# What is the correct way to use Base.OncePerThread?

**URL:** <https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597>\
**Category:** General Usage\
**Tags:** multithreading, memory, task\
**Created:** [December 16, 2025, 9:14pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597 "2025-12-16T21:14:14Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![AwesomeQuest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/awesomequest/32/38910_2.png) [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)\
**Post date:** [December 16, 2025, 9:14pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/1 "2025-12-16T21:14:14Z")

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I want to use Base.OncePerThread but it’s not clear what the intended usage is.

Is the following a correct / safe / intended way to use Base.OncePerThread ?

```julia-auto
finalset = Set()
tempsets = OncePerThread() do
    return Set()
end
Threads.@threads for i in 1:100
    push!(tempsets(), rand("qwe", 3) |> String)
end
usedtempsets = @view tempsets.xs[tempsets.ss .== 0x01]
union!(finalset, usedtempsets...)
finalset

```

If this is intended why is it such a pain to get the per-thread values out again?

Just `tempsets.xs[tempsets.ss .== 0x01]` without the `@view` errors and `OncePerThread` doesn’t support `iterate` so am I not supposed to access the memory after the threads are done with it?

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [December 16, 2025, 9:31pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/2 "2025-12-16T21:31:20Z")

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Contrary to what the name of the macro implies, `@threads` does not launch OS threads (which `OncePerThread` is referring to), but tasks (which is what `OncePerTask` is about). I believe `OncePerThread` is mostly intended for doing per-OS-thread initialization of third party C libraries that assume OS threads as the means of concurrency.

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**Author:** ![abraemer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraemer/32/51403_2.png) [@abraemer](https://discourse.julialang.org/u/abraemer)\
**Post date:** [December 16, 2025, 9:33pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/3 "2025-12-16T21:33:38Z")

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I think most of the pain here stems from the fact that Julia’s parallelism is really focused on Tasks not on threads.

That being said: using OhMyThreads.jl you can write:

```julia-auto
using OhMyThreads: tmap, chunks

tmpsets = tmap(chunks(1:100); n=Threads.nthreads()) do chunk
    tmpset = Set()
    for i in chunk
        push!(tempset, rand("qwe", 3) |> String)
    end
    return tmpset
end
reduce(union!, tmpsets)

```

Which reads and feels much clearer to me.

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<div class="post-metadata">

**Author:** ![AwesomeQuest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/awesomequest/32/38910_2.png) [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)\
**Post date:** [December 16, 2025, 9:36pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/4 "2025-12-16T21:36:11Z")

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Indeed, the reason I’m using `OncePerThread` in the example is because `Set` is not thread safe and I don’t want to use locks since I don’t need to.

So all `OncePerThread` is supposed to be doing in the example is allocating memory that each thread “owns” and then doing clean up after they are done by collecting all their work.

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<div class="post-metadata">

**Author:** ![AwesomeQuest](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/awesomequest/32/38910_2.png) [@AwesomeQuest](https://discourse.julialang.org/u/AwesomeQuest)\
**Post date:** [December 16, 2025, 9:49pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/5 "2025-12-16T21:49:13Z")

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That example doesn’t seem to work  
`chunks` complains that it needs a number of chunks and then `tmap` gives me an

```julia-auto
UndefVarError: `threadpool` not defined in `OhMyThreads.Implementation`

```

I’m using OhMyThreads v0.8.3

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**Author:** ![jakobnissen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jakobnissen/32/13477_2.png) [@jakobnissen](https://discourse.julialang.org/u/jakobnissen)\
**Post date:** [December 17, 2025, 8:55am UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/6 "2025-12-17T08:55:50Z")

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This is a pattern I often use, and I’m interested if OhMyThreads provide anything pattern for this. I couldn’t find any. Perhaps @Mason knows?

Note the specific issues here:

- The reduction operation is not thread safe, but OhMyThreads’ `@set reduce` seems to assume thread-safety.
- The reduction must happen after all the sets have been computed in parallel, and not at each iteration. Does OhMyThreads provide functionality for this?
- We require task-local state (a `Set` each)

Here is one solution. It works, but it’s not super efficient:

1. `trytake!` should really be in Base, and could use Channel internals to be faster
2. Here, I only create one set per concurrently running task. This may not be needed - perhaps one per total task would be fine.
3. There’s a bunch of overhead in the congested channel, which might be an issue.

```julia
using OhMyThreads: @tasks

function foo(n::Int, ntasks::Int=Threads.nthreads())
	ntasks < 1 && throw(ArgumentError("Must use at least one task"))
	ch = Channel{Set{String}}(Inf)
	@tasks for i in 1:n
	    @set ntasks = ntasks
	    set = @something trytake!(ch) Set{String}()
	    push!(set, String(rand("abcdefghijklmnopqrstuvwxyz", 3)))
	    put!(ch, set)
	end
	close(ch)
	@lock ch reduce(union!, ch, init=Set{String}())
end

function trytake!(ch::Channel)
    isempty(ch) && return nothing
    @lock ch begin
        isempty(ch) ? nothing : Some(take!(ch))
    end
end

```

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<div class="post-metadata">

**Author:** ![abraemer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraemer/32/51403_2.png) [@abraemer](https://discourse.julialang.org/u/abraemer)\
**Post date:** [December 17, 2025, 7:08pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/7 "2025-12-17T19:08:59Z")

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Sorry for missing the argument to `chunks`. I typed this on my phone and could not double check.

Anyhow, I believe the follow-up error to be a bug and opened an issue for that.

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<div class="post-metadata">

**Author:** ![abraemer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/abraemer/32/51403_2.png) [@abraemer](https://discourse.julialang.org/u/abraemer)\
**Post date:** [December 18, 2025, 4:52pm UTC](https://discourse.julialang.org/t/what-is-the-correct-way-to-use-base-onceperthread/134597/8 "2025-12-18T16:52:42Z")

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Not sure if this is still relevant for you but I fixed the bug and so my example should work with OhMyThreads.jl 0.8.4 which was tagged ~1h ago.