# Arrays of arrays and arrays of structures in CUDA kernels cause random errors

**URL:** https://discourse.julialang.org/t/arrays-of-arrays-and-arrays-of-structures-in-cuda-kernels-cause-random-errors/69739
**Category:** GPU
**Tags:** gpu, cuda
**Created:** [October 14, 2021, 10:41am UTC](https://discourse.julialang.org/t/arrays-of-arrays-and-arrays-of-structures-in-cuda-kernels-cause-random-errors/69739 "2021-10-14T10:41:45Z")
**Posts on this page:** 1
**Showing post:** 8

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### Author: ![fedoroff](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fedoroff/32/53209_2.png) [@fedoroff](https://discourse.julialang.org/u/fedoroff)
#### Post date: [October 17, 2021, 11:05am UTC](https://discourse.julialang.org/t/arrays-of-arrays-and-arrays-of-structures-in-cuda-kernels-cause-random-errors/69739/8 "2021-10-17T11:05:08Z")

</div>

Thank you, I have read the docs for [`GC.@preserve`](https://docs.julialang.org/en/v1/base/base/#Base.GC.@preserve) and this interesting discussion: [On the garbage collection](https://discourse.julialang.org/t/on-the-garbage-collection/35695). I hope now I understand what causes the error in my case. However, it is not clear for me how I should use `GC.@preserve` in case of array of arrays. Here I prepared a much more simple MWE:

```julia
using CUDA

function kernel(a, bb)
    id = threadIdx().x
    a[id] = sum(bb[id])
    return nothing
end

N = 10

a = CUDA.zeros(N)

b = Array{CuArray}(undef, N)
for i=1:N
    b[i] = CUDA.ones(2)
end

# This potentially can cause an error
bb = CuArray([cudaconvert(b[i]) for i=1:N])
@cuda threads=N kernel(a, bb)

# Option 1:
bb = CuArray([cudaconvert(b[i]) for i=1:N])
GC.@preserve b begin
    @cuda threads=N kernel(a, bb)
end

# Option 2:
btmp = [cudaconvert(b[i]) for i=1:N]
bb = CuArray(btmp)
GC.@preserve btmp begin
    @cuda threads=N kernel(a, bb)
end

```

In this example I do not understand what exactly I should preserve: the original array of CuArrays `b`, the temporary array of CuDeviceArrays `[cudaconvert(b[i]) for i=1:N]`, or both of them.

Sorry, it is very hard to debug such code, since GC manages to collect the original objects only if the kernel has been running long enough. Can I mimic the GC behaviour and cause the error by myself using e.g. something like `finilize(b)`?

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