# Distributing a function that uses FFTW

**URL:** <https://discourse.julialang.org/t/distributing-a-function-that-uses-fftw/69564>\
**Category:** General Usage\
**Tags:** fftw, distributed\
**Created:** [October 11, 2021, 2:36pm UTC](https://discourse.julialang.org/t/distributing-a-function-that-uses-fftw/69564 "2021-10-11T14:36:22Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![tictaccat](https://avatars.discourse-cdn.com/v4/letter/t/9f8e36/32.png) [@tictaccat](https://discourse.julialang.org/u/tictaccat)\
**Post date:** [October 11, 2021, 2:36pm UTC](https://discourse.julialang.org/t/distributing-a-function-that-uses-fftw/69564/1 "2021-10-11T14:36:23Z")

</div>

Hi! I’ve been having a lot of trouble using `pmap` with a function that uses FFTW with a planned FFT. Here is a minimum example, where I just try to perform a FFT on an array of matrices:

```julia
@everywhere using FFTW
using Distributed
addprocs(1)
pl = plan_fft(zeros(4, 4), flags=FFTW.MEASURE)
x = [zeros(4,4) for i in 1:2]
pmap(A -> pl * A, x)

```

I get a segmentation fault:

```nohighlight
      From worker 2:	signal (11): Segmentation fault
      From worker 2:	in expression starting at none:0
      From worker 2:	fftw_execute_dft at...
 From worker 2:	_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
      From worker 2:	jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
      From worker 2:	#7 at ./In[5]:3
      From worker 2:	_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
      From worker 2:	jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
      From worker 2:	jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
      From worker 2:	do_apply at /buildworker/worker/package_linux64/build/src/builtins.c:670
      From worker 2:	#106 at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:278
      From worker 2:	run_work_thunk at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:63
      From worker 2:	macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.6/Distributed/src/process_messages.jl:278 [inlined]
      From worker 2:	#105 at ./task.jl:411
      From worker 2:	unknown function (ip: 0x7ff9e1f9796c)
      From worker 2:	_jl_invoke at /buildworker/worker/package_linux64/build/src/gf.c:2237 [inlined]
      From worker 2:	jl_apply_generic at /buildworker/worker/package_linux64/build/src/gf.c:2419
      From worker 2:	jl_apply at /buildworker/worker/package_linux64/build/src/julia.h:1703 [inlined]
      From worker 2:	start_task at /buildworker/worker/package_linux64/build/src/task.c:833
      From worker 2:	Allocations: 8213770 (Pool: 8210931; Big: 2839); GC: 11

```

I’ve found a GitHub issue [here](https://github.com/tknopp/NFFT.jl/issues/17) that discusses this issue. However, I’m still mystified about how to solve it. I’ve tried plastering `@everywhere`’s everywhere and defining the providing function to pmap on every processor rather than making it anonymous. But I haven’t got things to work, much less figured out what would be the most appropriate way of doing this sort of thing in my larger scale program. I suppose I’m still a novice to how to write distributed code in Julia, in particular in how to pass information to workers (should I pass information through closures in pmap? or should I define things everywhere, and if so how without ruining my code? etc.) I would greatly appreciate your help!

---

<div class="post-metadata">

**Author:** ![tictaccat](https://avatars.discourse-cdn.com/v4/letter/t/9f8e36/32.png) [@tictaccat](https://discourse.julialang.org/u/tictaccat)\
**Post date:** [October 11, 2021, 2:57pm UTC](https://discourse.julialang.org/t/distributing-a-function-that-uses-fftw/69564/2 "2021-10-11T14:57:30Z")

</div>

The code seems to work if I do:

```julia
@everywhere using FFTW
using Distributed
addprocs(1)
@everywhere pl = plan_fft(zeros(4, 4), flags=FFTW.MEASURE)
x = [ones(4,4) for i in 1:2]
@everywhere f(A) = pl * A
pmap(A -> f(2*A), x)

```

Based on the GitHub issue, it seems like FFTW plans are not serializable, which causes the seg fault. This fixes the problem here, but what can I do in general? In my code, I do things such as create a struct which has an FFT plan as an attribute. Will I need to put an `@everywhere` on every line in the entire pipeline of code that contains my FFT plan? (e.g. in the example above, I needed to put an `@everywhere` on the function `f`. This seems really hard to do in my codebase, and also seems to go against Julia’s usual model of distributed computation. Please let me know if there’s a better way.