Why does a vector with 10 times more elements takes 2x-5x less time to pre-allocate?

General Usage Performance
7

Note that this is only the case on 1.10.4, in 1.11.1 I also got 3 allocations for both sizes.

This doesn’t seem to be true in 1.11: resize! essentially just calls _deleteend!, which is implemented as

function _deleteend!(a::Vector, delta::Integer)
    delta = Int(delta)
    len = length(a)
    0 <= delta <= len || throw(ArgumentError("_deleteend! requires delta in 0:length(a)"))
    newlen = len - delta
    for i in newlen+1:len
        @inbounds _unsetindex!(a, i)
    end
    setfield!(a, :size, (newlen,))
    return
end

(while in earlier versions it’s a ccall). So apart from changing the size attribute, it is also explicitly looping over all ‘deleted’ elements.

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