# Perf improvement suggestion for simple extrema

**URL:** <https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776>\
**Category:** Performance\
**Tags:** proposal\
**Created:** [February 17, 2020, 9:03pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776 "2020-02-17T21:03:54Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![wizebt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wizebt/32/12256_2.png) [@wizebt](https://discourse.julialang.org/u/wizebt)\
**Post date:** [February 17, 2020, 9:03pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/1 "2020-02-17T21:03:54Z")

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While working on large arrays (images) searching for the fastest min/max and extrema I struggled to findout that recent fast minimum and maximum implementation use reduce. However extrema can be improved

```julia
julia> versioninfo()
Julia Version 1.3.1
Commit 2d5741174c (2019-12-30 21:36 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Xeon(R) E-2186M CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 6

julia> x=rand(Float32,1000000);
julia> @btime extrema($x)
  2.187 ms (0 allocations: 0 bytes)
(5.9604645f-7, 0.9999994f0)
julia> function minmax(x)
       """
       Find min and max values in an Array, 3x faster than extrema
       """
               a = b = x[1]
               for i in 2:length(x)
                       if x[i] > b 
                               b = x[i]
                       elseif x[i] < a 
                               a = x[i]
                       end
               end
               a, b
       end
minmax (generic function with 1 method)
julia> @btime minmax($x)
  681.715 μs (0 allocations: 0 bytes)
(5.9604645f-7, 0.9999994f0)

```

Hope it can be of some use

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<div class="post-metadata">

**Author:** ![longemen3000](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/longemen3000/32/7298_2.png) [@longemen3000](https://discourse.julialang.org/u/longemen3000)\
**Post date:** [February 17, 2020, 9:39pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/2 "2020-02-17T21:39:15Z")

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```julia
julia> methods(extrema)
# 8 methods for generic function "extrema":
[1] extrema(s::BitSet) in Base at bitset.jl:418
[2] extrema(r::AbstractRange) in Base at range.jl:579
[3] extrema(x::Real) in Base at operators.jl:486
[4] extrema(A::AbstractArray; dims) in Base at multidimensional.jl:1531
[5] extrema(itr) in Base at operators.jl:451
[6] extrema(f, x::Real) in Base at operators.jl:487
[7] extrema(f, A::AbstractArray; dims) in Base at multidimensional.jl:1542
[8] extrema(f, itr) in Base at operators.jl:468

```

there isnt any optimized implementation of `extrema(x::AbstractArray),` so its good to do. the only thing that makes me worried is how to manage not standard indices

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<div class="post-metadata">

**Author:** ![tomaklutfu](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tomaklutfu/32/2411_2.png) [@tomaklutfu](https://discourse.julialang.org/u/tomaklutfu)\
**Post date:** [February 17, 2020, 10:08pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/3 "2020-02-17T22:08:16Z")

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I don’t know if `extrema` ignores `NaN` values but the loop version here ignores. So, in reality, you may have one more check and branching.

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<div class="post-metadata">

**Author:** ![tbeason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tbeason/32/15898_2.png) [@tbeason](https://discourse.julialang.org/u/tbeason)\
**Post date:** [February 17, 2020, 10:37pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/4 "2020-02-17T22:37:17Z")

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It has been an open secret for some time that `extrema` is slow in a number of scenarios. See [Extrema is slower than maximum + minimum · Issue #31442 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/31442)

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<div class="post-metadata">

**Author:** ![wizebt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wizebt/32/12256_2.png) [@wizebt](https://discourse.julialang.org/u/wizebt)\
**Post date:** [February 18, 2020, 2:30pm UTC](https://discourse.julialang.org/t/perf-improvement-suggestion-for-simple-extrema/34776/5 "2020-02-18T14:30:28Z")

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When extrema hits NaN it returns NaN of array eltype.  
My simple function is easy to modify to skip NAN values by inserting  
`isnan(I[i]) && continue`  
as first test and not much impact on perf