# Strange behaviour of addprocs on Julia 1.12-rc1

**URL:** <https://discourse.julialang.org/t/strange-behaviour-of-addprocs-on-julia-1-12-rc1/132094>\
**Category:** General Usage\
**Tags:** question, distributed\
**Created:** [September 4, 2025, 9:48am UTC](https://discourse.julialang.org/t/strange-behaviour-of-addprocs-on-julia-1-12-rc1/132094 "2025-09-04T09:48:12Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ufechner7](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ufechner7/32/51363_2.png) [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Post date:** [September 4, 2025, 9:48am UTC](https://discourse.julialang.org/t/strange-behaviour-of-addprocs-on-julia-1-12-rc1/132094/1 "2025-09-04T09:48:12Z")

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I used to use this call to add a process:

```julia-auto
using DistributedNext
addprocs(1)

```

I use the new process for plotting based on PyPlot, which normally works fine. I use a separate process for plotting because PyPlot doesn’t like multi-threading. So I do the calculations multi-threaded in the main process, and plotting single-threaded in a worker process.

But the tests on Github crash if I do that on Julia 1.12-rc1.

The crashes disappeared when I use this command:

```julia-auto
addprocs(1; exeflags=["-t 1", "--project", "--gcthreads=1,0"])

```

Any idea why? Shouldn’t it do exactly the same?

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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:** [September 5, 2025, 6:08am UTC](https://discourse.julialang.org/t/strange-behaviour-of-addprocs-on-julia-1-12-rc1/132094/2 "2025-09-05T06:08:44Z")

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Probably caused by [first point in NEWS.md](https://github.com/JuliaLang/julia/blob/v1.12.0-rc1/NEWS.md#language-changes):

> Julia now defaults to 1 “interactive” thread, in addition to the 1 default “worker” thread. i.e. `-t1,1`. This means in default configuration the main task and repl (when in interactive mode), which both run on thread 1, now run within the `interactive` threadpool. The libuv IO loop also runs on thread 1, helping efficient utilization of the worker threadpool used by `Threads.@spawn`. Asking for specifically 1 thread (`-t1`/`JULIA_NUM_THREADS=1`) or passing `0` will disable the interactive thread i.e. `-t1,0` or `JULIA_NUM_THREADS=1,0` , or `-tauto,0` etc. Asking for more than 1 thread will enable the interactive thread so `-t2` will set the equivalent of `-t2,1` ([#57087](https://github.com/JuliaLang/julia/issues/57087)).

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<div class="post-metadata">

**Author:** ![ufechner7](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ufechner7/32/51363_2.png) [@ufechner7](https://discourse.julialang.org/u/ufechner7)\
**Post date:** [September 5, 2025, 6:22am UTC](https://discourse.julialang.org/t/strange-behaviour-of-addprocs-on-julia-1-12-rc1/132094/3 "2025-09-05T06:22:45Z")

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Thanks!

But does it make sense to start Julia with an interactive thread when you create a worker process?