# How to implement multi-threading with external in-place mutable variables?

**URL:** <https://discourse.julialang.org/t/how-to-implement-multi-threading-with-external-in-place-mutable-variables/62610>\
**Category:** General Usage\
**Tags:** question\
**Created:** [June 9, 2021, 7:55am UTC](https://discourse.julialang.org/t/how-to-implement-multi-threading-with-external-in-place-mutable-variables/62610 "2021-06-09T07:55:32Z")\
**Posts on this page:** 1\
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**Author:** ![mlanghinrichs](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mlanghinrichs/32/50371_2.png) [@mlanghinrichs](https://discourse.julialang.org/u/mlanghinrichs)\
**Post date:** [June 9, 2021, 7:55am UTC](https://discourse.julialang.org/t/how-to-implement-multi-threading-with-external-in-place-mutable-variables/62610/1 "2021-06-09T07:55:32Z")

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Hey,  
I’m wondering how to implement a method with multi-threading that also allows for mutating external variables in-place. The idea is that each thread/base requires their “own” external variable for data race free mutations.

One solution I came up with is based on [`FLoops.jl`](https://juliafolds.github.io/FLoops.jl/dev/) (down below). The `@floop` macro can run the for-loop in single or multiple threads (`ex=SequentialEx()` or `ex=ThreadedEx()`, respectively). The `@init` macro, together with a `deepcopy()`, should handle data race free mutating variables. Does anyone understand what I want and is the code correct and safe?

```julia
using FLoops

# method to emulate some "runtime"
sleep2 = t -> (b=time(); while b+t > time() end)

function FL_ex3!(y, varexternal, niter; ex=ThreadedEx())
   @floop ex for i in 1:niter
       @init c = deepcopy(varexternal)
       sleep2(0.25)
       c .= (1.0*i, 2.0*i, 3.0*i)
       y[i, :] .= c
   end
   y
end

### sequential and multi-threading give the same output (as a check)
FL_ex3!(zeros(10, 3), zeros(3), 10, ex=SequentialEx()) == FL_ex3!(zeros(10, 3), zeros(3), 10)
# true

### compare performance multi-threaded vs. single-threaded
Threads.nthreads() # 10

using BenchmarkTools
@btime FL_ex3!(zeros(10, 3), zeros(3), 10)
# 250.124 ms (150 allocations: 10.45 KiB)

@btime FL_ex3!(zeros(10, 3), zeros(3), 10, ex=SequentialEx())
# 2.500 s (7 allocations: 960 bytes)

```

Multi-threading is a bit new to me, so sorry if the terminology is a bit off. Would be very glad for input!

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