# Type-instability because of @threads boxing variables

**URL:** https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395
**Category:** Performance
**Tags:** parallel
**Created:** [March 24, 2022, 2:17pm UTC](https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395 "2022-03-24T14:17:45Z")
**Posts on this page:** 1
**Showing post:** 21

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### Author: ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)
#### Post date: [March 25, 2022, 12:27pm UTC](https://discourse.julialang.org/t/type-instability-because-of-threads-boxing-variables/78395/21 "2022-03-25T12:27:13Z")

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Yes.  
You could try adding `let` blocks.  
Note that `Threads.@spawn` has sugar where using `$` on variables automatically creates let blocks for you:

```julia
julia> @macroexpand Threads.@spawn foo($x)
quote
    #= threadingconstructs.jl:229 =#
    let var"##394" = x
        #= threadingconstructs.jl:230 =#
        local var"#14#task" = Base.Threads.Task((()->begin
                            #= threadingconstructs.jl:226 =#
                            foo(var"##394")
                        end))
        #= threadingconstructs.jl:231 =#
        (var"#14#task").sticky = false
        #= threadingconstructs.jl:232 =#
        if $(Expr(:islocal, Symbol("##sync#48")))
            #= threadingconstructs.jl:233 =#
            Base.Threads.put!(var"##sync#48", var"#14#task")
        end
        #= threadingconstructs.jl:235 =#
        Base.Threads.schedule(var"#14#task")
        #= threadingconstructs.jl:236 =#
        var"#14#task"
    end
end

```

But this isn’t supported for `Threads.@threads`, although it’s probably easy enough to manually create `let` blocks, unless you have an absurd number of variables (fun fact: `let` used to be `O(N^2)` in number of variables, until very recent versions of Julia [1.8+?], where it is now `O(N)`, thanks to some code with thousands of `let` causing this to actually be problematic). This is the classic workaround.

You could also manually chunk your iteration range, perhaps separating the code that iterates over a chunk into a separate function, and use `@spawn`. I also often do this anyway when I want more control over task-local state, e.g. rngs. `@spawn` is of course more dynamic than `@threads`, which you may or may not want.

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