# Memory allocations are not very understandable

**URL:** <https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696>\
**Category:** Performance\
**Created:** [December 11, 2017, 9:25pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696 "2017-12-11T21:25:15Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 11, 2017, 9:25pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/1 "2017-12-11T21:25:15Z")

</div>

I am having a hard time understanding how memory is managed by Julia.

Why do the following lines allocate memory at all?

```julia
        - function winsMilestone(topSide, bottomSide, justPlayedCard::Player)
827581056 if (countnz(topSide) < 3 || countnz(bottomSide) < 3)
        0 return none
        - end
       end

```

Knowing that topSide and bottomSide are of the same type and are views on a UInt8 Array?

---

<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:** [December 11, 2017, 9:29pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/2 "2017-12-11T21:29:54Z")

</div>

That looks like it might be allocation caused by compilation itself. Did you follow the instructions to do `Profile.clear_malloc_data()` from [https://docs.julialang.org/en/stable/manual/profile/#Memory-allocation-analysis-1](https://docs.julialang.org/en/stable/manual/profile/#Memory-allocation-analysis-1) ?

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 11, 2017, 10:16pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/3 "2017-12-11T22:16:00Z")

</div>

Yes, I did.

---

<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:** [December 11, 2017, 10:52pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/4 "2017-12-11T22:52:07Z")

</div>

Hm, that’s really interesting. It will be easier to help if you can post a full reproducible example. Can you create a standalone demo of the issue?

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 9:09am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/5 "2017-12-12T09:09:03Z")

</div>

Here is a minimal example:

```julia
function winsMilestone(topSide, bottomSide)
    a = 0

    if (countnz(topSide) < 3 || countnz(bottomSide) < 3)
         return 1
    end

    return 2
end

function test(n_iter)
    A = zeros(UInt8,(7, 9))
    for i=1:n_iter
        for j=1:9
            @views winsMilestone(A[1:3, j], A[4:7, j])
        end
    end
end

test(1)

Profile.clear_malloc_data()
test(1000)

```

And with memory annotations:

```julia
        - function winsMilestone(topSide, bottomSide)
        - a = 0
        - 
  1152000 if (countnz(topSide) < 3 || countnz(bottomSide) < 3)
        0 return 1
        - end
        - 
        0 return 2
        - end
        - 
        - function test(n_iter)
      304 A = zeros(UInt8,(7, 9))
        0 for i=1:n_iter
        0 for j=1:9
        0 @views winsMilestone(A[1:3, j], A[4:7, j])
        - end
        - end
        - end
        - 
        - test(1)
        - 
        - Profile.clear_malloc_data()
        - test(1000)

```

The line `a=0` is to show that memory is not associated with the first line of the function.  
The problem is probably not related to `countnz` since if the tested expression in the `if` by the corresponding disjunction on all the elements of the array, the problem is the same.

Thanks.

---

<div class="post-metadata">

**Author:** ![jmert](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmert/32/3161_2.png) [@jmert](https://discourse.julialang.org/u/jmert)\
**Post date:** [December 12, 2017, 9:56am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/6 "2017-12-12T09:56:25Z")

</div>

I think it’s just the allocation tracker misattributing the allocation. If you create the views before the call, then `winsMilestone` shows no allocation:

```julia
function winsMilestone(topSide, bottomSide)
    a = 0

    if (countnz(topSide) < 3 || countnz(bottomSide) < 3)
         return 1
    end

    return 2
end

function test(n_iter)
    A = zeros(UInt8,(7, 9))
    for i=1:n_iter
        for j=1:9
            A1 = @view A[1:3, j]
            A2 = @view A[4:7, j]
            winsMilestone(A1, A2)
        end
    end
end

test(1)

Profile.clear_malloc_data()
test(1000)

```

you end up with the `*.mem` output:

```julia
        - function winsMilestone(topSide, bottomSide)
        - a = 0
        -
        0 if (countnz(topSide) < 3 || countnz(bottomSide) < 3)
        0 return 1
        - end
        -
        0 return 2
        - end
        -
        - function test(n_iter)
      304 A = zeros(UInt8,(7, 9))
        0 for i=1:n_iter
        0 for j=1:9
   576000 A1 = @view A[1:3, j]
   576000 A2 = @view A[4:7, j]
        0 winsMilestone(A1, A2)
        - end
        - end
        - end
        -
        - test(1)
        -
        - Profile.clear_malloc_data()
        - test(1000)
        -
        -

```

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 10:33am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/7 "2017-12-12T10:33:51Z")

</div>

Why would a view allocate so much memory?  
Also, why is A allocated using 304 bytes when the actual data it holds is 63 bytes (plus size and type information)?

Thanks,

---

<div class="post-metadata">

**Author:** ![jmert](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jmert/32/3161_2.png) [@jmert](https://discourse.julialang.org/u/jmert)\
**Post date:** [December 12, 2017, 11:14am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/8 "2017-12-12T11:14:47Z")

</div>

> Why would a view allocate so much memory?

Because it’s not a zero-cost abstraction, and the view object needs space to describe what the view is covering.

```julia-repl
julia> A = zeros(UInt8, 7, 9);

julia> a = view(A, 1:3, 1);

julia> sizeof(a)
48

julia> fieldnames(a)
4-element Array{Symbol,1}:
 :parent 
 :indexes
 :offset1
 :stride1

julia> typeof(a)
SubArray{UInt8,1,Array{UInt8,2},Tuple{UnitRange{Int64},Int64},true}

```

In your example, 576,000 bytes / 9,000 iterations = 64 bytes whereas `sizeof` returns 48 bytes, but the difference is probably a matter of the underlying memory manager aligning the objects to word boundaries.

> Also, why is A allocated using 304 bytes when the actual data it holds is 63 bytes (plus size and type information)?

Probably type information plus memory alignment, but you’ll probably need to dig into the C internals (or have someone who already knows the anwer comment) to verify if that accounts for all of the difference.

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 12, 2017, 11:21am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/9 "2017-12-12T11:21:13Z")

</div>

FWIW, allocation of `views` should in 0.7 be elided much more frequently than in 0.6.

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 11:28am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/10 "2017-12-12T11:28:29Z")

</div>

@jmert, is the only possible optimization to inline `winsMilestone` by end and doing the disjunction directly on the A array?

@kristoffer.carlsson, is it already implemented?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 12, 2017, 11:56am UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/11 "2017-12-12T11:56:11Z")

</div>

Yes it just got merged. You might need to `@inline` `winsMilestone` but I am not sure.

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 1:01pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/12 "2017-12-12T13:01:43Z")

</div>

Maybe Julia is just not a match for my needs.

[Edit] But I wish it is not the case.

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 1:59pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/13 "2017-12-12T13:59:38Z")

</div>

Testing Julia from master branch (easy compilation BTW), I have:

```julia
        - using Profile
        - 
        - @inline function winsMilestone(topSide, bottomSide)
        - a = 0
        - 
        0 if (count(!iszero, topSide) < 3 || count(!iszero, bottomSide) < 3)
        0 return 1
        - end
        - 
        0 return 2
        - end
        - 
        - function test(n_iter)
        0 A = zeros(UInt8,(7, 9))
        0 top_idx = collect(1:3)
        0 bottom_idx = collect(4:7)
        0 for i=1:n_iter
        0 for j=1:9
        0 A1 = @view A[top_idx, j]
        0 A2 = @view A[bottom_idx, j]
        0 winsMilestone(A1, A2)
        - end
        - end
        - end
        - 
        - test(1)
        - 
        - Profile.clear_malloc_data()
        - test(1000)

```

It seems a bit odd. Isn’t it?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 12, 2017, 2:01pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/14 "2017-12-12T14:01:31Z")

</div>

`@time` also prints the number of allocations, you can compare to that 0.6.

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 2:17pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/15 "2017-12-12T14:17:23Z")

</div>

julia master:

```julia
0.000153 seconds (18.13 k allocations: 572.787 KiB)

```

julia 0.6:

```julia
0.000870 seconds (54.09 k allocations: 1.929 MiB)

```

It is better, indeed (I don’t know if the two version where compiled with the same options).

Is memory allocation tracking broken with julia 0.7?

---

<div class="post-metadata">

**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [December 12, 2017, 2:19pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/16 "2017-12-12T14:19:13Z")

</div>

On today’s master, with the deprecation fixed and an `@inline` added it’s fixed:

```julia
julia> function test(n_iter)
           A = zeros(UInt8,(7, 9))
           for i=1:n_iter
               for j=1:9
                   A1 = @view A[1:3, j]
                   A2 = @view A[4:7, j]
                   winsMilestone(A1, A2)
               end
           end
       end                                                                                                                                                               
test (generic function with 1 method)                                                                                                                                    

julia> @inline function winsMilestone(topSide, bottomSide)
           a = 0

           if (count(!iszero, topSide) < 3 || count(!iszero, bottomSide) < 3)
                return 1
           end

           return 2
       end
winsMilestone (generic function with 1 method)

julia> @time test(1000) # warum-up                                                                                                                                          
  0.085711 seconds (132.28 k allocations: 7.346 MiB, 7.89% gc time)                                                                                                      

julia> @time test(1000)                                                                                                                                                  
  0.000033 seconds (5 allocations: 304 bytes)                                                                                                                            

```

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 2:31pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/17 "2017-12-12T14:31:23Z")

</div>

I can reproduce it. I got tricked by @time annotation that also need to be pre-compiled.

Anyway the generated mem file is now wrong since the 304 bytes are not annoted when A is bound:

```julia
        - using Profile
        - 
        - @inline function winsMilestone(topSide, bottomSide)
        - a = 0
        - 
        0 if (count(!iszero, topSide) < 3 || count(!iszero, bottomSide) < 3)
        0 return 1
        - end
        - 
        0 return 2
        - end
        - 
        - function test(n_iter)
        0 A = zeros(UInt8,(7, 9))
        - #top_idx = collect(1:3)
        - #bottom_idx = collect(4:7)
        0 for i=1:n_iter
        0 for j=1:9
        - #A1 = @view A[top_idx, j]
        - #A2 = @view A[bottom_idx, j]
        0 @views winsMilestone(A[1:3, j], A[4:7, j])
        - end
        - end
        - end
        - 
        - @time test(1)
        - 
        - Profile.clear_malloc_data()
        - @time test(1000)

```

It will make code harder to debug.

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 12, 2017, 2:38pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/18 "2017-12-12T14:38:20Z")

</div>

I think these 5 allocations are from the the stuff that `@time` expands to, which will be in global scope.

Try

```julia
f() = @time(1000)
f()
f()

```

---

<div class="post-metadata">

**Author:** ![rvlander](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rvlander/32/2863_2.png) [@rvlander](https://discourse.julialang.org/u/rvlander)\
**Post date:** [December 12, 2017, 2:43pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/19 "2017-12-12T14:43:11Z")

</div>

```julia
julia> f() = @time(1000)
f (generic function with 1 method)

julia> f()
  0.000000 seconds
1000

julia> f()
  0.000000 seconds
1000

```

Don’t they come from the call to `zeros`?

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [December 12, 2017, 2:44pm UTC](https://discourse.julialang.org/t/memory-allocations-are-not-very-understandable/7696/20 "2017-12-12T14:44:06Z")

</div>

Since you don’t return anything, everything is probably optimized away.

edit; Or wait, you will return something from the last evaluation in the for loop I guess.

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