# Why is there an allocation?
**URL:** https://discourse.julialang.org/t/why-is-there-an-allocation/85405
**Category:** General Usage
**Tags:** memory-allocation
**Created:** [August 6, 2022, 4:55pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405 "2022-08-06T16:55:33Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 4:55pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/1 "2022-08-06T16:55:33Z")
```julia
module mbas001
elxs = fill(Matrix{Float64}(undef, 3, 3), 7)
for _i in eachindex(elxs)
elxs[_i] = rand(3, 3)
end
ex = fill(zero(Float64), 3, 3)
copy2ex!(ex, elxs, j, k) = let
elxsc = elxs[c]
for _i in axes(ex, 2)
ex[1, _i] = elxsc[j, _i];
ex[2, _i] = elxsc[k, _i];
end
ex
end
c = 4
@time let
ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
end
@time copy2ex!(ex, elxs, 1, 2)
@time copy2ex!(ex, elxs, 1, 2)
@time copy2ex!(ex, elxs, 1, 2)
end
```
produces
```julia
julia> include("C:\\Users\\pkonl\\Documents\\00WIP\\HelmBEM.jl\\test\\littletest.jl")
WARNING: replacing module mbas001.
0.000023 seconds (6 allocations: 224 bytes)
0.018632 seconds (9.56 k allocations: 517.691 KiB, 99.73% compilation time)
0.000017 seconds (6 allocations: 96 bytes)
0.000016 seconds (6 allocations: 96 bytes)
Main.mbas001
```
What puzzles me is why the function `copy2ex!` allocates memory.
It happens in the loop
```julia
for _i in axes(ex, 2)
ex[1, _i] = elxsc[j, _i];
ex[2, _i] = elxsc[k, _i];
end
```
Which, in my understanding, should be simply assignment of one matrix location the value of another matrix location. In other words, scalars.
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [August 6, 2022, 4:57pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/2 "2022-08-06T16:57:13Z")
`c` is a non-const global variable.
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 5:06pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/3 "2022-08-06T17:06:21Z")
Wow. I missed that! Thanks.
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 5:08pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/4 "2022-08-06T17:08:44Z")
Now if I could understand why the memory allocation happens here:
```julia
ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
```
There are still 6 ALLOCATIONS:
```julia
WARNING: replacing module mbas001.
0.000025 seconds (6 allocations: 224 bytes)
0.022149 seconds (6.53 k allocations: 337.683 KiB, 99.85% compilation time)
0.000015 seconds
0.000003 seconds
Main.mbas001
```
The code looks like this now:
```julia
module mbas001
elxs = fill(Matrix{Float64}(undef, 3, 3), 7)
for _i in eachindex(elxs)
elxs[_i] = rand(3, 3)
end
ex = fill(zero(Float64), 3, 3)
copy2ex!(ex, elxs, c, j, k) = let
elxsc = elxs[c]
for _i in axes(ex, 2)
ex[1, _i] = elxsc[j, _i];
ex[2, _i] = elxsc[k, _i];
end
ex
end
@time let
c = 4
ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
end
c = 4
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
end
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [August 6, 2022, 5:21pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/5 "2022-08-06T17:21:40Z")
> [@PetrKryslUCSD](#):
>
> `elxs = fill(Matrix{Float64}(undef, 3, 3), 7)`
const
> [@PetrKryslUCSD](#):
>
> `ex = fill(zero(Float64), 3, 3) `
const
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [August 6, 2022, 5:22pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/6 "2022-08-06T17:22:22Z")
> [@PetrKryslUCSD](#):
>
> ```julia
> @time let
> c = 4
> ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
> end
>
> ```
```julia
@time @views let
```
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 5:32pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/7 "2022-08-06T17:32:41Z")
> [@jling](#):
>
> `@views `
Now everything is a local variable. Still, there are allocations.
```julia
module mbas001
let
elxs = fill(Matrix{Float64}(undef, 3, 3), 7)
for _i in eachindex(elxs)
elxs[_i] = rand(3, 3)
end
ex = fill(zero(Float64), 3, 3)
copy2ex!(ex, elxs, c, j, k) = let
elxsc = elxs[c]
for _i in axes(ex, 2)
ex[1, _i] = elxsc[j, _i];
ex[2, _i] = elxsc[k, _i];
end
ex
end
@time @views let
c = 4
ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
end
c = 4
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
end
end
```
gives
```julia
WARNING: replacing module mbas001.
0.000013 seconds (15 allocations: 512 bytes)
0.020667 seconds (6.54 k allocations: 338.026 KiB, 99.88% compilation time)
0.000005 seconds (9 allocations: 352 bytes)
0.000003 seconds (9 allocations: 352 bytes)
Main.mbas001
```
Edit: Note that I forgot to remove the `views`. Everything is local now.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [August 6, 2022, 6:00pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/8 "2022-08-06T18:00:57Z")
Doesn’t `@time` itself allocate? I always use `@btime` and variable interpolation for reliable allocation estimates.
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 6:02pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/9 "2022-08-06T18:02:17Z")
It actually worked (sort of) in [https://discourse.julialang.org/t/why-is-there-an-allocation/85405/4:](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/4:) there were zero allocations with the function.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [August 6, 2022, 6:05pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/10 "2022-08-06T18:05:37Z")
`@btime` seems to have improved over time, but it might still be an idea to use BenchmarkTools to be more certain.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [August 6, 2022, 6:28pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/11 "2022-08-06T18:28:51Z")
> [@PetrKryslUCSD](#):
>
> …
please implement all the changes I proposed:
```julia
julia> module mbas001
const elxs = fill(Matrix{Float64}(undef, 3, 3), 7)
for _i in eachindex(elxs)
elxs[_i] = rand(3, 3)
end
const ex = fill(zero(Float64), 3, 3)
copy2ex!(ex, elxs, c, j, k) = let
elxsc = elxs[c]
for _i in axes(ex, 2)
ex[1, _i] = elxsc[j, _i];
ex[2, _i] = elxsc[k, _i];
end
ex
end
@time @views let
c = 4
ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
end
c = 4
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
@time copy2ex!(ex, elxs, c, 1, 2)
end
0.000001 seconds
0.012696 seconds (7.42 k allocations: 388.971 KiB, 99.93% compilation time)
0.000003 seconds
0.000002 seconds
Main.mbas001
```
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 6:30pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/12 "2022-08-06T18:30:18Z")
Thank you, I appreciate your suggestion, but really I want to have only local variables.
Testing in the global scope was obviously a mistake.
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [August 6, 2022, 7:00pm UTC](https://discourse.julialang.org/t/why-is-there-an-allocation/85405/13 "2022-08-06T19:00:30Z")
It is possible that `@time` allocates when it is used in a local scope. Tracking allocations
with everything in local scope:
```julia
- module mbas001
- let
- elxs = fill(Matrix{Float64}(undef, 3, 3), 7)
- for _i in eachindex(elxs)
- elxs[_i] = rand(3, 3)
- end
- ex = fill(zero(Float64), 3, 3)
- copy2ex!(ex, elxs, c, j, k) = let
0 elxsc = elxs[c]
0 for _i in axes(ex, 2)
0 ex[1, _i] = elxsc[j, _i];
0 ex[2, _i] = elxsc[k, _i];
0 end
0 ex
- end
- let
- c = 4
- ex[1,:] = elxs[c][1,:]; ex[2,:] = elxs[c][2,:];
- end
- c = 4
- copy2ex!(ex, elxs, c, 1, 2)
- copy2ex!(ex, elxs, c, 1, 2)
- copy2ex!(ex, elxs, c, 1, 2)
- end
- end
```