# Mapslices very slow
**URL:** https://discourse.julialang.org/t/mapslices-very-slow/86826
**Category:** GPU
**Created:** [September 6, 2022, 2:12am UTC](https://discourse.julialang.org/t/mapslices-very-slow/86826 "2022-09-06T02:12:50Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)
#### Post date: [September 6, 2022, 2:12am UTC](https://discourse.julialang.org/t/mapslices-very-slow/86826/1 "2022-09-06T02:12:50Z")
new to CUDA
`mapsilces` seems to run very slow.
`reduce` is very fast but requires an accumulator (e.g. +) instead of a function (e.g. sum).
I’d like to apply the `linear_interpolation` function to slices of a matrix (or tensor).
Can a GPU do this faster than CPU?
Would I have to re-write the `linear_interpolation` function as a custom Kernel?
I’d rather not do this
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### Author:  [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)
#### Post date: [September 6, 2022, 7:13am UTC](https://discourse.julialang.org/t/mapslices-very-slow/86826/2 "2022-09-06T07:13:29Z")
`mapslices` is known to be slow, at least on CPU
>
>
> Consider the following piece of code:
> \`\`\`
> n=10^6;
> X = randn(1,n);
> V(x) = 0.5… \* dot(x,x);
> f(V, X) = \[V((@view X\[:,n\])) for n = 1:size(X,2) \]
> \`\`\`
> Timing on this reveals:
> \`\`\`
> julia\> @time mapslices(V, X, 1);
> 1.177690 seconds (12.00 M allocations: 267.024 MiB, 6.91% gc time)
> julia\> @time f(V,X);
> 0.047745 seconds (1.00 M allocations: 53.406 MiB, 5.58% gc time)
> \`\`\`
> which is substantially different.
>
> Note that in this simple example, my data could be a column vector; however, this is a surrogate for problems where I have time series data and \`\`X\`\` is d x n with 1\< d \<\< n.
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### Author:  [@maleadt](https://discourse.julialang.org/u/maleadt)
#### Post date: [September 8, 2022, 6:18am UTC](https://discourse.julialang.org/t/mapslices-very-slow/86826/3 "2022-09-08T06:18:25Z")
`*slices` (mapslices, eachslice) functions are not supported, because they take a function that transforms a slice and as such cannot be compiled into a single kernel. Instead, we need to call the function N times (for every slice) which results in many small kernels being launched.
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### Author:  [@Lincoln\_Hannah](https://discourse.julialang.org/u/Lincoln_Hannah)
#### Post date: [September 13, 2022, 11:56am UTC](https://discourse.julialang.org/t/mapslices-very-slow/86826/4 "2022-09-13T11:56:53Z")
many thanks:)