[LinearAlgebra] Initialise number of BLAS threads with jl_effective_threads (#55574)

This is a safer estimate than `Sys.CPU_THREADS` to avoid oversubscribing
the machine when running distributed applications, or when the Julia
process is constrained by external controls (`taskset`, `cgroups`,
etc.).

Fix #55572

Author: giordano

NEWS.md

@@ -128,6 +128,8 @@ Standard library changes
   between different eigendecomposition algorithms ([#49355]).
 * Added a generic version of the (unblocked) pivoted Cholesky decomposition
   (callable via `cholesky[!](A, RowMaximum())`) ([#54619]).
+* The number of default BLAS threads now respects process affinity, instead of
+  using total number of logical threads available on the system ([#55574]).
 
 #### Logging
 

stdlib/LinearAlgebra/src/LinearAlgebra.jl

@@ -843,9 +843,9 @@ function __init__()
     # https://github.com/xianyi/OpenBLAS/blob/c43ec53bdd00d9423fc609d7b7ecb35e7bf41b85/README.md#setting-the-number-of-threads-using-environment-variables
     if !haskey(ENV, "OPENBLAS_NUM_THREADS") && !haskey(ENV, "GOTO_NUM_THREADS") && !haskey(ENV, "OMP_NUM_THREADS")
         @static if Sys.isapple() && Base.BinaryPlatforms.arch(Base.BinaryPlatforms.HostPlatform()) == "aarch64"
-            BLAS.set_num_threads(max(1, Sys.CPU_THREADS))
+            BLAS.set_num_threads(max(1, @ccall(jl_effective_threads()::Cint)))
         else
-            BLAS.set_num_threads(max(1, Sys.CPU_THREADS ÷ 2))
+            BLAS.set_num_threads(max(1, @ccall(jl_effective_threads()::Cint) ÷ 2))
         end
     end
 end

test/threads.jl

@@ -359,3 +359,18 @@ end
         @test jl_setaffinity(0, mask, cpumasksize) == 0
     end
 end
+
+# Make sure default number of BLAS threads respects CPU affinity: issue #55572.
+@testset "LinearAlgebra number of default threads" begin
+    if AFFINITY_SUPPORTED
+        allowed_cpus = findall(uv_thread_getaffinity())
+        cmd = addenv(`$(Base.julia_cmd()) --startup-file=no -E 'using LinearAlgebra; BLAS.get_num_threads()'`,
+                     # Remove all variables which could affect the default number of threads
+                     "OPENBLAS_NUM_THREADS"=>nothing,
+                     "GOTO_NUM_THREADS"=>nothing,
+                     "OMP_NUM_THREADS"=>nothing)
+        for n in 1:min(length(allowed_cpus), 8) # Cap to 8 to avoid too many tests on large systems
+            @test readchomp(setcpuaffinity(cmd, allowed_cpus[1:n])) == string(max(1, n ÷ 2))
+        end
+    end
+end