[julia-users] help with performance

[Frederico Novaes]

Hi,

I'm trying to write better (faster) code in Julia, and here's an example 
that I would like to understand where the difference in performance comes 
from:

function test_1(μ,Δσ)     
            
    ub = int(μ+Δσ)
    lb = int(μ-Δσ)
    
    n_el = ub - lb
    
    disc_dist = Array(Float64,n_el)

    n = lb
    for i = 1:n_el 
        disc_dist[i] = μ*σ*n 
        n += 1
    end


    # Return
    return disc_dist
end


function test_2(μ,σ,x_σ)     
            
    ub = int(μ+x_σ*σ)
    lb = int(μ-x_σ*σ)
    
    n_el = ub - lb

    disc_dist = Array(Float64,n_el)

    n = lb
    for i = 1:n_el 
        disc_dist[i] = μ*σ*n
        n += 1
    end

    # Return
    return disc_dist
end

function loop_states_1(n_states,μ,Δσ)    
    for i_state = 1:n_states
        result = test_1(μ,Δσ)
    end    
end

function loop_states_2(n_states,μ,σ,x_σ)    
    for i_state = 1:n_states
        result = test_2(μ,σ,x_σ)
    end    
end

# Parameters
σ = 4
μ = 22.3
x_σ = 2.0
Δσ = x_σ*σ
n_states = 500000


And then, when benchmarking:
@time test_1(μ,Δσ);

elapsed time: 2.4147e-5 seconds (1024 bytes allocated)

@time test_2(μ,σ,x_σ);
elapsed time: 1.382e-5 seconds (256 bytes allocated)

@time loop_states_1(n_states,μ,Δσ)
elapsed time: 1.598021522 seconds (472000096 bytes allocated, 32.01% gc time)

@time loop_states_2(n_states,μ,σ,x_σ)
elapsed time: 0.122917637 seconds (88000080 bytes allocated, 56.84% gc time)


The only difference between functions test_1 and test_2 is that the first one 
takes as arguments (μ,Δσ) and the latter (μ,σ,x_σ). 
Also the definitions for lb and ub. The output is the same. I've been reading 
about type instability, annotations, ... but I can't see where the difference 
in performance comes from. 

Any help ?