# LoadError: UndefVarError: \`@threads\` not defined in \`Main\`

**URL:** <https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216>\
**Category:** New to Julia\
**Created:** [January 26, 2025, 4:42am UTC](https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216 "2025-01-26T04:42:19Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Mohamad\_Badiez](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mohamad_badiez/32/213635_2.png) [@Mohamad\_Badiez](https://discourse.julialang.org/u/Mohamad_Badiez)\
**Post date:** [January 26, 2025, 4:42am UTC](https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216/1 "2025-01-26T04:42:19Z")

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Hi everybody,  
I just want to stress test on all of my CPU threads with the multiplication of two random matrices but I got the following error:

 ![Screenshot 2025-01-25 122207](https://global.discourse-cdn.com/julialang/original/3X/5/0/509fdae548695bbf587f4dea9a69b5027fcee81f.jpeg)

What should I do as a Julia new user?  
Thank you

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**Author:** ![GHTaarn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ghtaarn/32/216007_2.png) [@GHTaarn](https://discourse.julialang.org/u/GHTaarn)\
**Post date:** [January 26, 2025, 5:03am UTC](https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216/2 "2025-01-26T05:03:13Z")

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You need to either write `Threads.@threads` or import the symbol. The correct way to import the symbol would be `using .Threads: @threads`, but as a beginner, simply `using .Threads` would be okay as well.

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**Author:** ![GHTaarn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ghtaarn/32/216007_2.png) [@GHTaarn](https://discourse.julialang.org/u/GHTaarn)\
**Post date:** [January 26, 2025, 5:19am UTC](https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216/3 "2025-01-26T05:19:49Z")

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Also, if you have not done so please read [Please read: make it easier to help you](https://discourse.julialang.org/t/please-read-make-it-easier-to-help-you/14757) (especially point 2 about posting code).

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<div class="post-metadata">

**Author:** ![eldee](https://avatars.discourse-cdn.com/v4/letter/e/b5a626/32.png) [@eldee](https://discourse.julialang.org/u/eldee)\
**Post date:** [January 26, 2025, 9:54am UTC](https://discourse.julialang.org/t/loaderror-undefvarerror-threads-not-defined-in-main/125216/5 "2025-01-26T09:54:36Z")

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Also note that `Threads.nthreads()` just returns the number of threads you’re using (e.g. `8`). It does not enable anything. The `-t` flag you pass to the `julia` executable is what enables multithreading.

Furthermore, `A[i, :]` and `B[:, j]` make copies. Assuming that is not what you want, you should use `view`s:

```julia
# These are all equivalent
C[i, j] = dot(view(A, i, :), view(B, :, j))
C[i, j] = dot(@view(A[i, :]), @view(B[:, j]))
@views C[i, j] = dot(A[i, :], B[:, j])

```