# Segmentation Fault with FFTW plans and multi-threading

**URL:** <https://discourse.julialang.org/t/segmentation-fault-with-fftw-plans-and-multi-threading/116777>\
**Category:** General Usage\
**Tags:** fftw, multithreading\
**Created:** [July 8, 2024, 2:44pm UTC](https://discourse.julialang.org/t/segmentation-fault-with-fftw-plans-and-multi-threading/116777 "2024-07-08T14:44:26Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![leonardromano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/leonardromano/32/210519_2.png) [@leonardromano](https://discourse.julialang.org/u/leonardromano)\
**Post date:** [July 8, 2024, 2:44pm UTC](https://discourse.julialang.org/t/segmentation-fault-with-fftw-plans-and-multi-threading/116777/1 "2024-07-08T14:44:26Z")

</div>

I am trying to optimize a routine that performs FFTs on timeseries of various 3D boxes with fixed but different sizes (typically 256 - 512 cells per dimension).

For each box I thus had in mind to plan an FFT using FFTWs “plan\_fft()” function and on top of that I was planning to use multithreading.

An example code could look like this

```julia
using FFTW # v.1.8.0

FFTW.set_num_threads(Threads.nthreads()) # In my case 32

FFTs = [plan_fft(randn(Float32, rand(256:512), rand(256:512), rand(256:512)), flags=FFTW.MEASURE) for _ in 1:32]

```

This code runs perfectly fine on a single thread, but with multithreading it throws a segmentation fault after a seemingly random number of iterations:

```julia
[1599023] signal (11.1): Segmentation fault
in expression starting at REPL[7]:1
spawn_apply at /home/lromano/.julia/artifacts/e95ca94c82899616429924e9fdc7eccda275aa38/lib/libfftw3f.so (unknown line)
#2 at /home/lromano/.julia/packages/FFTW/6nZei/src/providers.jl:58
unknown function (ip: 0x7c154ff2674f)
_jl_invoke at /cache/build/builder-amdci4-6/julialang/julia-release-1-dot-10/src/gf.c:2894 [inlined]
ijl_apply_generic at /cache/build/builder-amdci4-6/julialang/julia-release-1-dot-10/src/gf.c:3076
jl_apply at /cache/build/builder-amdci4-6/julialang/julia-release-1-dot-10/src/julia.h:1982 [inlined]
start_task at /cache/build/builder-amdci4-6/julialang/julia-release-1-dot-10/src/task.c:1238
Allocations: 45645903 (Pool: 45640544; Big: 5359); GC: 16
Segmentation fault

```

Has anyone run into the same or similar issues with FFTW and knows how to fix them?

I need to add that my code was running perfectly fine until Thursday night and actually has already finished processing ~2/3 of my data. Then suddenly on Friday it stopped working and I now cannot even move beyond the “plan\_fft” stage.  
I tried running on a single thread over the weekend but it crashed when applying the FFT operator.

---

<div class="post-metadata">

**Author:** ![jipolanco](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jipolanco/32/12129_2.png) [@jipolanco](https://discourse.julialang.org/u/jipolanco)\
**Post date:** [July 8, 2024, 3:47pm UTC](https://discourse.julialang.org/t/segmentation-fault-with-fftw-plans-and-multi-threading/116777/2 "2024-07-08T15:47:14Z")

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This looks similar to [this FFTW.jl issue](https://github.com/JuliaMath/FFTW.jl/issues/236).

I have also run into this issue recently. One workaround which has been suggested is to use the MKL FFTW interface. I tried this some time ago and initially run into performance issues when using MKL and threads. If it helps, after some trial and error I’ve written [here](https://jipolanco.github.io/VortexPasta.jl/dev/tips/parallelisation/#Using-MKL-FFT-routines) about the “proper” way of using the MKL provider which avoids these issues.