Thread affinitization: pinning Julia threads to cores

General Usage
10

I’ve been trying around with ThreadPinning.jl related to this post here Current OpenBLAS Versions (January 2022) do not support Intel gen 11 performantly? - #30 by fgerick .

I have some trouble understanding this behaviour:

#julia -t 48 
julia> using BenchmarkTools, Hwloc, ThreadPinning, LinearAlgebra, Octavian; A = rand(5_000,5_000); B = similar(A);

julia> BLAS.get_num_threads()
96

julia> BLAS.set_num_threads(48)

julia> @btime mul!($B, $A, $A);
  626.977 ms (0 allocations: 0 bytes)

julia> BLAS.set_num_threads(24)

julia> @btime mul!($B, $A, $A);
  377.458 ms (0 allocations: 0 bytes)

julia> @btime matmul!($B, $A, $A);
  521.579 ms (0 allocations: 0 bytes)

julia> pinthreads(:compact)

julia> @btime matmul!($B, $A, $A);
  223.912 ms (0 allocations: 0 bytes)

julia> @btime matmul!($B, $A, $A);
  233.511 ms (0 allocations: 0 bytes)

julia> pinthreads(:compact)

julia> @btime matmul!($B, $A, $A);
  188.991 ms (0 allocations: 0 bytes)

julia> @btime matmul!($B, $A, $A);
  186.621 ms (0 allocations: 0 bytes)

julia> @btime mul!($B, $A, $A);
  380.789 ms (0 allocations: 0 bytes)

The OpenMP threads are not affected by ThreadPinning as I understand. However, why do I see different benchmarks for the pure Julia code in Octavian’s matmul! after pinthreads(:compact) the first time and the second time? Is there a way to start Julia on just one socket, without having to pin afterwards?