Use Base.min / Base.max in MPI reductions (#2054)

* use Base.min/max in MPI.Allreduce

MPI.jl's reduce currently does not work for custom operators (such as Trixi's
min/max) on ARM

* add comments

* explain workdaround

* typo

* Apply suggestions from code review

Co-authored-by: Hendrik Ranocha <[email protected]>

* switch to macos-latest in mpi tests

* remove arch specification for macos-latest

macos-latest is 14, which is ARM

* readd arch, required by julia-actions/setup-julia

* back to macos-13 and x64

---------

Co-authored-by: Hendrik Ranocha <[email protected]>

Author: benegee

src/auxiliary/math.jl

@@ -284,6 +284,11 @@ end
 # when using `@fastmath`, which we also get from
 # [Fortran](https://godbolt.org/z/Yrsa1js7P)
 # or [C++](https://godbolt.org/z/674G7Pccv).
+#
+# Note however that such a custom reimplementation can cause incompatibilities with other
+# packages. Currently we are affected by an issue with MPI.jl on ARM, see
+# https://github.com/trixi-framework/Trixi.jl/issues/1922
+# The workaround is to resort to Base.min / Base.max when using MPI reductions.
 """
     Trixi.max(x, y, ...)
 

src/callbacks_step/analysis.jl

@@ -434,7 +434,8 @@ function (analysis_callback::AnalysisCallback)(io, du, u, u_ode, t, semi)
             res = maximum(abs, view(du, v, ..))
             if mpi_isparallel()
                 # TODO: Debugging, here is a type instability
-                global_res = MPI.Reduce!(Ref(res), max, mpi_root(), mpi_comm())
+                # Base.max instead of max needed, see comment in src/auxiliary/math.jl
+                global_res = MPI.Reduce!(Ref(res), Base.max, mpi_root(), mpi_comm())
                 if mpi_isroot()
                     res::eltype(du) = global_res[]
                 end

src/callbacks_step/analysis_dg2d_parallel.jl

@@ -131,7 +131,8 @@ function calc_error_norms(func, u, t, analyzer,
     global_l2_error = Vector(l2_error)
     global_linf_error = Vector(linf_error)
     MPI.Reduce!(global_l2_error, +, mpi_root(), mpi_comm())
-    MPI.Reduce!(global_linf_error, max, mpi_root(), mpi_comm())
+    # Base.max instead of max needed, see comment in src/auxiliary/math.jl
+    MPI.Reduce!(global_linf_error, Base.max, mpi_root(), mpi_comm())
     total_volume = MPI.Reduce(volume, +, mpi_root(), mpi_comm())
     if mpi_isroot()
         l2_error = convert(typeof(l2_error), global_l2_error)

src/callbacks_step/analysis_dg3d_parallel.jl

@@ -49,7 +49,8 @@ function calc_error_norms(func, u, t, analyzer,
     global_l2_error = Vector(l2_error)
     global_linf_error = Vector(linf_error)
     MPI.Reduce!(global_l2_error, +, mpi_root(), mpi_comm())
-    MPI.Reduce!(global_linf_error, max, mpi_root(), mpi_comm())
+    # Base.max instead of max needed, see comment in src/auxiliary/math.jl
+    MPI.Reduce!(global_linf_error, Base.max, mpi_root(), mpi_comm())
     total_volume = MPI.Reduce(volume, +, mpi_root(), mpi_comm())
     if mpi_isroot()
         l2_error = convert(typeof(l2_error), global_l2_error)

src/callbacks_step/stepsize_dg2d.jl

@@ -54,7 +54,8 @@ function max_dt(u, t, mesh::ParallelTreeMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -70,7 +71,8 @@ function max_dt(u, t, mesh::ParallelTreeMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -154,7 +156,8 @@ function max_dt(u, t, mesh::ParallelP4estMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -170,7 +173,8 @@ function max_dt(u, t, mesh::ParallelP4estMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -186,7 +190,8 @@ function max_dt(u, t, mesh::ParallelT8codeMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -202,7 +207,8 @@ function max_dt(u, t, mesh::ParallelT8codeMesh{2},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end

src/callbacks_step/stepsize_dg3d.jl

@@ -130,7 +130,8 @@ function max_dt(u, t, mesh::ParallelP4estMesh{3},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -146,7 +147,8 @@ function max_dt(u, t, mesh::ParallelP4estMesh{3},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -162,7 +164,8 @@ function max_dt(u, t, mesh::ParallelT8codeMesh{3},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end
@@ -178,7 +181,8 @@ function max_dt(u, t, mesh::ParallelT8codeMesh{3},
                       typeof(constant_speed), typeof(equations), typeof(dg),
                       typeof(cache)},
                 u, t, mesh, constant_speed, equations, dg, cache)
-    dt = MPI.Allreduce!(Ref(dt), min, mpi_comm())[]
+    # Base.min instead of min needed, see comment in src/auxiliary/math.jl
+    dt = MPI.Allreduce!(Ref(dt), Base.min, mpi_comm())[]
 
     return dt
 end