@ end
 
 function SystemBuffer(active_size, buffer_size::Integer)
     # Using a `BitVector` is not an option as writing to it is not thread-safe.
-    # Also, to ensure thread-safe particle activation, we use an `atomic_cas` operation.
-    # Thus, `active_particle` is defined as a `Vector{UInt32}` because CUDA.jl
-    # does not support atomic operations on `Bool`.
-    # https://github.com/JuliaGPU/CUDA.jl/blob/2cc9285676a4cd28d0846ca62f0300c56d281d38/src/device/intrinsics/atomics.jl#L243
-    active_particle = vcat(fill(UInt32(true), active_size),
-                           fill(UInt32(false), buffer_size))
+    active_particle = vcat(fill(true, active_size), fill(false, buffer_size))
     eachparticle = collect(eachindex(active_particle))
 
     return SystemBuffer(active_particle, Ref(active_size), eachparticle, buffer_size)
@@ -49,9 +44,8 @@ end
 
     # TODO: Parallelize (see https://github.com/trixi-framework/TrixiParticles.jl/issues/810)
     # Update the number of active particles and the active particle indices
-    buffer.active_particle_count[] = sum(active_particle)
-    buffer.eachparticle[1:buffer.active_particle_count[]] .= findall(x -> x == true,
-                                                                     active_particle)
+    buffer.active_particle_count[] = count(active_particle)
+    buffer.eachparticle[1:buffer.active_particle_count[]] .= findall(active_particle)
 
     return buffer
 end
@@ -64,29 +58,6 @@ end
     return view(buffer.eachparticle, 1:buffer.active_particle_count[])
 end
 
-@inline function activate_next_particle(system)
-    (; active_particle) = system.buffer
-
-    for particle in eachindex(active_particle)
-        if PointNeighbors.Atomix.@atomic(active_particle[particle]) == false
-            # After we go into this condition, another thread might still activate this particle
-            # before we do. Therefore, we use an atomic swap, which activates the particle,
-            # but also returns the old value.
-            # If the old value is `true`, the particle was active before and we need to continue.
-            # Note: This doesn't work with Metal.jl. No error is thrown, but the operation is simply ignored.
-            # An atomic compare-and-swap operation is probably implemented for Metal.jl here:
-            # https://github.com/JuliaGPU/Metal.jl/blob/caf299690aa52448ee72ffc5688939b157fc1ba2/src/device/intrinsics/atomics.jl#L42
-            was_active = PointNeighbors.Atomix.@atomicswap active_particle[particle] = true
-
-            if was_active == false
-                return particle
-            end
-        end
-    end
-
-    error("No buffer particles available")
-end
-
 @inline function deactivate_particle!(system, particle, u)
     (; active_particle) = system.buffer
 
@@ -96,7 +67,8 @@ end
         u[dim, particle] = eltype(system)(1e16)
     end
 
-    # `activate_next_particle!` and `deactivate_particle!` are never called on the same buffer inside a kernel,
+    # `deactivate_particle!` and `active_particle[particle] = true`
+    # are never called on the same buffer inside a kernel,
     # so we don't have any race conditions on this `active_particle` vector.
     active_particle[particle] = false
 

src/general/semidiscretization.jl

@@ -964,6 +964,8 @@ function set_system_links(system::OpenBoundarySPHSystem, semi)
                                  system.density,
                                  system.volume,
                                  system.pressure,
+                                 system.boundary_candidates,
+                                 system.fluid_candidates,
                                  system.boundary_zone,
                                  system.reference_velocity,
                                  system.reference_pressure,

src/schemes/boundary/open_boundary/system.jl

@@ -36,7 +36,7 @@ Open boundary system for in- and outflow particles.
     This is an experimental feature and may change in future releases.
     It is GPU-compatible (e.g., with CUDA.jl and AMDGPU.jl), but currently **not** supported with Metal.jl.
 """
-struct OpenBoundarySPHSystem{BM, ELTYPE, NDIMS, IC, FS, FSI, ARRAY1D, BZ, RV,
+struct OpenBoundarySPHSystem{BM, ELTYPE, NDIMS, IC, FS, FSI, ARRAY1D, BC, FC, BZ, RV,
                              RP, RD, B, UCU, C} <: System{NDIMS}
     boundary_model       :: BM
     initial_condition    :: IC
@@ -47,6 +47,8 @@ struct OpenBoundarySPHSystem{BM, ELTYPE, NDIMS, IC, FS, FSI, ARRAY1D, BZ, RV,
     density              :: ARRAY1D # Array{ELTYPE, 1}: [particle]
     volume               :: ARRAY1D # Array{ELTYPE, 1}: [particle]
     pressure             :: ARRAY1D # Array{ELTYPE, 1}: [particle]
+    boundary_candidates  :: BC      # Array{UInt32, 1}: [particle]
+    fluid_candidates     :: FC      # Array{UInt32, 1}: [particle]
     boundary_zone        :: BZ
     reference_velocity   :: RV
     reference_pressure   :: RP
@@ -58,18 +60,21 @@ end
 
 function OpenBoundarySPHSystem(boundary_model, initial_condition, fluid_system,
                                fluid_system_index, smoothing_length, mass, density, volume,
-                               pressure, boundary_zone, reference_velocity,
+                               pressure, boundary_candidates, fluid_candidates,
+                               boundary_zone, reference_velocity,
                                reference_pressure, reference_density, buffer,
                                update_callback_used, cache)
     OpenBoundarySPHSystem{typeof(boundary_model), eltype(mass), ndims(initial_condition),
                           typeof(initial_condition), typeof(fluid_system),
-                          typeof(fluid_system_index), typeof(mass), typeof(boundary_zone),
-                          typeof(reference_velocity), typeof(reference_pressure),
-                          typeof(reference_density), typeof(buffer),
-                          typeof(update_callback_used),
+                          typeof(fluid_system_index), typeof(mass),
+                          typeof(boundary_candidates), typeof(fluid_candidates),
+                          typeof(boundary_zone), typeof(reference_velocity),
+                          typeof(reference_pressure), typeof(reference_density),
+                          typeof(buffer), typeof(update_callback_used),
                           typeof(cache)}(boundary_model, initial_condition, fluid_system,
                                          fluid_system_index, smoothing_length, mass,
-                                         density, volume, pressure, boundary_zone,
+                                         density, volume, pressure, boundary_candidates,
+                                         fluid_candidates, boundary_zone,
                                          reference_velocity, reference_pressure,
                                          reference_density, buffer, update_callback_used,
                                          cache)
@@ -153,9 +158,13 @@ function OpenBoundarySPHSystem(boundary_zone::BoundaryZone;
 
     smoothing_length = initial_smoothing_length(fluid_system)
 
+    boundary_candidates = fill(false, nparticles(initial_condition))
+    fluid_candidates = fill(false, nparticles(fluid_system))
+
     return OpenBoundarySPHSystem(boundary_model, initial_condition, fluid_system,
                                  fluid_system_index, smoothing_length, mass, density,
-                                 volume, pressure, boundary_zone, reference_velocity_,
+                                 volume, pressure, boundary_candidates, fluid_candidates,
+                                 boundary_zone, reference_velocity_,
                                  reference_pressure_, reference_density_, buffer,
                                  update_callback_used, cache)
 end
@@ -278,37 +287,67 @@ end
 update_open_boundary_eachstep!(system, v_ode, u_ode, semi, t) = system
 
 function check_domain!(system, v, u, v_ode, u_ode, semi)
-    (; boundary_zone) = system
+    (; boundary_zone, boundary_candidates, fluid_candidates) = system
     fluid_system = corresponding_fluid_system(system, semi)
 
     u_fluid = wrap_u(u_ode, fluid_system, semi)
     v_fluid = wrap_v(v_ode, fluid_system, semi)
 
+    boundary_candidates .= false
+
     # Check the boundary particles whether they're leaving the boundary zone
     @threaded semi for particle in each_moving_particle(system)
         particle_coords = current_coords(u, system, particle)
 
         # Check if boundary particle is outside the boundary zone
         if !is_in_boundary_zone(boundary_zone, particle_coords)
-            convert_particle!(system, fluid_system, boundary_zone, particle,
-                              v, u, v_fluid, u_fluid)
+            boundary_candidates[particle] = true
         end
     end
 
+    crossed_boundary_particles = findall(boundary_candidates)
+    available_fluid_particles = findall(==(false), fluid_system.buffer.active_particle)
+
+    @assert length(crossed_boundary_particles)<=length(available_fluid_particles) "Not enough fluid buffer particles available"
+
+    # Convert open boundary particles in the fluid domain to fluid particles
+    @threaded semi for i in eachindex(crossed_boundary_particles)
+        particle = crossed_boundary_particles[i]
+        particle_new = available_fluid_particles[i]
+
+        convert_particle!(system, fluid_system, boundary_zone, particle, particle_new,
+                          v, u, v_fluid, u_fluid)
+    end
+
     update_system_buffer!(system.buffer, semi)
     update_system_buffer!(fluid_system.buffer, semi)
 
+    fluid_candidates .= false
+
     # Check the fluid particles whether they're entering the boundary zone
     @threaded semi for fluid_particle in each_moving_particle(fluid_system)
         fluid_coords = current_coords(u_fluid, fluid_system, fluid_particle)
 
         # Check if fluid particle is in boundary zone
         if is_in_boundary_zone(boundary_zone, fluid_coords)
-            convert_particle!(fluid_system, system, boundary_zone, fluid_particle,
-                              v, u, v_fluid, u_fluid)
+            fluid_candidates[fluid_particle] = true
         end
     end
 
+    crossed_fluid_particles = findall(fluid_candidates)
+    available_boundary_particles = findall(==(false), system.buffer.active_particle)
+
+    @assert length(crossed_fluid_particles)<=length(available_boundary_particles) "Not enough boundary buffer particles available"
+
+    # Convert fluid particles in the open boundary zone to open boundary particles
+    @threaded semi for i in eachindex(crossed_fluid_particles)
+        particle = crossed_fluid_particles[i]
+        particle_new = available_boundary_particles[i]
+
+        convert_particle!(fluid_system, system, boundary_zone, particle, particle_new,
+                          v, u, v_fluid, u_fluid)
+    end
+
     update_system_buffer!(system.buffer, semi)
     update_system_buffer!(fluid_system.buffer, semi)
 
@@ -317,21 +356,21 @@ end
 
 # Outflow particle is outside the boundary zone
 @inline function convert_particle!(system::OpenBoundarySPHSystem, fluid_system,
-                                   boundary_zone::BoundaryZone{OutFlow}, particle, v, u,
-                                   v_fluid, u_fluid)
+                                   boundary_zone::BoundaryZone{OutFlow}, particle,
+                                   particle_new, v, u, v_fluid, u_fluid)
     deactivate_particle!(system, particle, u)
 
     return system
 end
 
 # Inflow particle is outside the boundary zone
 @inline function convert_particle!(system::OpenBoundarySPHSystem, fluid_system,
-                                   boundary_zone::BoundaryZone{InFlow}, particle, v, u,
-                                   v_fluid, u_fluid)
+                                   boundary_zone::BoundaryZone{InFlow}, particle,
+                                   particle_new, v, u, v_fluid, u_fluid)
     (; spanning_set) = boundary_zone
 
     # Activate a new particle in simulation domain
-    transfer_particle!(fluid_system, system, particle, v_fluid, u_fluid, v, u)
+    transfer_particle!(fluid_system, system, particle, particle_new, v_fluid, u_fluid, v, u)
 
     # Reset position of boundary particle
     for dim in 1:ndims(system)
@@ -343,8 +382,8 @@ end
 
 # Buffer particle is outside the boundary zone
 @inline function convert_particle!(system::OpenBoundarySPHSystem, fluid_system,
-                                   boundary_zone::BoundaryZone{BidirectionalFlow}, particle,
-                                   v, u, v_fluid, u_fluid)
+                                   boundary_zone::BoundaryZone{BidirectionalFlow},
+                                   particle, particle_new, v, u, v_fluid, u_fluid)
     relative_position = current_coords(u, system, particle) - boundary_zone.zone_origin
 
     # Check if particle is in- or outside the fluid domain.
@@ -356,7 +395,7 @@ end
     end
 
     # Activate a new particle in simulation domain
-    transfer_particle!(fluid_system, system, particle, v_fluid, u_fluid, v, u)
+    transfer_particle!(fluid_system, system, particle, particle_new, v_fluid, u_fluid, v, u)
 
     # Reset position of boundary particle
     for dim in 1:ndims(system)
@@ -368,19 +407,21 @@ end
 
 # Fluid particle is in boundary zone
 @inline function convert_particle!(fluid_system::FluidSystem, system,
-                                   boundary_zone, particle, v, u, v_fluid, u_fluid)
+                                   boundary_zone, particle, particle_new,
+                                   v, u, v_fluid, u_fluid)
     # Activate particle in boundary zone
-    transfer_particle!(system, fluid_system, particle, v, u, v_fluid, u_fluid)
+    transfer_particle!(system, fluid_system, particle, particle_new, v, u, v_fluid, u_fluid)
 
     # Deactivate particle in interior domain
     deactivate_particle!(fluid_system, particle, u_fluid)
 
     return fluid_system
 end
 
-@inline function transfer_particle!(system_new, system_old, particle_old,
+@inline function transfer_particle!(system_new, system_old, particle_old, particle_new,
                                     v_new, u_new, v_old, u_old)
-    particle_new = activate_next_particle(system_new)
+    # Activate new particle
+    system_new.buffer.active_particle[particle_new] = true
 
     # Transfer densities
     density = current_density(v_old, system_old, particle_old)