# Multithreading throw segmentfault occationally

**URL:** <https://discourse.julialang.org/t/multithreading-throw-segmentfault-occationally/43132>\
**Category:** General Usage\
**Tags:** question\
**Created:** [July 15, 2020, 3:39pm UTC](https://discourse.julialang.org/t/multithreading-throw-segmentfault-occationally/43132 "2020-07-15T15:39:30Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Roger-luo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/roger-luo/32/3399_2.png) [@Roger-luo](https://discourse.julialang.org/u/Roger-luo)\
**Post date:** [July 15, 2020, 3:39pm UTC](https://discourse.julialang.org/t/multithreading-throw-segmentfault-occationally/43132/1 "2020-07-15T15:39:30Z")

</div>

Hi I’m wondering if this is some kind of known issue, basically I have one simulation task for each Task, and I spawn 72 threads to execute these tasks, then my code throw a segment fault from the BLAS when I create 160 tasks using the `Threads.@spawn` , I tried to find which task causes the segment fault exactly, so

1. I manually split these 160 tasks into 80 tasks and another 80 tasks, I don’t get the segment fault anymore…
2. then I use `--check-bounds=yes` check if there is anything out of bounds, but I don’t get the segment fault anymore either

The entire codebase is quite huge, thus I cannot find a small readable script to reproduce it, and since there is no error message from Julia (but from a BLAS function) I can’t figure out which is causing it,  
any idea what I can use to trace this error further?

the segment fault trace looks like

```julia
signal (11): Segmentation fault
in expression starting at :1
unknown function (ip: 0x7fa766645941)
zgemv_n_SKYLAKEX at /home/ubuntu/packages/julias/julia-1.5/bin/../lib/julia/libopenblas64_.so (unknown line)
zgemv_64_ at /home/ubuntu/packages/julias/julia-1.5/bin/../lib/julia/libopenblas64_.so (unknown line)
gemv! at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/blas.jl:626
gemv! at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/matmul.jl:470
mul! at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/matmul.jl:66 [inlined]
mul! at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/matmul.jl:208 [inlined]
#expv!#27 at /home/ubuntu/.julia/packages/ExponentialUtilities/XXu86/src/krylov_phiv.jl:118

```

I can always reproduce the segment fault by running the entire thing (all 160 tasks on 72 threads) without setting `--check-bounds=yes`

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<div class="post-metadata">

**Author:** ![Roger-luo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/roger-luo/32/3399_2.png) [@Roger-luo](https://discourse.julialang.org/u/Roger-luo)\
**Post date:** [July 15, 2020, 4:25pm UTC](https://discourse.julialang.org/t/multithreading-throw-segmentfault-occationally/43132/2 "2020-07-15T16:25:40Z")

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for more detailed configuration I have set the `BLAS.set_num_threads(1)` and the thread spawning part looks like (but the actual code is much longer…)

```julia
# 160 jobs here, each will be running as a Task
# all the jobs are independent from each other
# (in principal they can also be running in different processes)
jobs = generate_jobs(configs) 
@sync for i in 1:length(jobs)
   @spawn begin
           run(jobs[i])
           # some post process
    end
end

```

The BLAS call happens inside each job.

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<div class="post-metadata">

**Author:** ![Roger-luo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/roger-luo/32/3399_2.png) [@Roger-luo](https://discourse.julialang.org/u/Roger-luo)\
**Post date:** [July 15, 2020, 5:09pm UTC](https://discourse.julialang.org/t/multithreading-throw-segmentfault-occationally/43132/3 "2020-07-15T17:09:04Z")

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OK as suggested from slack, I set `JULIA_COPY_STACKS=1` and now it works without segment faults! I’m not sure about why however.