# Is map(f,collection) memory inefficient?

**URL:** https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268
**Category:** Internals & Design
**Tags:** array
**Created:** [August 7, 2017, 10:41pm UTC](https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268 "2017-08-07T22:41:59Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![sambitdash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sambitdash/32/1377_2.png) [@sambitdash](https://discourse.julialang.org/u/sambitdash)
#### Post date: [August 7, 2017, 10:41pm UTC](https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268/1 "2017-08-07T22:41:59Z")

</div>

Hi All,

I used a 1MB array and applied map on it for some computation. It shows me an estimated allocation of 8MB. Can this happen or it’s some issue in reporting of `BenchmarkTools` package?

From pure intuition one can say the allocation should be the size of the input array. 8 times is a bit unlikely.

```julia
julia> @benchmark map(x -> 2x, arr)
BenchmarkTools.Trial: 
  memory estimate: 8.00 MiB
  allocs estimate: 3
  --------------
  minimum time: 1.992 ms (0.00% GC)
  median time: 2.309 ms (0.00% GC)
  mean time: 2.592 ms (7.45% GC)
  maximum time: 54.632 ms (93.92% GC)
  --------------
  samples: 1923
  evals/sample: 1

```

regards,

Sambit

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<div class="post-metadata">

### Author: ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)
#### Post date: [August 7, 2017, 10:44pm UTC](https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268/2 "2017-08-07T22:44:21Z")

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Since `arr` is a global variable at the REPL, its type could change at any time. So using it in a benchmark will give misleading results because Julia also has to deal with the potential type-instability. You need to splice in the current value with `$` like this:

```julia
@benchmark(x -> 2x, $arr)

```

Does that change your allocation results at all?

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<div class="post-metadata">

### Author: ![sambitdash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sambitdash/32/1377_2.png) [@sambitdash](https://discourse.julialang.org/u/sambitdash)
#### Post date: [August 7, 2017, 10:46pm UTC](https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268/3 "2017-08-07T22:46:26Z")

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No change

```julia
julia> @benchmark map(x -> 2x, $arr)
BenchmarkTools.Trial: 
  memory estimate: 8.00 MiB
  allocs estimate: 3
  --------------
  minimum time: 2.166 ms (0.00% GC)
  median time: 2.438 ms (0.00% GC)
  mean time: 2.970 ms (9.40% GC)
  maximum time: 57.251 ms (93.92% GC)
  --------------
  samples: 1679
  evals/sample: 1

```

---

<div class="post-metadata">

### Author: ![sambitdash](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sambitdash/32/1377_2.png) [@sambitdash](https://discourse.julialang.org/u/sambitdash)
#### Post date: [August 7, 2017, 10:59pm UTC](https://discourse.julialang.org/t/is-map-f-collection-memory-inefficient/5268/4 "2017-08-07T22:59:09Z")

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Please ignore. Didn’t realize the 2x will be a UInt and not UInt8.

```julia
julia> @benchmark map(x -> UInt8(2x), $arr)
BenchmarkTools.Trial: 
  memory estimate: 1.00 MiB
  allocs estimate: 3
  --------------
  minimum time: 936.873 μs (0.00% GC)
  median time: 951.316 μs (0.00% GC)
  mean time: 978.969 μs (1.52% GC)
  maximum time: 1.825 ms (0.00% GC)
  --------------
  samples: 5101
  evals/sample: 1

```

This fixes the issue.