# Where do these allocations come from?

**URL:** <https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549>\
**Category:** Performance\
**Tags:** memory-allocation\
**Created:** [May 9, 2023, 12:46pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549 "2023-05-09T12:46:31Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [May 9, 2023, 12:46pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549/1 "2023-05-09T12:46:31Z")

</div>

I want to write a function `g(w, x...)` with the following properties: If none of the arguments following `w` is a vector, then the tuple `x` is appended to `w` (a vector). For each of the other vector arguments, the function should loop over it, replacing the vector in turn by each of its elements. (This is a toy example.) For instance,

```julia
julia> w = Vector{NTuple{3,Int}}()
julia> g(w, [1,2],3,[4,5])
4-element Vector{Tuple{Int64, Int64, Int64}}:
 (1, 3, 4)
 (1, 3, 5)
 (2, 3, 4)
 (2, 3, 5)

```

Instead of writing a `@generated` function that produces the right number of loops for all possible inputs, I’m trying to have the compiler generate efficient code via multiple dispatch:

```julia
f(w, t) = push!(w, t)
f(w, t, x, y...) = f(w, (t..., x), y...)
f(w, t, v::Vector, y...) = for x in v ; f(w, t, x, y...) end
g(w, y...) = begin f(w, (), y...); w end

```

However, this leads to more allocations than a hard-coded version. For computing

```julia
v = [1,2]
w = Vector{NTuple{3,Int}}()
g(w, v,v,v) # as above
w = Vector{NTuple{3,Int}}()
g0(w, v,v,v) # hard-coded, see below

```

here is the result of using `--track-allocation=user`:

```julia
        - f(w, t) = push!(w, t)
       48 f(w, t, x, y...) = f(w, (t..., x), y...)
      336 f(w, t, v::Vector, y...) = for x in v ; f(w, t, x, y...) end
       48 g(w, y...) = begin f(w, (), y...); w end

        - function g0(w, v1::Vector, v2::Vector, v3::Vector)
        0 for x1 in v1, x2 in v2, x3 in v3
      208 push!(w, (x1,x2,x3))
        0 end
        0 w
        - end

```

Where do the additional allocations come from and how can they be avoided?

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [May 9, 2023, 2:49pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549/2 "2023-05-09T14:49:30Z")

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`push!` may need to allocate memory, because the input vector may not have enough space reserved. There’s n way around ultimately allocating that memory. If you know the target size ahead of time, you can suggest that the array holds enough space reserved with `sizehint!`.

---

<div class="post-metadata">

**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [May 9, 2023, 4:04pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549/3 "2023-05-09T16:04:11Z")

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This type of allocation is the same for `g` and `g0` because they push the same number of elements of the same type. This should account for the 208 allocated bytes one can see for `g0`. But where do the 224 _additional_ bytes for `g` come from?

---

<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [May 9, 2023, 4:11pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549/4 "2023-05-09T16:11:39Z")

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It’s possible that these come from the allocation of intermediaries and dispatch.

[https://docs.julialang.org/en/v1/manual/performance-tips/#Be-aware-of-when-Julia-avoids-specializing](https://docs.julialang.org/en/v1/manual/performance-tips/#Be-aware-of-when-Julia-avoids-specializing)

---

<div class="post-metadata">

**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [May 9, 2023, 4:29pm UTC](https://discourse.julialang.org/t/where-do-these-allocations-come-from/98549/5 "2023-05-09T16:29:13Z")

</div>

Thanks, that’s it! For the record, here is a version without additional allocations:

```julia
f(w, t) = push!(w, t)
f(w, t, x, y::Vararg{Any,N}) where N = f(w, (t..., x), y...)
f(w, t, v::Vector, y::Vararg{Any,N}) where N = for x in v ; f(w, t, x, y...) end
g(w, y::Vararg{Any,N}) where N = begin f(w, (), y...); w end

```