# @printf strange performance behavior

**URL:** https://discourse.julialang.org/t/printf-strange-performance-behavior/76013
**Category:** Performance
**Tags:** question, formatting
**Created:** [February 8, 2022, 10:46am UTC](https://discourse.julialang.org/t/printf-strange-performance-behavior/76013 "2022-02-08T10:46:12Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![\_bernhard](https://avatars.discourse-cdn.com/v4/letter/_/bc79bd/32.png) [@\_bernhard](https://discourse.julialang.org/u/_bernhard)
#### Post date: [February 8, 2022, 10:46am UTC](https://discourse.julialang.org/t/printf-strange-performance-behavior/76013/1 "2022-02-08T10:46:13Z")

</div>

Hello everybody!

I recently encountered some strange behavior related to printing numbers as ASCII in Julia.  
Tested were both Julia 1.6 and 1.7, the following timings are from 1.6.

```julia
using Printf
using BenchmarkTools

function combined_print(arr)
    buf = IOBuffer()
    print(buf, arr[1], " ", arr[2], " ", arr[3], " ", arr[3], " ", arr[5], " ", arr[6], " ", arr[7], " ", arr[8], " ", arr[9], " ", arr[10], "\n")
end

function separated_print(arr)
    buf = IOBuffer()
    print(buf,arr[1], " ")
    print(buf,arr[2], " ")
    print(buf,arr[3], " ")
    print(buf,arr[4], " ")
    print(buf,arr[5], " ")
    print(buf,arr[6], " ")
    print(buf,arr[7], " ")
    print(buf,arr[8], " ")
    print(buf,arr[9], " ")
    print(buf, arr[10], "\n")
end

function combined_printf(arr)
    buf = IOBuffer()
    @printf buf "%17.16lf %17.16lf %17.16lf %17.16lf %17.16lf %17.16lf %17.16lf %17.16lf %17.16lf %17.16lf\n" arr[1] arr[2] arr[3] arr[3] arr[5] arr[6] arr[7] arr[8] arr[9] arr[10]
end

function separated_printf(arr)
    buf = IOBuffer()
    @printf buf "%17.16lf " arr[1]
    @printf buf "%17.16lf " arr[2]
    @printf buf "%17.16lf " arr[3]
    @printf buf "%17.16lf " arr[4]
    @printf buf "%17.16lf " arr[5]
    @printf buf "%17.16lf " arr[6]
    @printf buf "%17.16lf " arr[7]
    @printf buf "%17.16lf " arr[8]
    @printf buf "%17.16lf " arr[9]
    @printf buf "%17.16lf\n" arr[10]
end

@btime combined_print(a) setup=(a=rand(10))
  1.913 μs (36 allocations: 5.09 KiB)

@btime separated_print(a) setup=(a=rand(10))
  1.994 μs (36 allocations: 5.25 KiB)

@btime combined_printf(a) setup=(a=rand(10))
  1.958 μs (29 allocations: 4.91 KiB)

@btime separated_printf(a) setup=(a=rand(10))
  1.688 μs (26 allocations: 4.94 KiB)

```

So to summarize, `@printf` is generally faster than `print`, which does surprise me, but the part I really don’t understand is, why printing only a single number per `@printf` statement as in `separated_printf` is faster than doing it all in a single `@printf` statement as in `combined_printf`.

Thanks for any ideas, as to what happens here.

EDIT: I corrected a mistake, in my benchmark, as the functions using `print` did print all 17 decimal places, whereas `@printf "%lf"` did not. This eliminates the general difference between `@printf` and `print`, but it remains, that `separated_printf` is faster than `combined_printf` and `*_print`.

---

<div class="post-metadata">

### Author: ![gbaraldi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gbaraldi/32/22101_2.png) [@gbaraldi](https://discourse.julialang.org/u/gbaraldi)
#### Post date: [February 8, 2022, 1:52pm UTC](https://discourse.julialang.org/t/printf-strange-performance-behavior/76013/2 "2022-02-08T13:52:16Z")

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If I had to guess you are seeing some sort of overhead from the vararg function that isn’t there on the non vararg one.