Performance disadvantages of structs?

New to Julia
12

Yes, they are much better :slight_smile: They are allocated on a stack, while mutable structs are allocated in a heap. It means, that all operations like creating/reading are much slower for mutable structures

For example

using BenchmarkTools
using Setfield

mutable struct Foo
    a::Int
end

struct Bar
    a::Int
end

Creation operation

function f1()
    map(i -> Foo(i), 1:10000)
end

function f2()
    map(i -> Bar(i), 1:10000)
end

@btime f1();
  # 50.576 μs (10002 allocations: 234.45 KiB)
@btime f2();
  # 8.269 μs (2 allocations: 78.20 KiB)

Reading operations

function g(v)
    res = 0
    for el in v
        res += el.a
    end
    res
end

vfoo = f1()
vbar = f2()

@assert g(vfoo) == g(vbar)
@btime g($vfoo)
  # 5.394 μs (0 allocations: 0 bytes)
@btime g($vbar)
  # 689.520 ns (0 allocations: 0 bytes)

Even updating is faster with immutable structures

function h1!(v)
    for el in v
        el.a *= 2
    end
end

function h2!(v)
    for (i, el) in pairs(v)
        @set! el.a *= 2
        v[i] = el
    end
end

@btime h1!(v) setup=(v = deepcopy($vfoo));
  # 8.399 μs (0 allocations: 0 bytes)
@btime h2!(v) setup=(v = deepcopy($vbar));
  # 5.959 μs (0 allocations: 0 bytes)

It’s a little more complicated to write updating function, but as you can see, it’s worth it.

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