# Using @btime and printing a Vector{Vector} yields infinite loop
**URL:** https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289
**Category:** General Usage
**Tags:** package
**Created:** [February 25, 2025, 12:34pm UTC](https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289 "2025-02-25T12:34:09Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Bernd\_Blasius](https://discourse.julialang.org/u/Bernd_Blasius)
#### Post date: [February 25, 2025, 12:34pm UTC](https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289/1 "2025-02-25T12:34:09Z")
The following code produces an infinite loop
```julia
using BenchmarkTools
v = Vector{Vector}(undef,1)
@btime println(v)
```
Is this a bug or intended behaviour?
---
### Author:  [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [February 25, 2025, 12:38pm UTC](https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289/2 "2025-02-25T12:38:09Z")
I can’t replicate this, I get (after a lot of output, because it’s not really sensible to call `@btime` on `println`):
```julia
(...)
Vector[#undef]
Vector[#undef]
Vector[#undef]
Vector[#undef]
293.800 μs (340 allocations: 69.92 KiB)
```
`Chairmarks` is a bit faster here, so I guess `BenchmarkTools` uses too many samples which are expensive as you’re spamming the console:
```julia
using Chairmarks
@b println($v)
```
---
### Author:  [@Bernd\_Blasius](https://discourse.julialang.org/u/Bernd_Blasius)
#### Post date: [February 25, 2025, 1:21pm UTC](https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289/3 "2025-02-25T13:21:34Z")
Ah, right. The loop is not infinite, but will stop after quite a lot of output.
Note that I did not call `@btime` on `println` directly, but on a function that (beside other things) also calls `println`
```julia
function foo(v)
# do stuff ...
println(v)
end
@btime foo(v)
```
So what is the recommended way to `@btime` such functions? Erase all `println` statements, or use Chairmarks instead?
---
### Author:  [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [February 25, 2025, 2:38pm UTC](https://discourse.julialang.org/t/using-btime-and-printing-a-vector-vector-yields-infinite-loop/126289/4 "2025-02-25T14:38:35Z")
There’s not really a difference between BenchmarkTools and Chairmarks here, except that for your example Chairmarks chooses fewer samples and finishes faster (although that’s generally a goal of Chairmarks).
In my view, if you’re using `@btime` or `@b` you’re doing so because you are looking at a performance-critical part of your code. That is inherently incompatible with printing to the terminal, in general I would say but especially in Julia, see:
> [@Julia seems an order of magnitude slower than Python when printing to the terminal, because of issue with "sleep"](https://discourse.julialang.org/t/julia-seems-an-order-of-magnitude-slower-than-python-when-printing-to-the-terminal-because-of-issue-with-sleep/78151/3):
>
> Some references: [JuliaLang/julia#36639](https://github.com/JuliaLang/julia/issues/36639), [JuliaLang/julia#43176](https://github.com/JuliaLang/julia/issues/43176), [Why is printint to a terminal slow (Discourse)](https://discourse.julialang.org/t/why-is-printing-to-a-terminal-slow/42987).
So yes, I’d say if you want to benchmark the runtime of some piece of code for which you care about performance, don’t let that code print to the terminal.