Grow matrix versus pre-alloc

matrices.jl

function expand_matrix(ms)
   T = eltype(eltype(ms))
   m2 = Matrix{T}(undef, 0, length(first(ms)))
   for i in eachindex(ms)
       m2 = [m2; ms[i]']
   end
   return m2
end

function prealloc_matrix(ms)
   T = eltype(eltype(ms))
   l = length(first(ms))
   rows = Vector{Vector{T}}(undef, 0)
   sizehint!(rows, l)
   for r in ms
       push!(rows, r)
   end
   m2 = Matrix{T}(undef, length(rows), l)
   for i in eachindex(rows)
       m2[i,:] .= rows[i]
   end
   return m2
end

julia> @benchmark prealloc_matrix(ms) setup=(ms=[zeros(10) for _ in 1:2000])
BenchmarkTools.Trial: 
  memory estimate:  290.48 KiB
  allocs estimate:  2012
  --------------
  minimum time:     54.812 μs (0.00% GC)
  median time:      60.610 μs (0.00% GC)
  mean time:        72.654 μs (12.13% GC)
  maximum time:     1.297 ms (94.20% GC)
  --------------
  samples:          10000
  evals/sample:     1

julia> @benchmark expand_matrix(ms) setup=(ms=[zeros(10) for _ in 1:2000])
BenchmarkTools.Trial: 
  memory estimate:  152.96 MiB
  allocs estimate:  7797
  --------------
  minimum time:     22.296 ms (8.96% GC)
  median time:      24.119 ms (9.42% GC)
  mean time:        25.338 ms (13.09% GC)
  maximum time:     80.244 ms (68.21% GC)
  --------------
  samples:          190
  evals/sample:     1

julia> @benchmark expand_matrix(ms) setup=(ms=[zeros(100) for _ in 1:2000])
BenchmarkTools.Trial: 
  memory estimate:  1.49 GiB
  allocs estimate:  7981
  --------------
  minimum time:     255.960 ms (9.87% GC)
  median time:      262.024 ms (10.06% GC)
  mean time:        271.074 ms (12.67% GC)
  maximum time:     320.406 ms (24.57% GC)
  --------------
  samples:          19
  evals/sample:     1

julia> @benchmark prealloc_matrix(ms) setup=(ms=[zeros(100) for _ in 1:2000])
BenchmarkTools.Trial: 
  memory estimate:  1.67 MiB
  allocs estimate:  2009
  --------------
  minimum time:     406.528 μs (0.00% GC)
  median time:      435.928 μs (0.00% GC)
  mean time:        496.345 μs (8.97% GC)
  maximum time:     42.289 ms (98.87% GC)
  --------------
  samples:          6744
  evals/sample:     1

Around 500x speedup, not bad :-P