Assign values to a subarray and it take a long time and large allocation

General Usage Performance
,
1

I have functions such as

function max_(v::Matrix{Float64})
    m,idx=findmax(v,dims=2)
    Dt=vec(getindex.(idx,2))
    return m,Dt
end

function max_!(Vs::AbstractVector{Float64},Ds::AbstractVector{Float64},v::Matrix{Float64})
    Vs,idx=findmax(v,dims=2)
    Ds=vec(getindex.(idx,2))
end

I run this

mat=[[1,2.,3] [4.,5,6] [1.,2.,3.]]

Vs=zeros(3,3);Ds=zeros(3,3);

@time max_!((@view Vs[:,1]),(@view Ds[:,1]),mat)

It takes

0.057112 seconds (154.04 k allocations: 8.670 MiB, 99.92% compilation time)

and

then run this

@time Vs[:,1],Ds[:,1]=max_(mat)

It takes

0.037935 seconds (34.87 k allocations: 1.926 MiB, 99.73% compilation time)

If I try

@time V,D=max_(mat)

I get

0.000014 seconds (10 allocations: 528 bytes)

How could that be? Why is it so low to give values to a subarray?

2

you’re measuring compilation time, mostly :wink:
EDIT: second run here takes

julia> @time max_!((@view Vs[:,1]),(@view Ds[:,1]),mat);
0.000022 seconds (14 allocations: 656 bytes)
3

My bad, sorry! Contribute a meaningless question ::

4

These are perhaps just toy functions, but note that max_! does not modify the Vs and Ds input arguments. You need Ds .= vec for that.

1 Like
5

I did not know .= do necessary to change the value. Thank you for reminding! And these are toy functions.