# Performance hit when using callbacks

**URL:** <https://discourse.julialang.org/t/performance-hit-when-using-callbacks/5107>\
**Category:** General Usage\
**Created:** [July 28, 2017, 9:40am UTC](https://discourse.julialang.org/t/performance-hit-when-using-callbacks/5107 "2017-07-28T09:40:30Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![krcools](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/krcools/32/10212_2.png) [@krcools](https://discourse.julialang.org/u/krcools)\
**Post date:** [July 28, 2017, 9:40am UTC](https://discourse.julialang.org/t/performance-hit-when-using-callbacks/5107/1 "2017-07-28T09:40:30Z")

</div>

I am trying to assemble a matrix using multiple processes. To make sure each process writes to the part of the matrix it owns, I am using a callback as one of the arguments to the single process jobs launched by the driver.

```julia
function assemble()
    M = N = 1000
    A = zeros(M,N)
    store = (v,m,n) -> (A[m,n] += v)

    P = procs()
    length(P) > 1 && (P = P[2:end])
    splits = [round(Int,s) for s in linspace(0, M, length(P)+1)]

    @sync begin
        for (i,p) in enumerate(P)
            start, stop = splits[i]+1, splits[i+1]
            storei = (v,m,n) -> store(v,start+m-1,n)
            Mi = stop - start + 1
            @async remotecall_wait(assemblechunk, p, Mi, N, storei)
    end end
    A
end

function assemblechunk(M,N,store)
    for m in 1:M
        for n in 1:N
            v = randn()
            store(v,m,n)
end end end

assemble(); @time assemble();

```

I am getting a lot of allocations and a serious slowdown of execution by doing this. The output of `@time` is:

```julia
0.167571 seconds (5.45 M allocations: 90.761 MiB, 8.63% gc time)

```

When I replace `storei` by `store` (which I know gives not the desired result), the penalty disappears:

```julia
0.011816 seconds (115 allocations: 7.662 MiB)

```

Any idea where this originates from? Or even how to start analysing this?

---

<div class="post-metadata">

**Author:** ![krcools](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/krcools/32/10212_2.png) [@krcools](https://discourse.julialang.org/u/krcools)\
**Post date:** [July 28, 2017, 9:56am UTC](https://discourse.julialang.org/t/performance-hit-when-using-callbacks/5107/2 "2017-07-28T09:56:23Z")

</div>

Ah…

[https://github.com/JuliaLang/julia/issues/15276](https://github.com/JuliaLang/julia/issues/15276)

Changing the corresponding line to the following fixed it:

```julia
start::Int, stop::Int = splits[i]+1, splits[i+1]

```