# Error in a parallel loop in Julia 0.7 rc2

**URL:** <https://discourse.julialang.org/t/error-in-a-parallel-loop-in-julia-0-7-rc2/12925>\
**Category:** General Usage\
**Tags:** parallel\
**Created:** [August 6, 2018, 6:55am UTC](https://discourse.julialang.org/t/error-in-a-parallel-loop-in-julia-0-7-rc2/12925 "2018-08-06T06:55:36Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Alexander-Barth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alexander-barth/32/3692_2.png) [@Alexander-Barth](https://discourse.julialang.org/u/Alexander-Barth)\
**Post date:** [August 6, 2018, 6:55am UTC](https://discourse.julialang.org/t/error-in-a-parallel-loop-in-julia-0-7-rc2/12925/1 "2018-08-06T06:55:36Z")

</div>

Since Julia version 0.7.0-rc2, I have an error in a parallel loop.  
Unfortunately, I am not able to narrow this error down to a minimum code snippet.

The error message (from a clean julia restart) is every time different. Here is an example:

```julia
  Closest candidates are:
    +(::Any, ::Any, !Matched::Any, !Matched::Any...) at operators.jl:502
  add_sum(::Type, ::Core.Compiler.Const) at ./reduce.jl:21
  _mapreduce(::typeof(identity), ::typeof(Base.add_sum), ::IndexLinear, ::Array{Any,1}) at ./reduce.jl:311
  _mapreduce_dim at ./reducedim.jl:305 [inlined]
  #mapreduce#542 at ./reducedim.jl:301 [inlined]
  mapreduce at ./reducedim.jl:301 [inlined]
  _sum at ./reducedim.jl:650 [inlined]
  _sum at ./reducedim.jl:649 [inlined]
  #sum#544 at ./reducedim.jl:645 [inlined]
  sum at ./reducedim.jl:645 [inlined]
  #DIVAndjog#190(::Array{Core.LineInfoNode,1}, ::Base.Iterators.Pairs{Symbol,Any,NTuple{5,Symbol},NamedTuple{(:moddim, :MEMTOFIT, :QCMETHOD, :RTIMESONESCALES, :velocity),Tuple{Array{Float64,1},Int64,Tuple{},Tuple{},Tuple{}}}}, ::Function, ::BitArray{3}, ::Tuple{Array{Float64,3},Array{Float64,3},Array{Float64,3}}, ::Tuple{Array{Float64,3},Array{Float64,3},Array{Float64,3}}, ::Tuple{Array{Float64,1},Array{Float64,1},Array{Float64,1}}, ::Array{Float64,1}, ::Tuple{Array{Float64,3},Array{Float64,3},Array{Float64,3}}, ::Array{Float64,1}, ::Array{Any,1}, ::Array{Any,1}, ::Int64) at /home/abarth/projects/Julia/divand.jl/src/DIVAndjog.jl:36

```

Or this:

```julia
  LoadError: ReadOnlyMemoryError()
   ... (the last 2 lines are repeated 1 more time)
   ... (the last 2 lines are repeated 1 more time)
   ... (the last 2 lines are repeated 8 more times)
   ... (the last 20 lines are repeated 1 more time)
   ... (the last 2 lines are repeated 13 more times)

```

Or just:

Speicherzugriffsfehler (which translates to memory access error) without any Julia error message.

The parallel loop in question looks like this:

```julia
    @sync @parallel for iwin = 1:size(windowlist,1)
    [...]
    end

```

In my test I run the parallel loop with only 1 CPU. The error persists if I replace `@parallel` by `@distributed`, but it does away when I make to loop serial (i.e. delete `@sync @parallel`).

The error is not 100% reproducible. Sometimes (maybe 1 out of 10) the code passes.

Here is a link the error on travis with the full message:

> **[Travis CI - Test and Deploy with Confidence](https://app.travis-ci.com/gher-ulg/DIVAnd.jl/jobs/412384891)**
>
> Travis CI enables your team to test and ship your apps with confidence. Easily sync your projects with Travis CI and you'll be testing your code in minutes.

It uses the branch Alex-0.7 from [GitHub - gher-uliege/DIVAnd.jl: DIVAnd performs an n-dimensional variational analysis of arbitrarily located observations](https://github.com/gher-ulg/DIVAnd.jl) . The error is triggered by the script test\_product.jl called by runtest.jl.

The last time where the parallel loop worked was 0.7.0-rc1.3 (Commit fe9a0752aa) and the first time I saw this issue was on  
0.7.0-rc1.14 (Commit ea5871a804). For what it is worth, here are the changes between those version:

> **[Comparing fe9a0752aa...ea5871a804 · JuliaLang/julia](https://github.com/JuliaLang/julia/compare/fe9a0752aa...ea5871a804)**
>
> The Julia Programming Language. Contribute to JuliaLang/julia development by creating an account on GitHub.

---

<div class="post-metadata">

**Author:** ![Alexander-Barth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alexander-barth/32/3692_2.png) [@Alexander-Barth](https://discourse.julialang.org/u/Alexander-Barth)\
**Post date:** [August 6, 2018, 9:59am UTC](https://discourse.julialang.org/t/error-in-a-parallel-loop-in-julia-0-7-rc2/12925/2 "2018-08-06T09:59:26Z")

</div>

I also get these kinds of errors:

```julia
GC error (probable corruption) :
Allocations: 64456802 (Pool: 64443516; Big: 13286); GC: 144
<?#0x7f863b07a580::(nil)>
0x7f864c249010: Queued root: 0x7f865d7488d0 :: 0x7f865a834c50 (bits: 3)
        of type Core.TypeMapEntry
0x7f864c249028: Queued root: 0x7f863b39d590 :: 0x7f865a835c10 (bits: 3)
        of type Core.MethodInstance
0x7f864c249040: Queued root: 0x7f863b39db90 :: 0x7f865a835c10 (bits: 3)
        of type Core.MethodInstance
0x7f864c249058: Queued root: 0x7f863b39e290 :: 0x7f865a835c10 (bits: 3)
        of type Core.MethodInstance
0x7f864c249070: Queued root: 0x7f863b39ea10 :: 0x7f865a835c10 (bits: 3)
        of type Core.MethodInstance
[...]

```

Maybe it is a memory error in Julia?

---

<div class="post-metadata">

**Author:** ![Alexander-Barth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/alexander-barth/32/3692_2.png) [@Alexander-Barth](https://discourse.julialang.org/u/Alexander-Barth)\
**Post date:** [August 8, 2018, 10:01am UTC](https://discourse.julialang.org/t/error-in-a-parallel-loop-in-julia-0-7-rc2/12925/3 "2018-08-08T10:01:29Z")

</div>

This bug is fixed in Julia 0.7.0 rc 3.