# Profiling in VSCode returns some red bars with "Flag: GC" - what do they mean?
**URL:** https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570
**Category:** General Usage
**Created:** [February 1, 2024, 1:07pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570 "2024-02-01T13:07:05Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 1, 2024, 1:07pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/1 "2024-02-01T13:07:05Z")
Hello!
When using `@profview` in Julia I observe that there exists both blue and red bars. The blue bars I think are fine, they just mention how much time is spent. But what about the red ones?
Is this something I should resolve, can I fix it in some way?
Kind regards
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 1, 2024, 2:37pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/2 "2024-02-01T14:37:52Z")
Red indicates type instability. Read the julia docs about it for an intro, and use tools like @code\_warntype to debug (or Cthulhu’s @descend\_code\_warntype for trickier problems with deep call stacks).
Orange indicates that garbage collection (GC) has occured. Usually if you have type instability you will also have garbage collection because compiling is allocating.
Anywhere that memory is allocated (e.g. creating or cloning a vector/array) can trigger the garbage collector.
Depending on your problem, you might not really need to care about type instability of GC or allocations, so long as your code works, but if it is performance critical then eliminating type instability is the top priority, followed by reducing allocations.
---
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 1, 2024, 4:14pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/3 "2024-02-01T16:14:28Z")
Okay I went in on some of the functions giving me this GC warning and this the output. I don’t see any `Any` but line %6 has a yellow color.
```julia
function updatexᵢⱼ!(xᵢⱼ, list, points)
#if length(xᵢⱼ) != length(list) resize!(xᵢⱼ, length(list)) end
for (iter, L) in enumerate(list)
i = L[1]; j = L[2];
xᵢⱼ[iter] = points[i] - points[j]
end
end
```
```julia
MethodInstance for SPHExample.SimulationEquations.updatexᵢⱼ!(::Vector{SVector{3, Float64}}, ::Vector{Tuple{Int64, Int64, Float64}}, ::Vector{SVector{3, Float64}})
from updatexᵢⱼ!(xᵢⱼ, list, points) @ SPHExample.SimulationEquations e:\git\SPHExample\src\SimulationEquations.jl:221
Arguments
#self#::Core.Const(SPHExample.SimulationEquations.updatexᵢⱼ!)
xᵢⱼ::Vector{SVector{3, Float64}}
list::Vector{Tuple{Int64, Int64, Float64}}
points::Vector{SVector{3, Float64}}
Locals
@_5::Union{Nothing, Tuple{Tuple{Int64, Tuple{Int64, Int64, Float64}}, Tuple{Int64, Int64}}}
@_6::Int64
L::Tuple{Int64, Int64, Float64}
iter::Int64
j::Int64
i::Int64
Body::Nothing
1 ─ %1 = SPHExample.SimulationEquations.enumerate(list)::Base.Iterators.Enumerate{Vector{Tuple{Int64, Int64, Float64}}}
│ (@_5 = Base.iterate(%1))
│ %3 = (@_5 === nothing)::Bool
│ %4 = Base.not_int(%3)::Bool
└── goto #4 if not %4
2 ┄ %6 = @_5::Tuple{Tuple{Int64, Tuple{Int64, Int64, Float64}}, Tuple{Int64, Int64}}
│ %7 = Core.getfield(%6, 1)::Tuple{Int64, Tuple{Int64, Int64, Float64}}
│ %8 = Base.indexed_iterate(%7, 1)::Core.PartialStruct(Tuple{Int64, Int64}, Any[Int64, Core.Const(2)])
│ (iter = Core.getfield(%8, 1))
│ (@_6 = Core.getfield(%8, 2))
│ %11 = Base.indexed_iterate(%7, 2, @_6::Core.Const(2))::Core.PartialStruct(Tuple{Tuple{Int64, Int64, Float64}, Int64}, Any[Tuple{Int64, Int64, Float64}, Core.Const(3)])
│ (L = Core.getfield(%11, 1))
│ %13 = Core.getfield(%6, 2)::Tuple{Int64, Int64}
│ (i = Base.getindex(L, 1))
│ (j = Base.getindex(L, 2))
│ %16 = Base.getindex(points, i)::SVector{3, Float64}
│ %17 = Base.getindex(points, j)::SVector{3, Float64}
│ %18 = (%16 - %17)::SVector{3, Float64}
│ Base.setindex!(xᵢⱼ, %18, iter)
│ (@_5 = Base.iterate(%1, %13))
│ %21 = (@_5 === nothing)::Bool
│ %22 = Base.not_int(%21)::Bool
└── goto #4 if not %22
3 ─ goto #2
4 ┄ return nothing
```
Thanks for the explanation of the colors. I care a lot about this since this is performance critical code and I am just uncertain how such a simple function cannot call the garbage collector correctly… I am probably doing something not so clever somewhere.
Kind regards
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 5, 2024, 2:16pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/4 "2024-02-05T14:16:12Z")
I don’t see anything that should allocate there, it all looks good, can you make a minimal example where it still does allocate?
---
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 5, 2024, 2:22pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/5 "2024-02-05T14:22:19Z")
You commented the perfect time, was just testing something. Basically using `TimerOutputs` package I am tracking each function - I think there is a bug in the package somewhere, since I see a low number of allocs for functions when `BenchmarkTools` would say zero. But back on track the function `updatexᵢⱼ!` returns 0 allocs:
But I get the GC flag for this function and two others as well;
And I just don’t know why.
Kind regards
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 5, 2024, 2:23pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/6 "2024-02-05T14:23:44Z")
Is that snippet run in global scope? I.e. in the REPL?
---
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 5, 2024, 2:25pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/7 "2024-02-05T14:25:32Z")
In regards to minimal example, if you clone this [GitHub - AhmedSalih3d/SPHExample at optimize-code-execution](https://github.com/AhmedSalih3d/SPHExample/tree/optimize-code-execution) and run `example\MainSimulation.jl` script it should work easily after `Pkg.instantiate()`. Note, I don’t expect you to do this, can take 10-15 minutes of your time to run simulation, but here is the full code for anyone curious.
> Is that snippet run in global scope? I.e. in the REPL?
It is run inside of ´RunSimulation` function found here [SPHExample/example/MainSimulation.jl at optimize-code-execution · AhmedSalih3d/SPHExample · GitHub](https://github.com/AhmedSalih3d/SPHExample/blob/optimize-code-execution/example/MainSimulation.jl) and then this function is called at the bottom of the script. So should not be in global scope.
Kind regards
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 6, 2024, 12:10pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/8 "2024-02-06T12:10:59Z")
Is it possible that LoadParticlesFromCSV is type unstable? That could make most of your function “RunSimulation” type unstable.
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 6, 2024, 12:11pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/9 "2024-02-06T12:11:47Z")
Also, I’m unfamiliar with @timeit, but could it be a source of allocations?
---
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 6, 2024, 12:38pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/10 "2024-02-06T12:38:54Z")
Yes, `LoadParticlesFromCSV` has a lot of errors, also type errors. Some perhaps to CSV it self - would this propagate down later? The defined vectors etc. don’t have type errors and the dataframe from CSV is not sed later…
`@timeit` is from `TimerOutputs.jl` and can have a few allocations it self, but I use it in a lot of place and only see GC in a few.
Kind regards
---
### Author:  [@Larbino1](https://discourse.julialang.org/u/Larbino1)
#### Post date: [February 6, 2024, 12:53pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/11 "2024-02-06T12:53:38Z")
If the type of the “points” vector cannot be inferred then that would cause dynamic dispatch every time you call a function with it as an argument (but that would not propagate problems down into the function)
---
### Author:  [@mstewart](https://discourse.julialang.org/u/mstewart)
#### Post date: [February 6, 2024, 2:02pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/12 "2024-02-06T14:02:26Z")
I tried out your `updatexᵢⱼ!` function on some made up vectors. With `@code_warntype` and using Julia 1.10.0, I see the yellow under locals at `@_5` but not at `%6`. Maybe that is some difference across versions in how `@code_warntype` presents the same information? In any event, I think that `Union` is just the result of iteration returning `nothing` at the end of the iteration: [@code\_warntype gives yellow ::Union{Nothing, Tuple{Int64,Int64}} in for loops](https://discourse.julialang.org/t/code-warntype-gives-yellow-union-nothing-tuple-int64-int64-in-for-loops/52814). The yellow on the line that calls `updatexᵢⱼ!` in your image of the code looks like some information provided by VS Code, which I am not using, so I don’t know how that works. I’m not an expert at tracking down allocations, but it all looks harmless to me, and it definitely doesn’t seem to be allocating on my system.
---
### Author:  [@Ahmed\_Salih](https://discourse.julialang.org/u/Ahmed_Salih)
#### Post date: [February 6, 2024, 2:09pm UTC](https://discourse.julialang.org/t/profiling-in-vscode-returns-some-red-bars-with-flag-gc-what-do-they-mean/109570/13 "2024-02-06T14:09:22Z")
@Larbino1 got it, then that function to load in data at start of simulation should not be an issue.
@mstewart, thanks! I agree 🙂