# Printing in a background process
**URL:** https://discourse.julialang.org/t/printing-in-a-background-process/81453
**Category:** General Usage
**Tags:** question
**Created:** [May 22, 2022, 8:34am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453 "2022-05-22T08:34:02Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@FedeClaudi](https://discourse.julialang.org/u/FedeClaudi)
#### Post date: [May 22, 2022, 8:34am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/1 "2022-05-22T08:34:02Z")
Hi everyone,
I’m trying to do something that feels like should be easy but I can’t figure it out.
Basically, let’s say I have a function (`doer`) which takes a while to run and I want to have another function `printer` printing stuff to stdout while `doer` is running - without messing with `doer` itself.
The motivation for this is that in [Term.jl](https://github.com/FedeClaudi/Term.jl) I need to have a function rendering progress bars information while user’s code runs. I can’t edit the user’s code and I want the progress bar rendering to happen continuously in the background.
This is a MWE of what I have so far:
```julia
mutable struct Status
on::Bool
end
function printer(s::Status)
println("Printer turning on")
while s.on
println("PRINTING")
sleep(0.01)
end
println("Printer turning off")
end
function doer()
# println("Doer getting started")
s = Status(true)
# @async printer(s)
Base.Threads.@spawn printer(s)
for k in 1:50000
x = rand(100, 100)
y = x.^2 .- rand(100, 100)
end
s.on = false
nothing
end
print("\n\n")
doer()
```
My expectation is that `printer` should be running in the background while `doer` finishes its stuff, but this is what the output lokos like:
```julia
Printer turning on
Printer turning off
```
I’ve tried using `@async` and `Task` and `@spawn`… same result. The only thing that makes it work is adding a `sleep` call in the loop in `doer`.
Btw: `Threads.nthreads() # 8`.
Any idea on what I’m doing wrong?
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [May 22, 2022, 10:08am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/2 "2022-05-22T10:08:46Z")
Try
```julia
for k in 1:50000
x = rand(100, 100)
y = x.^2 .- rand(100, 100)
yield() # <-- Allows switching over to other tasks.
end
```
Or better yet, thread the loop as well
```julia
Threads.@spawn begin
for k in 1:50000
x = rand(100, 100)
y = x.^2 .- rand(100, 100)
end
s.on = false
end
```
---
### Author:  [@ericphanson](https://discourse.julialang.org/u/ericphanson)
#### Post date: [May 22, 2022, 10:13am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/3 "2022-05-22T10:13:29Z")
I first tried accumulating from the loop so it couldn’t be removed by dead code elimination. That didn’t seem to make a difference. Then I remembered that GC pauses all threads, and I noticed the code was very allocating; I made the operations in the loop non-allocating, and suddenly got tons of printing.
```julia
using Random
function doer()
# println("Doer getting started")
s = Status(true)
# @async printer(s)
Base.Threads.@spawn printer(s)
accum = 0.0
x = zeros(100, 100)
y = similar(x)
for k in 1:50000
rand!(x)
@. y = x^2 - rand()
accum += sum(y)
end
s.on = false
return accum
end
```
edit: oops! This was just because I forgot to import `Random`; I lost the error in the sea of printing which never gets turned off… when I fix it, I don’t see the printing.
---
### Author:  [@ericphanson](https://discourse.julialang.org/u/ericphanson)
#### Post date: [May 22, 2022, 10:15am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/4 "2022-05-22T10:15:21Z")
> [@FedeClaudi](#):
>
> The motivation for this is that in [Term.jl](https://github.com/FedeClaudi/Term.jl) I need to have a function rendering progress bars information while user’s code runs. I can’t edit the user’s code and I want the progress bar rendering to happen continuously in the background.
This might need [Add threadpool support to runtime by jpsamaroo · Pull Request #42302 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/pull/42302#issuecomment-1072785204) which is in Julia 1.9.
---
### Author:  [@chengchingwen](https://discourse.julialang.org/u/chengchingwen)
#### Post date: [May 22, 2022, 10:18am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/5 "2022-05-22T10:18:41Z")
`@spawn doer()` also seems to work, I guess thread 1 have higher priority?
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [May 22, 2022, 10:30am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/6 "2022-05-22T10:30:58Z")
Which Julia version are you on? Your GC-free loop is just as blocking for me on Julia 1.7.2 (Windows).
---
### Author:  [@ericphanson](https://discourse.julialang.org/u/ericphanson)
#### Post date: [May 22, 2022, 10:32am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/7 "2022-05-22T10:32:22Z")
I just realized I had a simple mistake; I edited the post. It is not working for me either now, 1.7.2 MacOS.
---
### Author:  [@FedeClaudi](https://discourse.julialang.org/u/FedeClaudi)
#### Post date: [May 22, 2022, 10:48am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/8 "2022-05-22T10:48:27Z")
Thanks everyon for looking into this, I really appreciate the help.
@skleinbo 'and @chengchingwen had it right: wrapping `doer` or its content in another `@spaw` makes it work like a charm, I’ve also added a `wait` on the task generated by `@spawn doer()` and it does exactly what I need. Progress bar in Term.jl won’t be hanging any more!
@ericphanson thanks for the help, unfortunately I can’t ensure that users’ code (which would be in `doer`) is not allocating!
---
### Author:  [@chengchingwen](https://discourse.julialang.org/u/chengchingwen)
#### Post date: [May 22, 2022, 10:53am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/9 "2022-05-22T10:53:15Z")
Maybe it’s worth reading how [ProgressMeter](https://github.com/timholy/ProgressMeter.jl) do or even wrap that? It also support multi-threading and is thread-safe.
---
### Author:  [@feanor12](https://discourse.julialang.org/u/feanor12)
#### Post date: [May 22, 2022, 11:20am UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/10 "2022-05-22T11:20:04Z")
A similar topic was already discussed here:
> [@No real multithreading possible?](https://discourse.julialang.org/t/no-real-multithreading-possible/77920):
>
> Hey there, I am desperately trying to figure out how to do some “real” multi-threadding in Julia. What do I mean by that: My program consists of three tasks: the main task and two separate task I start. I need to communicate from the main task to the two separte tasks via channels. This is how I start my desired background task: c = Channel{Bool}(1, spawn=true) do c # create new channel with buffersize 1 status = false while isopen(c) if isready(c) # determine whether the chann…
---
### Author:  [@FedeClaudi](https://discourse.julialang.org/u/FedeClaudi)
#### Post date: [May 22, 2022, 12:51pm UTC](https://discourse.julialang.org/t/printing-in-a-background-process/81453/11 "2022-05-22T12:51:07Z")
Thanks, I looked for related topics but had missed this one.