# Unexpected behavior when using Threads.@spawn
**URL:** https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497
**Category:** General Usage
**Tags:** multithreading
**Created:** [September 4, 2023, 9:08am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497 "2023-09-04T09:08:34Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@chiion](https://discourse.julialang.org/u/chiion)
#### Post date: [September 4, 2023, 9:08am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/1 "2023-09-04T09:08:34Z")
Hi all,
Since `Threads.@spawn` is encouraged over `@async` according to doc, so I decided to convert all `@async` to `Threads.@spawn` when reusing some of my old code.
However, I’m getting some unexpected behaviors, for example when updating `GLMakie` figure.
Here is a MWE where a figure is being updated for `3` seconds and then stop: (julia: 1.9.3, GLMakie: v0.8.8)
```julia
using GLMakie
function main()
p = Observable(Point2f[(NaN, NaN)])
f, _ = scatter(p)
xlims!(-10, 10)
ylims!(-10, 10)
display(f)
exit = Threads.Atomic{Bool}(false)
@sync begin
# @async always works here
Threads.@spawn begin
while !exit[]
p[] = [Point2f(randn(2)) for _ in 1:1000]
sleep(0.1)
end
end
Threads.@spawn begin
sleep(3)
exit[] = true
end
end
end
```
- If julia start with only 1 thread, then everything works; figure updates normally
- If julia start with more than 1 threads, then the figure will only being updated for a short duration (less than 3 second). The more thread julia has, the quicker the figure stop updating.
- If `@async` is used to replace the first `Threads.@spawn`, then it always works regardless how many threads julia has
Am I using `Threads.@spawn` correctly? Or is it something else?
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 4, 2023, 9:45am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/2 "2023-09-04T09:45:32Z")
Your code outright crashes Julia on MacOS 13.5.1 😕
```julia
[13745] signal (11.2): Segmentation fault: 11
in expression starting at REPL[10]:1
glBindBuffer at /System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib (unknown line)
glBindBuffer at /Users/skleinbo/.julia/packages/ModernGL/yNrOu/src/functionloading.jl:64 [inlined]
bind at /Users/skleinbo/.julia/packages/GLMakie/toRaR/src/GLAbstraction/GLBuffer.jl:32
Allocations: 15546846 (Pool: 15543498; Big: 3348); GC: 22
[1] 13744 segmentation fault julia --threads=4
```
However, moving `display(f)` inside the first task, makes it work nicely!
---
### Author:  [@chiion](https://discourse.julialang.org/u/chiion)
#### Post date: [September 4, 2023, 10:07am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/3 "2023-09-04T10:07:32Z")
Interesting, I was also getting segfault on my desktop before. But after some updates (graphic driver, julia 1.9.2→1.9.3, …) it stops crashing. However, the problem of figure noting getting updated still remains.
---
### Author:  [@chiion](https://discourse.julialang.org/u/chiion)
#### Post date: [September 5, 2023, 4:26am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/4 "2023-09-05T04:26:54Z")
Correction: I checked again, the MWE still segfault with the same error on my PC (linux 6.1.50-1-lts) but `@async` version works fine. Moving `display(f)` inside the first task did not work for me.
On my laptop the `Threads.@async` did not segfault, but the figure is still not updating properly. Really strange.
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 5, 2023, 8:26am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/5 "2023-09-05T08:26:28Z")
Hmm, like I said, running `display` in the same task as the updating loop, i.e.
```julia
Threads.@spawn begin
display(f)
while !exit[]
p[] = [Point2f(randn(2)) for _ in 1:1000]
sleep(0.1)
end
end
```
works fine for me. A plot window opens and is updated for three seconds.
Maybe @sdanisch can help.
---
### Author:  [@chiion](https://discourse.julialang.org/u/chiion)
#### Post date: [September 5, 2023, 8:38am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/6 "2023-09-05T08:38:50Z")
For me, I have to start with `julia -t 1` for this to work.
If julia start with more than 1 thread, the figure will either not being updated or just segfault with the same error in your previous post.
But if `@async` is used, then everything is complete fine, regardless where you put the `display`.
So I guess the question is why `Threads.@spawn` and `@async` behave so differently in this case. I would expect then to be the same (in this example at least).
---
### Author:  [@skleinbo](https://discourse.julialang.org/u/skleinbo)
#### Post date: [September 5, 2023, 9:08am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/7 "2023-09-05T09:08:33Z")
Yes, I understand. As I said, it works for me with `-t 4` or such.
Check on which threads the respective tasks are running with `Threads.threadid()` during the loop. Maybe they migrate or something.
`@async` schedules a task on the calling thread, i.e. in this case always on thread 1.
```julia
function main()
p = Observable(Point2f[(NaN, NaN)])
f, _ = scatter(p)
xlims!(-10, 10)
ylims!(-10, 10)
#display(f)
exit = Threads.Atomic{Bool}(false)
@sync begin
# @async always works here
Threads.@spawn begin
display(f)
while !exit[]
@info "Rand task on thread $(Threads.threadid())"
p[] = [Point2f(randn(2)) for _ in 1:1000]
sleep(0.2)
end
end
Threads.@spawn begin
@info "Exit task on thread $(Threads.threadid())"
sleep(2)
exit[] = true
end
end
end
main (generic function with 1 method)
julia> main()
[ Info: Exit task on thread 2
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
[ Info: Rand task on thread 1
Task (done) @0x0000000147fe1c30
```
I suspect something in the pipeline from Observables to OpenGL isn’t thread safe, but I couldn’t tell what.
---
### Author:  [@chiion](https://discourse.julialang.org/u/chiion)
#### Post date: [September 5, 2023, 9:47am UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/8 "2023-09-05T09:47:50Z")
Thank you, I think that’s the most likely explanation.
Guess I’ll have to use `@async` with any plotting related code for now just to be safe and make my own code to be thread safe for using `Threads.@spawn`
---
### Author:  [@vchuravy](https://discourse.julialang.org/u/vchuravy)
#### Post date: [September 5, 2023, 2:03pm UTC](https://discourse.julialang.org/t/unexpected-behavior-when-using-threads-spawn/103497/9 "2023-09-05T14:03:22Z")
Maybe also fill an issue with GLMakie?