# "@." can induce poor performance sometimes

**URL:** <https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170>\
**Category:** Performance\
**Created:** [May 26, 2018, 5:25pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170 "2018-05-26T17:25:10Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![1115](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/1115/32/4465_2.png) [@1115](https://discourse.julialang.org/u/1115)\
**Post date:** [May 26, 2018, 5:25pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/1 "2018-05-26T17:25:10Z")

</div>

`@.` macro, it will put a `.` behind some user defined function even though it is applited on a scalar and there is no available `bmask.`.

Interestingly, it causes a lot of memory allocation, inducing a poor performance.

```julia
julia> bss = collect(0:1<<16-1)
julia> @benchmark @. (bss & bmask(7)) << 4
BenchmarkTools.Trial: 
  memory estimate: 2.98 MiB
  allocs estimate: 162832
  --------------
  minimum time: 13.145 ms (0.00% GC)
  median time: 13.418 ms (0.00% GC)
  mean time: 13.716 ms (1.65% GC)
  maximum time: 53.083 ms (73.50% GC)
  --------------
  samples: 365
  evals/sample: 1

julia> @benchmark (bss .& bmask(7)) .<< 4
BenchmarkTools.Trial: 
  memory estimate: 512.17 KiB
  allocs estimate: 6
  --------------
  minimum time: 62.952 μs (0.00% GC)
  median time: 75.028 μs (0.00% GC)
  mean time: 95.000 μs (13.37% GC)
  maximum time: 43.102 ms (99.42% GC)
  --------------
  samples: 10000
  evals/sample: 1

```

why?

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [May 26, 2018, 5:39pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/2 "2018-05-26T17:39:40Z")

</div>

Firstly, can you supply the `bmask` function? I cannot find it anywhere.

Secondly, remember to always interpolate global variables with `$` when using BenchmarkTools, like this:

```julia
@benchmark @. ($bss & bmask(7)) << 4

```

The difference here should be between `bmask(7)` and `bmask.(7)`, so you should just benchmark those two.

---

<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [May 26, 2018, 5:49pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/3 "2018-05-26T17:49:04Z")

</div>

> [@1115](#):
>
> `julia> bss = collect(0:1<<16-1)`

One more thing: you should almost never collect ranges like this. It wastes memory, is mostly slower, and is unnecessarily verbose. Just use

```julia
julia> bss = 0:1<<16-1

```

---

<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [May 26, 2018, 5:58pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/4 "2018-05-26T17:58:40Z")

</div>

This is just part of the broadcasting specification. All functions can be “dotted” and all dotted functions participate in the fusion. That means that Julia will only loop once over the output and will call all the dotted functions on the elements. This is even handy at times – for example you can use `A .+ rand.()` and the random addition will be different for each element.

Note that you can prevent a particular function from participating in the `@.` fusion with a `$`.

---

<div class="post-metadata">

**Author:** ![1115](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/1115/32/4465_2.png) [@1115](https://discourse.julialang.org/u/1115)\
**Post date:** [May 26, 2018, 7:48pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/5 "2018-05-26T19:48:15Z")

</div>

Thanks,  
1).

```julia
DInt = Int64
bmask(ibit::Int) = one(DInt) << (ibit-1)

```

**Remark** : if I do not alias `Int64` as `DInt`, using `bmask.` is OK, else, using `bmask.` causes allocation while `bmask` witthout dot has no allocation.

2).

```julia
julia> @benchmark ($bss .& bmask.(7)) .<< 4
BenchmarkTools.Trial: 
  memory estimate: 2.49 MiB
  allocs estimate: 130580
  --------------
  minimum time: 20.303 ms (0.00% GC)
  median time: 20.535 ms (0.00% GC)
  mean time: 20.986 ms (0.48% GC)
  maximum time: 29.362 ms (0.00% GC)
  --------------
  samples: 239
  evals/sample: 1

```

3). your suggestion on not using `collect` really helps a lot.

---

<div class="post-metadata">

**Author:** ![tkoolen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkoolen/32/1603_2.png) [@tkoolen](https://discourse.julialang.org/u/tkoolen)\
**Post date:** [May 26, 2018, 8:04pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/6 "2018-05-26T20:04:38Z")

</div>

You also want `const DInt = Int64`, see [https://docs.julialang.org/en/stable/manual/performance-tips/#Avoid-global-variables-1](https://docs.julialang.org/en/stable/manual/performance-tips/#Avoid-global-variables-1).

```julia
julia> const DInt = Int64
Int64

julia> bmask(ibit::Int) = one(DInt) << (ibit-1)
bmask (generic function with 1 method)

julia> bss = 0:1<<16-1
0:65535

julia> using BenchmarkTools

julia> @benchmark ($bss .& bmask.(7)) .<< 4
BenchmarkTools.Trial: 
  memory estimate: 512.08 KiB
  allocs estimate: 2
  --------------
  minimum time: 44.799 μs (0.00% GC)
  median time: 260.575 μs (0.00% GC)
  mean time: 287.050 μs (13.04% GC)
  maximum time: 3.557 ms (85.02% GC)
  --------------
  samples: 10000
  evals/sample: 1

```

---

<div class="post-metadata">

**Author:** ![1115](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/1115/32/4465_2.png) [@1115](https://discourse.julialang.org/u/1115)\
**Post date:** [May 26, 2018, 8:27pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/7 "2018-05-26T20:27:58Z")

</div>

You are right, now their performances are close.

---

<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [May 26, 2018, 9:47pm UTC](https://discourse.julialang.org/t/can-induce-poor-performance-sometimes/11170/8 "2018-05-26T21:47:24Z")

</div>

Wow, that’s cool!

```julia
julia> z = zeros(1,5);

julia> @. z + rand()
1×5 Array{Float64,2}:
 0.838248 0.40499 0.246746 0.979163 0.194097

julia> @. z + $rand()
1×5 Array{Float64,2}:
 0.142921 0.142921 0.142921 0.142921 0.142921

```

Makes sense, but I never thought about interpolating into a dot broadcast before.