# Memrory allocation

**URL:** <https://discourse.julialang.org/t/memrory-allocation/17669>\
**Category:** General Usage\
**Tags:** question\
**Created:** [November 18, 2018, 10:29am UTC](https://discourse.julialang.org/t/memrory-allocation/17669 "2018-11-18T10:29:01Z")\
**Posts on this page:** 8\
**Page:** 1

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**Author:** ![Ajaychat3](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@Ajaychat3](https://discourse.julialang.org/u/Ajaychat3)\
**Post date:** [November 18, 2018, 10:29am UTC](https://discourse.julialang.org/t/memrory-allocation/17669/1 "2018-11-18T10:29:01Z")

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Can someone explain why the memory allocation is differnet between foo and foo2 and increased time between foo2(21) and foo2(22).

```julia
using Distributed,SharedArrays,BenchmarkTools,StatsBase
function foo(m::Int64)
   z = SharedArray{Int64, 1}(m)
   for i = 1:m
       z[i] = i
   end
   StatsBase.mean(z)
end
foo (generic function with 1 method)
function foo2(m::Int64)
   z ::Float64 = 0.0
     for i = 1:m
        z +=i/m
   end
   z
end
foo2 (generic function with 1 method)
# function foo3(m::Int64)
# z=0.0
# for i = 1:m
# z += i/m
# end
# z
# end
@btime foo(10)
@btime foo(21)
@btime foo(22)
@btime foo2(10)
@btime foo2(21)
@btime foo2(22)
  27.077 μs (104 allocations: 4.66 KiB)
  27.077 μs (104 allocations: 4.66 KiB)
  27.897 μs (104 allocations: 4.66 KiB)
  1.230 ns (0 allocations: 0 bytes)
  1.230 ns (0 allocations: 0 bytes)
  21.743 ns (0 allocations: 0 bytes)
11.5
@btime foo2(100)
  100.228 ns (0 allocations: 0 bytes)
50.5
foo2(10)
5.5

```

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<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [November 18, 2018, 10:45am UTC](https://discourse.julialang.org/t/memrory-allocation/17669/2 "2018-11-18T10:45:08Z")

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1.2ns is so short the entire loop is probably compiled away by the compiler. After a certain limit on the loop length, it seems the compiler no longer does this?

Inspected the output of @code\_llvm or @code\_native?

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**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:** [November 18, 2018, 11:50am UTC](https://discourse.julialang.org/t/memrory-allocation/17669/3 "2018-11-18T11:50:34Z")

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Since you use constants, the compiler smart to constant fold the results and giving the results at runtime until it reaches a milestone. If constant folding is suppressed I get

```julia
julia> for i in 17:23
       println(i); @btime foo2($i)
       end
17
  25.539 ns (0 allocations: 0 bytes)
18
  26.766 ns (0 allocations: 0 bytes)
19
  27.859 ns (0 allocations: 0 bytes)
20
  29.131 ns (0 allocations: 0 bytes)
21
  30.288 ns (0 allocations: 0 bytes)
22
  31.514 ns (0 allocations: 0 bytes)
23
  32.530 ns (0 allocations: 0 bytes)

```

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<div class="post-metadata">

**Author:** ![Ajaychat3](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@Ajaychat3](https://discourse.julialang.org/u/Ajaychat3)\
**Post date:** [November 18, 2018, 12:52pm UTC](https://discourse.julialang.org/t/memrory-allocation/17669/4 "2018-11-18T12:52:27Z")

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> [@tomaklutfu](#):
>
> Since you use constants, the compiler smart to constant fold the results and giving the results at runtime until it reaches a milestone. If constant folding is suppressed I get
> 
> ```julia
> 
> ```

I have difficulty in understanding the concept of constant folding.  
You mean to say by prefixing the argument to foo2(), the constant folding is suppressed. Since I haven’t not used constant folding, there is something happening at foo2(22j which leads to a sudden jump in runtime. Is it related to architecture of compiler or the machine I am using (Win10 : i7)? Can it be altered by the user?

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<div class="post-metadata">

**Author:** ![Ajaychat3](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@Ajaychat3](https://discourse.julialang.org/u/Ajaychat3)\
**Post date:** [November 18, 2018, 1:01pm UTC](https://discourse.julialang.org/t/memrory-allocation/17669/5 "2018-11-18T13:01:11Z")

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I am yet to learn how to interpret the code\_llvm or code\_native output. Is there any simple and easy documentation around this?

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<div class="post-metadata">

**Author:** ![Ajaychat3](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@Ajaychat3](https://discourse.julialang.org/u/Ajaychat3)\
**Post date:** [November 18, 2018, 1:10pm UTC](https://discourse.julialang.org/t/memrory-allocation/17669/6 "2018-11-18T13:10:13Z")

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some more results. why the foo takes over?

```julia
@btime foo(45)
@btime foo2(45)

  27.487 μs (104 allocations: 4.66 KiB)
  44.717 ns (0 allocations: 0 bytes)

```

PS sorry it it not. I misunderstood the timing.

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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:** [November 18, 2018, 1:12pm UTC](https://discourse.julialang.org/t/memrory-allocation/17669/7 "2018-11-18T13:12:41Z")

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Constant fold is something like this, say there is hypothetical function `g(x) = x*foo2(10)` . When you call `g(...)`, the compiler is smart so the result of `foo2(10)` will be replaced the value itself. When using `@btime foo2(10)` vs `x=10; @btime foo2($x)`, the former just uses result of `foo2(10)` at runtime but the latter computes `foo2(x)` each time it is called.

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<div class="post-metadata">

**Author:** ![Ajaychat3](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@Ajaychat3](https://discourse.julialang.org/u/Ajaychat3)\
**Post date:** [November 18, 2018, 1:17pm UTC](https://discourse.julialang.org/t/memrory-allocation/17669/8 "2018-11-18T13:17:45Z")

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@tomaklutfu: Thanks for explaining.